After all the interplanetary battles are over, and the defender's space fleets have been reduced to ionized plasma or fled in panic, the final stage is entered. If the defender still resists, their planet is now the new target. The attacker will attempt to advance past the final defenses in order to loot, conquer, or destroy the defender's planet.

But please understand that bombing a planet back into the stone age is something that makes more sense in simplistic space operas, not in realpolitik.

A consideration for this:

If warfare is about causing the maximum destruction, these space siege scenarios make sense.

If warfare is about achieving political objectives by other means, you need to either leave someone to negotiate the surrender with, or leave something worth occupying.

If you're looking for numbers of boots on the ground to do occupation, look at the rules in Squadron Strike, which I had vetted by people who teach the US Army occupation duty and body counts.

One of the problems with wargames on this scale is that they're usually divorced from realpolitik.

A good way to illustrate this is that if the real world worked like most space gamers think planetary conquest worked, we'd'v'e given India a northern coast by making sure that Afghanistan had a mean altitude of 200 meters below datum...

Ken does have a good point. The motivation of the invaders puts limits on the allowed invasion techniques. If the invaders want slaves, it is counterproductive to kill every living thing on the defending planet. If the invaders want real estate, it is counterproductive to dust the planet with enough radioactive material to render it uninhabitable for the next ten thousand years. And so on.

The further underlying problem is: what do the aliens want? What is there that's easier to get by invading than by mining elsewhere in the solar system/local group/galaxy? The objective drives the means of invasion. Political domination is most easily achieved through infiltration — many politicians are easily bought or controlled through a combination of threats and gifts. (As Winchell posted while I typed!) Objectives beyond that seem like more trouble than they are worth.

James Sterrett

The lack of a logical reason for invasion is up to the author to devise a solution for. Some of the motivational questions can be side-stepped by assuming the invasion is not an alien one, but instead a hypothetical human interstellar empire attempting to invade a human colony world. The motivation of the empire can be something stupidly human like "gotta collect 'em all!". This is actually the motivation in Larry Niven and Jerry Pournelle's The Mote In God's Eye. In that novel, there once was a loosely allied human interstellar empire that collapsed in a bloody secession war. The new imperium rose from the ashes, grimly determined that such wars will not happen ever again, and all human worlds must be incorporated into the empire with no exceptions.

If one must have aliens invading because they want some crucial resource, I like to use an analogy. Ordinary resources are not worth it. I don't care what you saw in the TV show V, Markus Baur points out that aliens invading Terra to steal our water makes about as much sense as Eskimos invading Central America to steal their ice. The same goes for gold, uranium, or our women. But what if we hand-wave an unknown resource, something that our scientists have not even discovered yet? (Wow, Zzazel! Their planet is incredibly rich in polka-dotted quarks!)

Then us poor humans will find ourselves in the same spot as a primitive African tribe who does not understand why these Western stranger want to bulldoze their village in order to dig up the dirt. The westerners tell the tribesmen that the dirt is called "Coltain", from which they can extract something called "Tantalum", which is absolutely vital for something called a "Cell Phone." But to the tribesmen, it looks just like the same dirt that is everywhere else, and more specifically, in places that are not under their beloved village. This causes hard feelings, but unfortunately the westerners have something else called "automatic rifles".

Rick Robinson:

So I'm not sure there's really a place for space marines or not. It seems to me that, beyond what is essentially police work, space marines are only useful if you are ruthless enough to conquer planets, but not ruthless enough to do it by sheer terror.

Jon Brase:

As far as I'm concerned, planetary defenses can be pretty powerful and well protected (you can have big missiles, lots of propellant for them, big lasers, big generators and heat sinks for the lasers, and everything can be very well armored by the huge amount of rock available), to the point that to soften the defenses to where they can't stop space marines from landing means slagging the planet to the point that there isn't much of value left. So your options with an enemy planet are

  1. blockade them, and don't let them trade with anybody until they let you land marines and take over.
  2. threaten to slag them unless they let you land marines and take over.
  3. slag them.

Rick Robinson:

I pretty much agree. Given anything like the sort of techs we generally imagine here, a planetary landing and surface fight against serious resistance is horribly expensive and difficult. You have to spacelift large amounts of troops and munitions, then soft-land it all, fat slow targets coming down against defenders with surface concealment. The armament of your deep space warcraft may be be more or less useless against surface targets, requiring an additional force of fire support ships.

Sure, troops can land in remote areas — but then they are in remote areas, facing a long surface slog on a planet where the locals know their way around a lot better. And no matter where you land, shuttles coming down have a long re-entry trajectory during which they are extremely conspicuous and extremely vulnerable.

From a thread in SFConSim-l

As always it is absolutely crucial for the invaders to do their homework and perform an in-depth threat assessment before invading. In Christopher Anvil's novelette The Underhandler some aliens show up and decide to invade Terra. They quickly see that the global economy relies upon petroleum. So the strategy is to seize the oil and thus bring Terra to its knees. Well, let's see, the biggest concentration of petroleum is in this spot the humans call "the middle east." We'll send some troops in to seize it. How much resistance can they put up? It's not like everybody there is armed...

Which proved to be famous last words.

"On approach, we found a single-dominant-species planet varying in technological development from region to region. Electromagnetic surveillance revealed enormous differences in local languages and customs. It was obvious we couldn't have a binding agreement with such a collection of fragments. However, these differences seemed to offer excellent chances to defeat the locals one by one. And they were dependent on fossil oil to run their technology. Our analysis showed that a large part of this fossil oil came from a comparatively undeveloped region locally known as the Middleast. If we attacked this region, we might do several things:

"First, win a quick local victory.

"Second, overawe the rest of the factions.

"Third, paralyze these other factions at will, by withholding from them our portion of the fossil oil.

"Fourth, adroitly play off one faction against the other, using the fossil oil as a bargaining counter. "After all, they would be unprepared, and how could they possibly guess what would happen to them once we got control of the fossil oil?"

The voice came to a stop, and Kranf, frowning, said, "Then what?"

"Well—the relatively undeveloped region, for some reason, turned out to be overloaded with weapons. There was a little delay while they got over their surprise, then we got hit with everything—bullets, bombs, rockets, gas—it was like stealing meat out of the claws of a dozing thrakosnarr. Then the outside factions that we planned to finesse later on, came piling in. They had warplanes, long-range rocket-bombs, monster sea-borne floating fortresses, and every description of armored ground attack machine you can conceive of.

"Well, what could we do? We'd planned a neat surgical strike with minimum losses. Instead, the locals went berserk. There was no way we could hope to militarily fight it out on their terms—they had the whole resources of the planet at hand, and we only had what was with us.The obvious thing was to use our scientific superiority to hamstring them."


"We set up bioduplication bases, got out our stock of tailored pests, found out which ones seemed to fit, dropped around twenty thousand flights of sixteen-legged jangerls, stingbats, and burrowing trap-adders to poison and terrorize the natives, and give them a little warning that they'd better cooperate."

"How did that work?"

"Well, till we used the pests, there was tough resistance. After that, it got vicious. These split-up groups formed an alliance, got this native here, in front of us, to run things overall, and he got them actively working together. Before long, their measures and their counterblows were on a level we hadn't even imagined was possible. They even adopted a simplified common language to be able to hit us harder. Everything got worse after we started using pests. But what else could we do?"

"Be specific. What incident led you to call for help?"

"Well, we'd just landed two or three million forty-legged flatstings genetically engineered to kill natives, and the natives had come out with a dust that killed flatstings, and then they fired a swarm of missiles that came up off the planet and shot out into space. Only a few came anywhere near us, so we figured their control was breaking down. That was when I sent that report that we were getting the edge on them. Well, these missiles kept coming up, but we were happy to see them waste their firepower, figured we'd won, and called on them to surrender. About then, a missile streaked in from nowhere and hit us from behind. The next thing you knew, they were coming in from all directions, and we realized all these seemingly wasted missiles had been set to come back at us. There was no possible way we could defend against this.

"These things blew up the Moon Command Base, the bio-teams, the germ-synthesis labs, Tactical Combat Center, and Fleet Refit Base, and all that survived were our forces actually on the planet, and our ships in transit.

"That's when I called for help, sir. I've done my best, and every move I've made has been computer checked for maximum damage to them and maximum gain to us; but nothing worked. I'm out of my depth. Maybe somebody else can solve it."

From The Underhandler by Christopher Anvil (1990)

Orbital Bombardment

For an in-depth look at the topic, go to the indispensable Future War Stories.

If the concept of a huge cannon indirectly attacking targets over the horizon is "artillery", the concept of attacking planetary ground targets from orbit is "ortillery." (term was invented by Game Designer's Workshop)

Gravity Gauge

Two people throwing rocks at each other is pretty much a fair fight. If one person is on a hill, they have an advantage. And if one person is at the bottom of a well, that's not fair at all. By analogy, it is beyond unfair if one person is in orbit. The lucky one in orbit does not need to use bullets, missiles or nuclear weapons; a nice selection of rocks and boulders will do. Nudge a rock hard enough to de-orbit it, and it will strike with most of the kinetic energy difference between orbit and the ground. The poor slob on the ground, however, has to use huge rockets just to boost weapons up to the level of orbital person. This is called the gravity gauge.

Please note that "unfair" does NOT mean "impossible".


     "Have you ever wondered why the Patrol consists of nothing but officers—and student officers, cadets?"
     "Mmm, no, sir."
     "Naturally you wouldn't. We never wonder at what we grow up with. Strictly speaking, the Patrol is not a military organization at all."
     "I know, I know—you are trained to use weapons, you are under orders, you wear a uniform. But your purpose is not to fight, but to prevent fighting, by every possible means. The Patrol is not a fighting organization; it is the repository of weapons too dangerous to entrust to military men.
     "With the development last century of mass-destruction weapons, warfare became all offense and no defense, speaking broadly. A nation could launch a horrific attack but it could not even protect its own rocket bases. Then space travel came along.
     "The spaceship is the perfect answer in a military sense to the atom bomb, and to germ warfare and weather warfare. It can deliver an attack that can't be stopped—and it is utterly impossible to attack that spaceship from the surface of a planet."
     Matt nodded. "The gravity gauge."
     "Yes, the gravity gauge. Men on the surface of a planet are as helpless against men in spaceships as a man would be trying to conduct a rock-throwing fight from the bottom of a well. The man at the top of the well has gravity working for him.
     "We might have ended up with the tightest, most nearly unbreakable tyranny the world has ever seen. But the human race got a couple of lucky breaks and it didn't work out that way. It's the business of the Patrol to see that it stays lucky.
     "But the Patrol can't drop an atom bomb simply because some pipsqueak Hitler has made a power grab and might some day, when he has time enough, build spaceships and mass-destruction weapons. The power is too great, too awkward—it's like trying to keep order in a nursery with a loaded gun instead of a switch.
     "The space marines are the Patrol's switch."

(ed note: of course the gravity gauge isn't quite as much of a problem when the planetary defenses include huge laser weapons. Lasers had not been invented yet when the novel was written. But as Heinlein observes below, lasers are pretty worthless if you are being bombarded by house-sized bolders.)

From SPACE CADET by Robert Heinlein (1948)

While it is possible to target the enemy even if the only friendly observers are in orbit, accuracy will be much improved if there is a human or robot on the ground close to the target giving target coordinates. These are called artillery observers, spotters, forward observer, fire support specialist, or fister. Though I suppose in this case they will be called ortillery observers instead.

Of course ortillery shares with artillery the ever-present danger of "friendly fire. If your army units are on the planet battling enemy units, and you have ortillery assets in orbit, often you will need to call down ortillery strikes on hostile positions. But there are many assorted failure modes that will result in the strike hitting your units instead. Weapon malfunctions, ortillery operator mistakes, inaccurate target coordinates, there are many opportunities for things to go badly wrong.

Orbiting a string of nuclear weapons aimed at Earth would be an easy way of conquering the world. Or a Lunar missile base. This was why it was outlawed in the SALT II treaty of 1979. Robert Heinlein wrote about this in his novel Space Cadet and the short story "The Long Watch".

Or maybe it wasn't such a good idea in the first place. The blog Tales Of Future Past points out that neither the Moon nor Earth orbital bases turned out to offer any sort of advantage over surface-based missiles. Lunar bases are easy to target, require missiles with huge amounts of delta-V to deliver the nuclear weapon to the target on Earth, and will take days of transit time. Orbital bombs have utterly predictable orbits and can be seen by everybody (unlike ground based missiles), can only be sent to their target at infrequent intervals (unlike ground based missiles), and will require a deorbiting rocket with pretty much the same delta-V as a ground base missile. So what is the advantage? Please note that not all of these drawbacks apply to enemy spacecraft laying siege to Terra.

Attacking spacecraft dropping nuclear weapons would be somewhat like the situation faced by nations threatened by enemy intercontinental ballistic missiles except that in this case the weapons have no boost phase. The discredited Strategic Defense Initiative had all sorts of ideas of how to deal with the problem. For our purposes, ignore any solution that depends upon the boost phase (since there isn't any), space-based programs are "orbital fortresses", and ground-based programs are "planetary fortresses".

Ah, Luke Campbell points out that I'm wrong, there will be a boost phase.

Another detail — munitions will have a boost phase, otherwise they will just end up drifting next to the spacecraft that released them. So either the spacecraft boosts the munition using a gun or cannon or coilgun or something that can impart enough delta-V to de-orbit it, or the munition de-orbits itself in a similar fashion using a rocket.


For myself and my setting, I concluded that at least some aspects of the “kill it with nuclear fire” school are going to be more or less inescapable because of, as you point out, how good the planetary defenders have it.

What the Ley Accordsthe Eldraeverse equivalent of the Geneva Convention, essentially – actually says is that you can’t use planetary bombardment indiscriminately on civilian populations or to make terror strikes, and once you’ve disabled the orbital defenses and “own the high orbitals”, you’re supposed to ask for their surrender before you start firing on the legitimate military targets…

…because once you’ve started dropping heavy enough hellflowers (air-burst antimatter sterilization/EMP weapons), stoneburners (sub-ground-burst anti-bunker burrowing antimatter shaped-charges), and plain old k-rods (Rods From God) to take out deep-running submarines, crust-embedded fortresses, giant planetary lasers, etc., etc., there’s no way not to do major damage to the planet even if you’re not trying to, or indeed if you’re trying not to. If you’re lucky, you’ll get away with a few dozen simultaneous earthquakes/tsunamis/wildfires/hurricanes/massive radiation events/etc. worth of damage. If you’re unlucky, you throw enough debris into the air to give you a particle winter and a major extinction event. And either way, depending on how careful the planetary government is to keep its military facilities in the middle of nowhere, you’ve got megadeaths to gigadeaths.

The polities that are both (a) established galactic citizens, and (b) halfway civilized, all understand this, and that you’re supposed to surrender the planet when you lose the orbital defenses, because while you might not be able to take it back, you definitely can’t un-wreck it.

(Even if you intend to fight a guerilla war groundside afterwards and are willing to absorb the damage from that, you may still find it worthwhile to surrender any formal planetary defenses you invested in. At least that way they’re only going to be dropping tactical k-rods on you…

…but there’s no upside to engaging in a pissing contest with starship-class weapons and their planet-mounted equivalents when the planet is going to take all the collateral damage, and the fleet in orbit doesn’t have to worry about that.)

Thus, Imperial admirals hate having to fight galactic newbies (who might be under the impression that you can fight and win an orbit/ground battle without taking horrific collateral damage), or worse, the kind of fanatics who don’t mind taking their population and ecology along with them when they go. (Although, in practice, there’s usually someone in the latter’s command structure willing to introduce their leader to a bullet rather than let him initiate Ragnarok.) Even Caliéne “the Worldburner” Sargas-ith-Sargas, the IN’s mostly-tame sociopath, thinks it’s a little messy and inelegant.

From SLAG THEM! by Alistair Young (2015)

“You’re Liiriani, yes?” The recruiter eyed the tattered uniforms on those crowding into his prefab. “Ex-military. Wait… you’re Temple Guard? The ones left behind after the fall of Mantaniir?”

“Yeah. I was at Mantaniir. We all were.” The scarred veteran’s lip curled, and he spat. “Proud Mantaniir. Glorious Mantaniir. Mantaniir the Unfallen, Guardian of the Holies, all of that. Well, it didn’t fall, or we’d be dead. It was swept aside like it was nothing.”

“The first day could have been the last day. We —“

…were prepared, we were ready, we were the last line of defense for Iliir itself, and we knew they were coming at dawn. They’d told us that much. But we heard nothing. Saw nothing. Not until dawn.

We’d never fought a space war before. No-one understood what it meant that we’d lost the high orbitals. Not until the k-rods started falling, and then it was too late to help us. The minefields down-valley went in the first wave — to give us time to see what was killing us. The flak towers went in the next, along with communications and sensors. Then they started drunkwalking their shots around the valley, blasting walls, barracks, everything left of the fortress flat. What was left of us had run for the bunkers by then, and down through them into the deep tunnels. Couldn’t so much as get a shot off. We were down there for days — any time someone made a run for it, or poked so much as a nose-tip above ground, they dropped a k-rod on them. We had no power — if any generators started up, that bunker got a k-rod within minutes. Just hiding in the dark.

And then the machines hit us, wolves and spiders. From both sides — we heard later that their stormtroopers bypassed us and dropped on Iliir directly. Wolves, the little ones, ‘bots that run in packs, wall, ceiling, or floor, see in the dark, spit bullets or tear a man’s leg off themselves. And then the spiders, big eight-legged bastards with fire and cutting torches and rockets. All howling to each other like the gods below. And they wouldn’t die! Enough explosive might stop one, but if it wasn’t torn apart, it’d fix itself — or the rest of them would — and come after you again.

So we surrendered. The spiders herded us outside again, up among the craters, and fenced us in with electrowire. A couple of us tried to make a break for it. They didn’t get past the perimeter. Spiders didn’t care — they just sat there watching us, day and night. A couple of days later, one of their armor boys came by to look us over, and left us a crate of rat-bars and a medkit. Then he left us there with just the spiders to watch us. That was the only enemy we saw in the entire battle.

Two weeks later, we got word that the war was over, the Council had been captured, surrendered, were killed, one of those. The spiders all marched back into a shuttle and left us alone, then, so we scavenged what we could, tried to stay alive. A week after that, the new Council had all of us who’d let Iliir fall through our ‘heretical incompetence’ shoved aboard an old ore freighter and dumped us on this craphole planet.

“— are what’s left of the Liirian Temple Guard, yeah. Seventh Fist Ileer, commanding. And me an’ the boys’ll fight for you. Nothing else left for us now. But only if we’re fighting men. Nothing that don’t bleed and won’t die.”

— Sagivv’s Company recruitment interview, Márch, eight months after the Liir Conflict


The entire cost analysis raises the question of the alternative to invasion.  The basic strategy is to destroy the planet’s orbital defense and force the defender to surrender under threat of bombardment.  If the defender refuses, the attacker can land forces with impunity and support them with orbital bombardment.  The options for bombardment are the same as for normal space warfare: lasers and kinetics.

Lasers have a number of excellent qualities for attacking surface targets.  First, they respond at the speed of light, so troops on the ground don’t have to wait very long for fire support, and any potential target can’t move out of the way.  Second, their accuracy is very good, which is useful when attacking targets that are in a populated area.  However, lasers also have their drawbacks.  First and foremost, the atmosphere absorbs frequencies above the visible spectrum quite effectively, preventing lasers operating in those frequencies from attacking surface targets.  As such frequencies are the most efficient for attacking other spacecraft, it is entirely possible that specialized bombardment craft would be required.  While some lasers (such as FELs) can alter their frequency, the same does not apply to the optics involved, making such capability superfluous.  Secondly, the laser is a line-of-sight weapon, and is affected by the distance it must travel, both in total and through the atmosphere.  This sets tradeoffs between accuracy and time on station.  A laserstars in low orbit will obviously have the smallest spot sizes and best pointing accuracy, but it only sees the target for a few minutes on each pass.  Even when it can see the target, its low altitude means that for a significant portion of its orbit it will be shooting through a lot of the atmosphere.  Furthermore, the planet’s rotation will ensure that the next pass will have a groundtrack well away from the target (unless it is located on the equator or at the poles), which limits availability to a pass or two a day.  

Higher altitudes have lower accuracy due to increased range, but greater availability per pass, thanks to the higher angle.  Moreover, the orbit can be made elliptical, which ensures that more of the orbit can be spent over the target.  The theoretical optimum, ignoring the effects of range, is a geostationary orbit, which keeps the ship directly over the area of interest.  The angle of fire through the atmosphere is a potential problem when shooting at higher latitudes.  This can be partially solved through the use of Molniya orbits, which give improved coverage in such areas.  Targets near the pole do not suffer the normal problems of groundtrack movement, as the target is close to the groundtrack no matter what.

The use of multiple laser platforms can solve many of these problems, and the question then becomes finding the balance between coverage and the minimum number of platforms.  As a first approximation, somewhere between 12 and 24 bombardment platforms will be required for constant global coverage, although less could be used if stationed in very high orbit.  However, coverage itself might not be sufficient, particularly in hotly-contested areas, where multiple platforms would likely be required at all times.  Significant study has already been done in this area (although done with respect to missile defense instead of planetary bombardment, many of the underlying principles apply) and the author will not investigate that topic further.

Before a discussion of kinetic bombardment, a basic treatment of the projectiles used (orbit-to-surface kinetics, or OSKs) is in order.  A rule of thumb for OSKs is that the area density will need to be at least 10 tons/m (strictly speaking, the area density needs to be at least that of the atmosphere of the target body, and the numbers given are for Earth).  If the area density is below this threshold, the object will reach the ground at low velocity, while objects with area densities higher than 10 tons/m2 will still be moving quickly at impact, making them potential weapons.  To put that into perspective, an OSK made of iron will need to be about 1.27 m long to achieve the required area density (independent of diameter) while one made of tungsten will only need to be .52 m long.  This length in turn sets the minimum feasible diameter to avoid buckling, and ratios of 20 or 30 to 1 are probably feasible, giving a minimum mass on the order of 5 kg for the tungsten rod and 14 kg for the iron.  Buckling is unlikely to be a design driver, however, as a shape with a flared base is more stable on entry.  A larger OSK will, all else being equal, be more efficient than a smaller one, retaining more of its mass and velocity on impact.

Another potentially serious problem is entry heating.  An OSK depends upon retaining as much of its velocity as possible, which in turn means that it will be receiving large heat loads when it is low in the atmosphere.  When dealing with the heat loads present in OSKs, ablation will occur, and the projectile must be carefully designed to avoid having said ablation destroy its aerodynamic characteristics.  This problem could also occur if the projectile was damaged, which would ease the problem of surface defense.  If the projectile successfully impacts the target, it will tend to penetrate and bury itself in the target, limiting damage to the surrounding area.  In fact, this effect has been compared to a shaped charge.

All of the above applies mostly to OSKs that are long in proportion to their diameter.  If the projectile is instead about equal in length and diameter, it will tend to produce a crater and do more damage to the surrounding area.  The problem, however, is that this requires a large projectile.  For tungsten, the OSK will need to mass approximately 2.7 tons for cratering to occur, while an iron projectile will need to be 16.1 tons and a typical rocky projectile (3 g/cc) will be about 110 tons.  It should be noted that even for cratering kinetics, the damage is still downwards and outwards, which is not optimal for area effects such as attacking troops.  

It appears that the best way to achieve area effects with OSKs is to intentionally break up the OSK near the ground, producing something akin to a meteor airburst.  The best-known example of this phenomena is the Tunguska Event of 1908, but similar meteor “explosions” occur frequently.  The Chelyabinsk Meteor in February of 2013 is another excellent example of a projectile producing bomb-like effects when it breaks up, although it was much larger than would be practical for military use.  What occurs is that the sudden transition from a single, low-drag object to a large number of small, high-drag objects dumps a large amount of energy into the atmosphere in the form of heat, which produces effects akin to an explosion.  The exact form and magnitude of this effect is uncertain, but it appears that a fireball would be formed and would continue on the course of the OSK.  There would also be some blast, but the exact dynamics of the outcome are currently unknown to the author.  Either the blast or the fireball would be useful against area targets such as troops or light vehicles, which traditional OSKs are largely ineffective against.

While the OSK is in the lower atmosphere, it will be sheathed in plasma due to its velocity.  This prevents both the use of onboard sensors and communication with the outside world (although there have been suggestions that communications could be made possible).  This renders traditional OSKs useless against maneuvering targets, as they are, at best, inertially guided for the last few minutes of their flights.  It is possible that the plasma sheathe will provide protection against lasers, but the magnitude of the effect is unknown.

This raises another option, an OSK that intentionally dumps most of its energy and finishes its flight at only 2 km/s or so.  This actually increases penetration, and allows guided projectiles to be used.  Some form of heat shield would be required, but that is a fairly simple matter to build.  This sort of kinetic would be useful against targets like tanks, particularly if deployed from an “Orbital Death Pod” of some sort.  Other types of conventional munitions could also be carried in the ODPs, such as cluster munitions, nuclear weapons (although most OSK research is based on ICBM RVs) or even autonomous UAVs.  The main drawback to this type of OSK is that it is somewhat more vulnerable to planetary defenses, as it sheds most of its speed before impact, which potentially puts it in the kill envelope of many SAMs.  They are, however, less vulnerable than manned drop pods, as they can decelerate much more quickly, and thus hold their velocity lower in the atmosphere.

The deployment of OSKs is subject to some of the same constraints of orbital mechanics as are laser bombardment platforms, but there are significant differences in the effects of said constraints.  First off, OSKs, unlike lasers, do not have instant response times.  Times as low as 12 minutes have been cited, but these require large numbers (40-150) of satellites in low orbit to achieve continuous coverage, and the projectiles will enter the atmosphere at low velocity, reducing lethality.    A requirement for continuous coverage obviously does not apply for strategic bombardment, but it does suggest that small sub-busses might be used for ground support.  If the launching platform is at an altitude of 6000 km, the flight time will be approximately 75 minutes, while a geostationary launcher will have a flight time of about 12 hours, although these can be reduced somewhat with more delta-V.  The delta-V requirements for deployment vary significantly, depending on range and entry angle.  A high entry angle is desirable, to minimize the distance traversed in the lower atmosphere, but requires more delta-V than a shallow trajectory.  

Crossrange is another serious concern, and would require yet more delta-V.  The exact magnitudes involved vary significantly with the initial orbit and the time between launch and impact.  See Appendix B of Space Weapons, Earth Wars for more details.

All of this suggests that large rail/coilguns might be the weapons of choice for general planetary bombardment, massive concentrations of projectiles might not be necessary and the savings over missiles could be substantial.  The delta-V requirements (generally somewhere between 10 and 15 km/s) are well within the capabilities of coilguns, and the logistics advantages would be substantial.

The tactics required to neutralize planetary defenses (see Section 4) will vary based on the type of defense.  While a number of systems are fixed, and thus can be dealt with via massive firepower, mobile systems are a much tougher target to handle.  Hunting mobile targets is quite likely the only place where any sort of ground forces have a place.  These would be small teams, similar to the LRRPs (Long Range Reconnaissance Patrols) of Vietnam, tasked with locating hidden missile launchers (likely the truck-mounted missiles discussed in Section 4) and reporting their location to the bombardment forces.  It has been suggested that these teams instead be used to attack the launchers directly, but there are three major drawbacks to this.  The first is that the logistics burden of moving people is significantly higher than that of moving kinetics, and lasers have very low logistical requirements.  Even with low casualty rates, the mass economy of this strategy is dubious in the extreme.  Secondly, even if the mass efficiency is slightly better, the fact that human casualties are part of this scheme is likely to doom it.  Lastly, it is impossible to directly tell what gave a position away from a kinetic strike, but a ground raid is quite obvious.  The countermeasure is thus to move hunter-killer teams to the area and guard the other launchers.  If a kinetic strike takes out a platform, the defender has to decide between looking for LRRPs (or UGVs), looking for UAVs, putting more camouflage up, trying for better indirect camouflage (operational patterns and radio discipline, for instance), or hunting for spies.  It should be noted, however, that such teams more or less require low-orbit fire support, given the time lag in kinetic drops from high orbit.  

There might be an occasional target that cannot be killed directly from orbit, but there are several ways of dealing with such targets.  One possibility is a weapon similar to the W54Davy Crocket” atomic rocket launcher of the Cold War.  This has the advantage of operating outside the envelope of most defensive systems deployed against orbital attack.  The team operating it would still have the difficult task of getting it in place and getting away, but the chances of doing so are probably fairly good.  Another option is a cruise missile dropped well away from the defenses, which then flies in on its own.  Some versions of the Tomahawk cruise missile have ranges as long as 2,500 km, while the AGM-129 ACM has a range of 3,700 km.  These are significantly larger than the danger zone of even long-range SOMs, giving the missile more or less free entry into the atmosphere.  The warhead could be either nuclear or conventional, as required by the nature of the target.  Such missiles would remain vulnerable to air defenses, but their small size, relatively low cost, and lack of need to return to orbit make them an attractive proposition.  It has also been suggested that commando teams could be used directly against certain strategic targets, generally command and control, but the ease with which these can be defended against such attacks makes them a dubious proposition compared to orbital or remote bombardment.

Redirected asteroids are often proposed as a means of attacking planetary targets.  The usual logic goes that they are “free” and thus the cheap way of destroying a planet.  From a military point of view, however, asteroids are nearly useless.  First off, small asteroids are not suitable for use in this manner.  While purpose-built kinetics can be used to hit targets with minimal collateral damage, any natural asteroid of comparable size will either slow down to the point of uselessness or disintegrate in the upper atmosphere.  Larger asteroids (>1 km) are useless except for ecocide.  While this might occasionally be the goal, it falls outside the scope of this paper.  A medium-sized asteroid, such as the aforementioned Chelyabinsk Meteor, might have some uses, but it would be unreliable, and difficult to use, as that meteor is estimated to have mass 11,000 tons.  The damage mechanism would be the same as that proposed above for airbust kinetics, except for a lack of precise control and a much higher lower size limit.  (Oddly enough, when RAND studied this issue for Space Weapons, Earth Wars, they noted the use of asteroids for airbust, but neglected to apply it to man-made kinetics.)

The biggest problem with asteroids is their ease of use relative to nuclear weapons.  Hauling an asteroid from the belt is quite difficult, and truly silly.  More plausibly, there are a significant number of earth-crossing asteroids which might be suitable for this use.  The number available (and thus the minimum lag time) is based upon the amount of delta-V that can be applied, and when.  For example, 10-meter iron asteroids (about the minimum size that can be considered as a useful weapon) hit the Earth about 3 times a century.  If one is diverted that passes within 1.35 light-seconds (the distance of the moon), there are about two a week available.  Larger objects will pass by less often, although stony asteroids become viable weapons as the size increases.  These types of asteroids probably have windows of a few months.  Particularly in the case of smaller asteroids, the primary delay driver will be the diversion process.

The diversion itself will not take that long.  With a few months lead time, the delta-V required is generally in the tens or hundreds of m/s, which given projected mass driver technology would involve the use of 30% of the object as remass at the upper end.  The power requirements will likely force this acceleration to be made over a fairly long period, pushing the lag even higher.  If the asteroid is diverted later, the delta-V requirements skyrocket.  At one month out, 1 km/s is passed, at which point about half of the asteroid is being used as remass.

The time lag is the greatest drawback to the use of asteroids for any sort of bombardment.  Assuming that the defender is somewhere near technological parity, he will be able to deflect away any asteroid you can send towards him given enough time.  Depending on the exact nature of the asteroid in question, the time he has available could vary significantly.  If the incoming asteroid is intended to take out a city, even a small deflection quite late would send it over rural or uninhabited territory, greatly reducing damage.  This forces the attacker to guard the asteroid for months while it is moved into position, and the logistics costs of doing so are quite high.  There is a significant economy of scale in bigger asteroids, however.  They require the same protection as the smaller ones, and the cost scaling of the mass driver will be negligible compared to the cost of the naval forces deployed to protect it.  Furthermore, the asteroid itself cannot be deflected as easily at the last instant, either through a last-ditch effort to break through the fleet guarding it or after the fleet has pulled off to avoid following it into the atmosphere.

The above concepts are often associated with the name ‘Project Thor’, which is believed to be the Air Force’s original study of the idea of orbital bombardment.  However, the author has found no reason to believe this to be the case.  There was an official Project Thor, but it was a study of fragmentation ballistics, and had nothing to do with orbital bombardment.  Moreover, research has been unable to turn up any official studies of orbital kinetic bombardment that are above the level of that done in Space Weapons, Earth Wars.  The name Project Thor appears to have originated with Jerry Pournelle, who thought of the concept while working in operations research, and appears to have popularized the term.  The author believes it was his personal term, and in no way official.

The primary alternative for destroying strategic ground targets is nuclear weapons.  The cost of a comparable nuclear weapon is almost certain to be no greater than that of the asteroid-deflection operation, particularly when the fleet operations costs are factored in, and depending on the technologies involved, it is very likely to be significantly less.  The nuclear weapons are also more accurate and predictable, and have none of the lag problems associated with asteroids.  The only area where an asteroid wins over a nuclear weapon is in dealing with terminal defenses.  While a nuclear weapon (which would almost certainly resemble an ICBM RV) can be engaged and destroyed by ABM-class weapons, an asteroid would be virtually unaffected by any defenses after it entered the atmosphere.  A nuclear weapon fired at it might be able to disrupt it and cause it to dump its energy high in the atmosphere, but that might still have serious climatic effects, as well as the possibility of some damage on the surface, and the damage done by the nuclear weapon itself.

All in all, the disadvantages in redirecting even NEOs are likely to outweigh any possible cost advantages.  The only practical reason to do so might be for purposes of psychological effects.  The defenders would know exactly how difficult it is to use an asteroid as a weapon, and the fact that it was done anyway would send a very clear message.  Another vaguely possible cause would be a total ban on the use of nuclear weapons, but given that such a ban shows no signs of happening today, and would be next to impossible to enforce, and that nuclear-electric drives are assumed throughout this paper, this is unlikely.  As Space Weapons, Earth Wars put it: “Because much cheaper, more responsive weapons of mass destruction are readily available, this one is likely to remain safely in the realm of science fiction.”  For a more irreverent treatment of the issue, Google “Rocks are not free”.

A related issue that deserves a brief mention is the use of spacecraft as weapons against ground targets.  This is unlikely, to say the least.  As mentioned above, a sectional density of 10 tons/m2 is required to make it through the atmosphere with a reasonable amount of velocity remaining.  While common ships might reach this threshold, they are almost certain to fail structurally long before impact.  When they break up, the pieces are unlikely to retain sufficient sectional density to reach the ground.  The result is a high-altitude airburst, which, given expected vessel sizes, is unlikely to do significant damage.  It has even been suggested that vessels be intentionally designed to break up on atmospheric entry to reduce the risk to people on the ground.  Even if the ship was theoretically capable of doing damage to ground targets, there still remains the issue of actually hitting the target, and the prospects on that front are dubious at best.

The basic strategy of the attacker during a planetary invasion is to destroy enough of the surface defenses by bombardment to be able to dictate terms to the defender, or support troops should that be necessary.  The mechanism by which to do this is more or less that of moving into range and dueling with them.  This in turn reveals the locations of the fixed defenses, and forces the mobile ones to either fire their missiles (which then renders them ineffective for the rest of the siege) or risk revealing their location.  The weapons can be forced to fire to protect targets on the ground, as the spacecraft will have to fight them instead of conducting its bombardment mission.  The attacker would continue to do so until he had reduced the defenses to a point that he could seriously consider landing troops. This would not be the point at which all of the defenses had been eliminated, but the point at which the attacker could provide cover for a landing from his fleet, and do so with confidence that he could protect both his fleet and the drop pods.

Before the actual landing, the attacker would probably call the defenders and offer good terms for them if they were to surrender at this time, and tell them that the terms would become much worse if the landing went through.  Most defenders would probably surrender at this time, as they can no longer expect to resist with hope of long-term success.  If they refuse, they either consider death better than capitulation, or believe that they can defeat the landing.  At this juncture, a reputation for honesty and honor would greatly help the attacker.  Conversely, if the defender waits until the attacker begins to land and opens fire, the attacker will probably show no mercy and destroy the defender’s cities.  This holds the leaders hostage to the people, ensuring they go through with their side of the bargain.

It has been suggested that in some cases, the attackers cannot offer terms, primarily due to unreasonable political leadership.  This situation is outside the scope of this paper, but the other side of the same problem, an unreasonable defender, is not.  In this case, clearing as much of the orbital defenses as possible is vital, so that the ground troops can have access to continuous space support.  This should serve as a massive force multiplier, allowing a reasonable amount of troops to capture the world.  Occupation is likely to be a much bigger problem, but it is one that lies outside the scope of this paper.

by Byron Coffey (2016)

(ed note: this game is a sequel to the tabletop game StarForce: Alpha Centauri. Due to the odd background universe, literally the only valuable thing a planet has to offer is the colonists. Therefore in the game StarSoldier, the soldiers have to avoid causing civilian casualties at all costs.)

"Teleships" are starships, their faster-than-light movement is called "shifting". A "StarGate" is a sort of orbital fortress defending the planet from invading Teleships. The "Heissen Field" is a weapon that allows starships in orbit to render everybody on the planet unconsious. Everybody that is unprotected, defending StarSoldiers are unaffected.)



Undisputed control of local space is a doctrinal prerequisite to any attempt by a StarForce to induce a Heissen Field and land StarSoldiers on an unfriendly planet. It is therefore usually the case that only one side—the side attacking, in the strategic sense—that will be able to call upon off-surface support. And being extremely destructive, Orbital Ground Support is only utilized in extreme circumstances. In any event, the provision of support bombardment by even "unopposed" StarForces is somewhat problematic, as the presence of automated and StarSoldier manned defensive missile batteries and laser banks on the surface of the planet has the capacity to make things difficult for orbiting Teleships. A StarForce may not move or defend itself telesthetically within the proximity of the gravity fields which characterize solar systems, and so is dependent upon "conventional" kinetic drive (Energy Modulation Packs) and computer-directed laser interception for those tasks. Faced with a powerful surface defense utilized to capacity, Teleships generally adopt variable geometary orbits, which allow them to approach closely to the planet for brief and unpredictable passes. At least two StarForces (eight TeleShips) are required to provide effective ground support under such circumstances.


The rest of the city seemed to have died of neglect rather than violence. It certainly hadn't been bombed out. Harkaman thought most of the fighting had been done with subneutron bombs or Omega-ray bombs, that killed the people without damaging the real estate. Or bio-weapons; a man-made plague that had gotten out of control and all but depopulated the planet.

From SPACE VIKING by H. Beam Piper (1962)

Project Thor

Back before he was a science fiction author, Dr. Jerry Pournelle was working in operations research at Boeing. There he came up with the concept for Project Thor, aka "Rods from God". The USAF calls them "hypervelocity rod bundles.

(so it is not true that Project Thor was "invented by a science fiction writer", Dr. Pournelle had not yet started his writing career when he created it)

The weapons are rods of tungsten, ranging in size from that of a crowbar to that of a telephone pole (about 12 meters for all you young whipper snappers who have never seen a land-line phone). Each one has a small computer in the rear and control fins on the nose, i.e., they are dirt cheap and can be mass produced. Boost them into orbit, and each one can be deorbited to strike a specific target anywhere on Earth in a few minutes, striking it at about 3 to 9 kilometers per second. This is equal to 1 to 3 Ricks worth of damage, which means the unfortunate target will be on the receiving end of the equivalent of 3 kilograms of TNT for each kilogram of tungsten rod from god. Not bad for a crowbar. Especially since they are not covered under the SALT II treaty.

A 2003 USAF report describes rods that are 6.1 m × 0.3 m tungsten cylinder The report says that while orbital velocity is 9 kilometers pre second, the design under consideration would have slowed down to about 3 kilometers per second by the time it hit the target. The report estimates that the rod will impact with a force of 11.5 tons of TNT. The back of my envelope says that a cylinder that size composed of pure tungsten will have a mass of 8.3 metric tons, but the figures in the USAF report imply that the rod has a mass of 8.9 metric tons. Which is close enough for government work.

11.5 tons of TNT per rod is pretty pathetic, you might as well use a conventional bomb. This is because 3 kilometers per second is 1 Rick, which means each kilogram of rod is equal to one kilogram of TNT, so why not just drop TNT from a conventional bomber?

An article in Popular Science breathlessly suggests that the rods will strike the target at 11 kilometers per second. This is 13.4 Ricks, which will give the rod an impact of 120 metric tons of TNT. That's more like it, now we are getting into tactical nuclear weapons levels of damage. But the article does not explain how the rod is suppose to start at 9 km/s and strike at 11 km/s after being slowed by atmospheric friction. Popular Science left that as an exercise for the reader.

The rod is admittedly quite difficult for the enemy to defend against. It is moving like a bat out of hell, er, ah, has a very high closing velocity, and it has a tiny radar cross section.

The trouble is, the "plasma sheath" created by atmospheric re-entry prevents remote control of the rod. Radio cannot pass through the plasma, so the bar has to be inertially guided. Or not. A Russian scientist thinks they have found the key to allowing radio signals to pass through the plasma sheath. A related problem is that anything on the rod that is not made of tungsten is going to want to burn up in re-entry. Things like the guidance computer, sensors, and hypothetical remote control radio.

The main drawback to Project Thor is the prohibitive cost of boosting the rods into their patrol orbits. Of course if you have a space-faring civilization, the rods can be manufactured already in orbit, thus eliminating the boost cost. Which means any planetary nation without a presence in space is going to be at a severe disadvantage, but that is always true.

Another problem is maintaining the rods in orbit. Things are going to break down, so you either have to have a budget to boost replacements or have assets in orbit that can do maintenance.

Finally, no, this is not the same as the Magnetic Accelerator Cannon from the Halo games. That is a coil gun, Project Thor is more like a weaponized version of dropping a penny from the top of the Empire State building.

Predictably, some maniac made a "Rods from God" mod for the game Kerbal Space Program.

The basic weapon system consists of an orbiting element some 20 to 40 feet long. It requires a GPS receiver to locate itself; a means of taking it out of orbit; an atmospheric guidance system, such as a means of changing its center of gravity (moving weights, small fins, etc.), and a communication system to give it a target and activate the system. No warhead is wanted or needed. Thor will impact a target area at about 12,000 feet per second (3.7 km/s); that is sufficient kinetic energy to destroy most hard targets, with minimum collateral damage and of course no fall-out. Achievable accuracy has been estimated at ten to twenty feet CEP.

ed note: CEP is Circular Error of Probablility. It is the radius of a circle inside of which 50% of the missiles will fall.


A kinetic bombardment or a kinetic orbital strike is the hypothetical act of attacking a planetary surface with an inert projectile, where the destructive force comes from the kinetic energy of the projectile impacting at very high velocities. The concept originated during the Cold War.

The typical depiction of the tactic is of a satellite containing a magazine of tungsten rods and a directional thrust system. When a strike is ordered, the satellite would brake one of the rods out of its orbit and into a suborbital trajectory that intersects the target. As the rod approaches periapsis and the target due to gravity, it picks up immense speed until it begins decelerating in the atmosphere and reaches terminal velocity shortly before impact. The rods would typically be shaped to minimize air resistance and maximize terminal velocity. In science fiction, the weapon is often depicted as being launched from a spaceship, instead of a satellite.

Kinetic bombardment has the advantage of being able to deliver projectiles from a very high angle at a very high speed, making them extremely difficult to defend against. In addition, projectiles would not require explosive warheads, and—in the simplest designs—would consist entirely of solid metal rods, giving rise to the common nickname "Rods from God". Disadvantages include the technical difficulties of ensuring accuracy and the prohibitively high costs of positioning ammunition in orbit.

The Outer Space Treaty is designed to prohibit weapons of mass destruction in orbit or outer space; however, its text does not formally define what constitutes a weapon of mass destruction. Since the most common form of kinetic ammunition is inert tungsten rods, it is uncertain if kinetic bombardment is not prohibited by treaty.

Real life concepts and theories

During the Vietnam War, there was limited use of the Lazy Dog bomb, a steel projectile shaped like a conventional bomb but only about 25.4 mm (1") long and 9.525 mm (3/8") diameter. A piece of sheet metal was folded to make the fins and welded to the rear of the projectile. These were dumped from aircraft onto enemy troops and had the same effect as a machine gun fired vertically. Observers visiting a battlefield after an attack said it looked like the ground had been 'tenderized' using a gigantic fork. Bodies had been penetrated longitudinally from shoulder to lower abdomen.

Project Thor is an idea for a weapons system that launches telephone pole-sized kinetic projectiles made from tungsten from Earth's orbit to damage targets on the ground. Jerry Pournelle originated the concept while working in operations research at Boeing in the 1950s before becoming a science-fiction writer.

The system most often described is "an orbiting tungsten telephone pole with small fins and a computer in the back for guidance". The system described in the 2003 United States Air Force report was that of 20-foot-long (6.1 m), 1-foot-diameter (0.30 m) tungsten rods, that are satellite controlled, and have global strike capability, with impact speeds of Mach 10.

The time between deorbit and impact would only be a few minutes, and depending on the orbits and positions in the orbits, the system would have a worldwide range. There would be no need to deploy missiles, aircraft or other vehicles. Although the SALT II (1979) prohibited the deployment of orbital weapons of mass destruction, it did not prohibit the deployment of conventional weapons. The system is not prohibited by either the Outer Space Treaty or the Anti-Ballistic Missile Treaty.

The idea is that the weapon would naturally contain a large kinetic energy, because it moves at orbital velocities, at least 8 kilometers per second. As the rod would approach Earth it would necessarily lose most of the velocity, but the remaining energy would cause considerable damage. Some systems are quoted as having the yield of a small tactical nuclear bomb. These designs are envisioned as a bunker buster. As the name suggests, the 'bunker buster' is powerful enough to destroy a nuclear bunker. With 6–8 satellites on a given orbit, a target could be hit within 12–15 minutes from any given time, less than half the time taken by an ICBM and without the launch warning. Such a system could also be equipped with sensors to detect incoming anti-ballistic missile-type threats and relatively light protective measures to use against them (e.g. Hit-To-Kill Missiles or megawatt-class chemical laser).

In the case of the system mentioned in the 2003 Air Force report above, a 6.1 m × 0.3 m tungsten cylinder impacting at Mach 10 has a kinetic energy equivalent to approximately 11.5 tons of TNT (or 7.2 tons of dynamite). The mass of such a cylinder is itself greater than 9 tons, so the practical applications of such a system are limited to those situations where its other characteristics provide a clear and decisive advantage—a conventional bomb/warhead of similar weight to the tungsten rod, delivered by conventional means, provides similar destructive capability and is far more practical and cost effective.

The highly elongated shape and high mass are to enhance sectional density and therefore minimize kinetic energy loss due to air friction and maximize penetration of hard or buried targets. The larger device is expected to be quite effective at penetrating deeply buried bunkers and other command and control targets.

The weapon would be very hard to defend against. It has a very high closing velocity and small radar cross-section. Launch is difficult to detect. Any infrared launch signature occurs in orbit, at no fixed position. The infrared launch signature also has a much smaller magnitude compared to a ballistic missile launch. One drawback of the system is that the weapon's sensors would almost certainly be blind during atmospheric reentry due to the plasma sheath that would develop ahead of it, so a mobile target could be difficult to hit if it performed an unexpected maneuver. The system would also have to cope with atmospheric heating from re-entry, which could melt non-tungsten components of the weapon.

The phrase "Rods from God" is also used to describe the same concept. An Air Force report called them "hypervelocity rod bundles".

In science fiction

In the mid-1960s, popular science interest in orbital mechanics led to a number of science fiction stories which explored their implications. Among these was The Moon Is a Harsh Mistress by Robert A. Heinlein in which the citizens of the Moon bombard the Earth with rocks wrapped in iron containers which are in turn fired from an electromagnetic launch system at Earth-based targets.

In the 1970s and 1980s this idea was refined in science fiction novels such as Footfall by Larry Niven and Jerry Pournelle (the same Pournelle that first proposed the idea for military use in a non-fiction context), in which aliens use a Thor-type system. During the 1980s and 1990s references to such weapons became a staple of science fiction roleplaying games such as Traveller, Shadowrun and Heavy Gear (the latter game naming these weapons ortillery, a portmanteau of orbital artillery), as well as visual media including Babylon 5's "mass drivers" and the film Starship Troopers, itself an adaptation of a Heinlein novel of the same name.

The re-purposing of space colonies for use in kinetic bombardment (referred as a "colony drop") is a frequent element of the Gundam franchise and is central to the plots of Mobile Suit Gundam: Char's Counterattack and Mobile Suit Gundam 0083: Stardust Memory.

A smaller "crowbar" variant is mentioned in David's Sling by Marc Stiegler (Baen, 1988). Set in the Cold War, the story is based on the use of (relatively inexpensive) information-based "intelligent" systems to overcome an enemy's numerical advantage. The orbital kinetic bombardment system is used first to destroy the Soviet tank armies that have invaded Europe and then to take out Soviet ICBM silos prior to a nuclear strike.

In Neal Stephenson's Anathem a kinetic bombardment weapon is deployed from orbit to trigger the eruption of a dormant volcano.

From the mid-1990s, kinetic weapons as science fiction plot devices appeared in video games. Appearing in Bullfrog Productions' 1996 Syndicate Wars as a player-usable weapon, it also featured prominently in the plot of Tom Clancy's Endwar, Mass Effect 2 and Call of Duty: Ghosts, to name some.

The Warren Ellis comic Global Frequency (issue #12, "Harpoon", August 2004) featured the threat of kinetic spears, weapons designed to be dropped from satellites, heat up on re-entry, and strike the ground with the force of a tactical nuke, and as hot as the edge of the sun. Rather than being a weapon of war they were depicted as part of a 'die-back' protocol designed to reduce Earth's human population to a sustainable level.

In Daniel Suarez's book Freedom, a suborbital version of Thor is used composed of many small arrows or spikes for anti-personnel use.

In James S. A. Corey's The Expanse series, a radical group from within the Belter movement bombards Earth with high-speed asteroids, killing billions.

In 2013 a kinetic weapon bombardment system consisting of tungsten rods in an orbiting platform, codenamed Project: Zeus, was featured in the movie G.I. Joe: Retaliation, where it destroys London. However, the movie misrepresented physics by claiming the rod would not be "launched" or "fired" but merely "dropped". If it were released without force it would orbit the Earth in the same manner as the platform itself. In order for a rod to fall straight toward the center of Earth it would need to be launched away from the station with a tangential velocity equal in magnitude and opposite in direction from the orbiting station. This velocity would be in the range of approximately 7–8 km/s for satellites in low earth orbit, however, the actual velocity change needed to merely deorbit within half an orbit would be much less. Likely a few hundred m/s For a low-orbiting satellite, if even that much.

In John Birmingham's Stalin's Hammer (a part of his Axis of Time series), Soviet scientists use 21st century technology obtained from a fleet thrown back to World War II to create a satellite capable of launching tungsten rods from orbit and launch it in early 50's.

In David Weber's Honorverse series, kinetic strike weapons are a standard armament of all space navies that conduct orbit-to-ground operations. The version used by the Royal Manticoran Navy is a six-hundred-kilogram iron slug equipped with a small gravitic drive, capable of variable yields ranging from that of a large artillery shell to an intermediate-yield nuclear device, and packaged in six-shot satellites that are deployed from starship counter-missile tubes.

An episode of Justice league featured a satellite based weapon that combines this with a rail gun which used magnetic coils to attract and then launch meteors toward the earth's surface.

In Peter F. Hamilton's The Night's Dawn trilogy, "kinetic harpoons" are being used to bombard the surface of a planet. The book in which the event occurs also specifies how the staggering of the harpoons impact caused the shockwaves from the impacts to resonate and result in an artificial earthquake.

From the Wikipedia entry for KINETIC BOMBARDMENT

     "One more thing, Mr. President," Curtis said insistently.
     "Today's attack. I suppose you'll be sending in lots of armor."
     The President looked puzzled.
     "We'll do it right, Doctor," General Toland said. He turned to leave. "And I'd like to get at it."
     "Thor," Curtis said.
     Toland stopped. "What's that? It sounds like something I've heard of—"
     "Project Thor was recommended by a strategy analysis group back in the eighties," Curtis said. "flying crowbars." He sketched rapidly. "You take a big iron bar. Give it a rudimentary sensor, and a steerable vane for guidance. Put bundles of them in orbit. To use it, call it down from orbit, aimed at the area you're working on. It has a simple brain, just smart enough to recognize what a tank looks like from overhead. When it sees a tank silhouette, it steers toward it. Drop ten or twenty thousand of those over an armored division, and what happens?"
     "Holy sh*t," Toland said.
     "Are these feasible?" Admiral Carrell asked.
     "Yes, sir," Anson said. "They can seek out ships as well as tanks—"
     "But we never built them," Curtis said. "We were too cheap."
     "We would not have them now in any case," Carrell said. "General, perhaps you should give some thought to camouflage for your tanks—"
     "Or call off the attack until there's heavy cloud cover," Curtis said. "I'm not sure how well camouflage works. Another thing, look out for laser illumination. Thor could be built to home in that way."

ed note: Anson is Robert A. Heinlein. Curtis is Jerry Pournelle.

From Footfall by Larry Niven and Jerry Pournelle (1985)

Strategic Nuclear Weapons

As mentioned in the Space War section, nuclear weapons behave quite differently in airless space (and airless planets) than they do in a planetary atmosphere.

On a planet with an atmosphere the x-rays are absorbed by the atmosphere and become thermal radiation and atmospheric blast. The duration of thermal pulse increases with yield from about 1 second for 10 kilotons to 10 seconds for 1 megaton.

In space it is just x-rays and neutrons.

Percentage of total energy
In Atmosphere
Blast40% to 50%
Thermal Radiation30% to 50%
Ionizing Radiation
(Prompt Radiation)
(unless this is a neutron bomb)
(Residual Radiation)
5% to 10%
In Space
Soft x-rays80%
Gamma rays10%

Thermal Radiation

In the tables below the range between the detonation point and the affected target is called the "slant range." If the weapon detonates on the ground this is just the ground distance between the target and the explosion. However, nuclear weapons are commonly detonated at some height above the ground to increase their effect. Given the ground range and the detonation height, the slant range can be calculated by using the Pythagorean theorem:

Thermal Radiation Graph

from Physics and Nuclear Arms Today edited by David Hafemeister (1991)
  • Explosion Yield is the yield of the nuclear weapon in kilotons. 1,000 kilotons = 1 megaton
  • Slant Range is the distance between the target and the detonation point of the weapon, in miles.
  • Curves are thermal flux in calories per square centimeters.

The vertical red line is for 1 megaton (1,000 kilotons). Remember these have a pulse duration of 10 seconds.

  • 5 to 6 cal/cm2 for 10 seconds will cause second degree burns. (green line)
  • 8 to 10 cal/cm2 for 10 seconds will cause third degree burns. (blue line)
  • 20 to 25 cal/cm2 for 10 seconds will ignite clothing. (violet line)

The equation is:

Q ≈ 3000 * ( ƒ * τ * Y / D2 )


Q = thermal flux (cal/cm2)
ƒ = thermal energy fraction ( from 0.35 to 0.40 for air bursts, 0.18 for ground bursts)
τ = atmospheric transmission factor (0.6 to 0.7 at 5 miles, 0.05 to 0.1 at 40 miles. Even lower if foggy)
Y = nuclear weapon yield (megatons). Please note the graph above uses kilotons, not megatons
D = slant range (miles)
EffectsExplosive yield / detonation height
1 kt / 200 m20 kt / 540 m1 Mt / 2.0 km20 Mt / 5.4 km
Thermal radiation—ground range (km)
Third degree burns0.62.51238
Second degree burns0.83.21544
First degree burns1.14.21953
from Wikipedia article Effects of nuclear explosions
Note the table is using ground range, not slant range.


A bit less than half the nuclear weapon's energy becomes atmospheric blast. This has two effects: a sharp increase in atmospheric pressure ("overpressure"), and incredibly strong winds. The overpressure crushes objects and collapses buildings. The wind turns lightweight objects into dangerous projectiles.

In the complicated equations for figuring the area that suffers from a given overpressure, the area is proportional to Y2/3 (where Y is the weapon's yield). This is called the "equivalent megatonnage" of a nuclear weapon. Why do we care? The point is that the combined equivalent megatonnage of several low-yield weapons is greater than that of a single weapon with the same total yield. In other words five warheads (2 megatons each) will do more damage to a city than a single warhead (10 megatons).

OverpressurePhysical Effects
20 psiHeavily built concrete buildings are severely damaged or demolished.
10 psiReinforced concrete buildings are severely damaged or demolished.
Small wood and brick residences destroyed.
Most people are killed.
5 psiUnreinforced brick and wood houses destroyed.
Heavier construction severely damaged.
Injuries are universal, fatalities are widespread.
3 psiResidential structures collapse.
Serious injuries are common, fatalities may occur.
1 psiLight damage to commercial structures
Moderate damage to residences.
Window glass shatters
Light injuries from fragments occur.

Note that the same source says you need 40 psi before lethal effects are noted on people, which contradicts the 10 psi entry above. I don't know which to believe.

Peak overpressureMaximum Wind Velocity
50 psi934 mph
20 psi502 mph
10 psi294 mph
5 psi163 mph
2 psi70 mph
Tables from Atomic Archives

The x-axis is the slant range in feet, divided by the weapon yield in megatons rasied to the 1/3 power. Trace upward to intersect the curve, then to the left to find the peak overpressure in PSI.

The curve can be traced approximately by the formula:

z = Y1/3 / D

p = (22.4 * z3) + (15.8 * z3/2)


z = scaled yield (megatons1/3/mile)
Y = weapon yield (megatons)
D = slant distance (miles)
p = overpressure (lb/in2 or PSI)

EffectsExplosive yield / detonation height
1 kt / 200 m20 kt / 540 m1 Mt / 2.0 km20 Mt / 5.4 km
Blast—ground range (km)
Urban areas completely levelled
(20 psi or 140 kPa)
Destruction of most civilian buildings
(5 psi or 34 kPa)
Moderate damage to civilian buildings
(1 psi or 6.9 kPa)
Railway cars thrown from tracks and crushed
(62 kPa)
(values for other than 20 kt are extrapolated
using the cube-root scaling)
from Wikipedia article Effects of nuclear explosions
Note the table is using ground range, not slant range.

Things are more complicated when the detonation point is some distance above ground level.

The primary shock wave expands outward as a sphere from the weapon detonation point. If this is not a ground-burst, at some point the sphere will expand until it hits the ground. The shock wave is reflected upward from the ground. Since the shocked region inside the sphere is hotter and denser than the rest of the atmosphere, the reflected shock wave travels faster than the primary shock wave. For certain geometries, the reflected shock wave catches up with the primary shock wave and the two shock fronts merge. This is called the Mach Stem. The overpressure at the stem is typically twice that of the primary shock wave.

The area the Mach stem passes over is called the Mach reflection region. The area from ground zero to the start of the Mach reflection region is called the Regular reflection region. It only suffers from the passage of two separate shock waves with the standard overpressure. The Mach reflection region suffers the double overpressure caused by the Mach stem.

The chart below plots the regular reflection region and Mach reflection region, given the detonation distance from the ground. To use, you divide the burst height and the distance from ground zero by weapon kilotons raised to the 1/3 power.

For instance, if the weapon had a yield of 1,000 kilotons (1 megaton) and the weapon burst 2,000 feet above ground level, 2000 / (10001/3)

Scaled Height of Burst = burstHeight / yield1/3
Scaled Height of Burst = 2000 / 10001/3
Scaled Height of Burst = 2000 / 10
Scaled Height of Burst = 200

so on the plot for the vertical scale you would use the tick-mark at 200. By the same token, for the horizontal scale, the tick mark for 800 corresponds to 800 * 10 = 8,000 feet (where 10 = 10001/3).

The dotted line shows where the regular reflection region stops and the Mach reflection region begins.

The bulges in the overpressure curves show where you can optimize the height of burst for a given overpressure. For instance, look at the 15 lb/in2 curve. Find the point on the curve that gets the farthest to the right. Trace a line horizontally to the vertical scale and you'll see this happens at a scaled height of burst of 650 feet. For a 1,000 kiloton weapon this is a burst height of 6,500 feet.

In other words, a weapon bursting at 650 scaled feet of altitude will throw 15 PSI of overpressure out to 1,200 scaled feet from ground zero. But a weapon doing a ground burst with 0 scaled feet of altitude will only throw 15 PSI out to 800 scaled feet from ground zero.

from Physics and Nuclear Arms Today edited by David Hafemeister (1991)

Prompt Radiation

EffectsExplosive yield / detonation height
1 kt / 200 m20 kt / 540 m1 Mt / 2.0 km20 Mt / 5.4 km
Effects of instant nuclear radiation—slant range (km)
Lethal total dose (neutrons and gamma rays)
Total dose for acute radiation syndrome1.
from Wikipedia article Effects of nuclear explosions
"Lethal" is defined as a dose of 10 grays. "Acute radiation syndrome" is defined as a dose of 1 gray.

Residual Radiation

This is the radioactive fallout, radioactive dust that falls from the sky in a long plume extending downwind.

As a rule of thumb, the fallout is dangerous for about one to six months after the bomb blast.

Unless it was a salted bomb, then you are probabably looking at a hundred years or so. A salted bomb whose fallout emitted a dosage of 10 sieverts per hour would need about 25 half-lives to decay to safe levels (i.e., to a dosage below natural background radiation). For example, a salted bomb producing Cobalt-60 would have fallout with a half life of 5.2714 years. 25 half-lives would be 131.785 years. Tantalum-182 has a half-life of only 114.4 days, it would be safe in about 7.8 years.

Air bursts tend to produce lesser amounts of fallout, but which travel at high altitudes and can scatter itself all over the entire planet.

Ground bursts tend to produce more severe levels of fallout, but which only travel relatively short distances from the detonation site (several hundred kilometers). The Castle Bravo 15 megaton nuclear test made a plume about 500 kilometers downwind with a maximum width of 100 kilometers.

Water surface bursts are sort of in-between.

The Wikipedia article stated that the crater of a ground burst would have fallout emitting radiation at a dosage rate of 30 grays per hour, but failed to specify the yield of the weapon.

Specialized Nuclear Weapons

Enhanced Radiation Weapon
Also known as a "neutron bomb." This is a bomb optimized so most of the energy comes out as neutrons. In conventional nuclear weapons most of the energy comes out as x-rays. The idea is for the weapon to kill enemy troops and civilians while doing minimal damage to buildings and equipment.
Details are classified but the best I've found is the theoretical maximum for a neutron bomb is 80% of the energy is neutrons and 20% x-rays. For conventional nuclear weapons it is 80% soft X-rays, 10% gamma rays, 10% neutrons.
Salted Bomb
Also known as a "cobalt bomb." This is a bomb optimized to produce huge amounts of radioactive fallout in order to render large areas uninhabitable.
This is done by encasing the weapon in a jacket composed of some element that will easily be transmuted into a radioactive isotope by the weapon's neutron flux. Proposed elements for the jacket include cobalt-59, gold-198, tantalum-182, zinc-65, and sodium-24.
A conventional nuclear weapon typically generates fallout that will decay to safe levels in one to six months. A cobalt bomb whose fallout caused a dose rate of 10 sieverts per hour would take about 130 years (25 half-lives) to decay to safe levels (safe levels being defined as "less than natural background radiation").
The name "salted" comes from the expression "sowing the earth with salt".
Dirty Bomb
This is not a military weapon, it is an ineffectual terrorism device. It is a stick of dynamite or other small chemical explosive inside a container of powdered radioactive material. The only reason it is mentioned here is because it is sometimes confused with a salted bomb.
A dirty bomb might spread a bit of mildly radioactive dust over a building or two.
A salted bomb will spread highly radioactive fallout across half a continent.
The dreaded Nuclear Electromagnetic Pulse. A conventional nuclear weapon of several megatons yield is detonated at a high altitude (commonly mentioned as 400 kilometers). The resuling EMP does damage to every device using electricity within a huge range. The details are sketchy because they are still classified. For best results a planetary atmosphere is required.
The linked Wikipedia article has an overview of the convoluted details, including a useful quote from a 2010 Oak Ridge National Laboratory report on common EMP misconceptions.


If the attacker wants to just destroy the defender's civilization but does not want to necessarily make the defenders extinct or render the planet uninhabitable, asteroid bombardment might be just the thing. In a balkanized solar system, this is the reason for each space-faring nation to have their own Spaceguard. The idea is to prevent unauthorized changes in asteroid orbits. The idea for several national spaceguards is to keep all the spaceguards honest.

     "What we're afraid of is a massive meteorite impact, something of asteroid size."
     The alien was silent for a time. Reynolds busied himself at the bar. Suddenly the alien said, "Thuktun Flishithy—Message Bearer—was docked to a moonlet of the ringed planet for many years. This many." The alien's trunk emerged from the mud, and he flexed a clump of four digits, three times. "Pushing. We were not told why. I once heard officers call the mass chaytnf."
     "What does it mean?"
     "It means this part of a fi'." The alien rolled (and Sherry shied from a wave of mud). One broad clawed foot emerged.
     The sci-fi types all seemed to freeze in place; but Jenny didn't need their interpretation. Her hand closed painfully on Jack's arm. "My God. It's real. Of course, the Foot, they're planning to stomp us—"
     "They're talking too damn much."
     "Huh? The alien's talking a lot more than they are."
     The blurry voice from the TV set was saying, "It was not so large as many of the—asteroids—at the ringed planet. I think 8 to the 12th standard masses—"
     "Standard mass is your mass? About eight hundred pounds . . . Curtis took a pocket calculator out of his bush jacket. "Jesus! Twenty-seven billion tons!"
     Nat Reynolds said, "At . . . ten to twenty miles per second, that could—Harpanet, where are they going to drop it?"

(ed note: if I have not made a mistake in math, this will strike with force of about 1.25 x 1022 joules or 3,000,000 megatons. See boom table. Final crater diameter: 40.4 km, final crater depth: 901 meters, The volume of the target melted or vaporized is 78.2 km3. Roughly half the melt remains in the crater, where its average thickness is 144 meters. The novel says 4,000 megatons, so I'm only off by three orders of magnitude.)

     Commander Anton Villars stared through the periscope and tried to look calm. It wasn't easy. An hour before the message had come to USS Ethan Allen. The long-wave transmitters were reliable but slow. The message came in dots and dashes, code tapped out and taken down to be put through the code machines. It couldn't be orders to attack the Soviet Union. There was no Soviet Union. Villars had been prepared to launch his Poseidon missiles against an unseen enemy in space. Instead:
     Safe? From four thousand megatons? There wasn't any safety. Villars' urge was to submerge and flee at flank speed.
     Off to starboard, the island of Rodriguez blazed with the colors of life. Jungle had long since given way to croplands. In the center bare rock reared sharply, a peak a third of a mile high. Waves broke over a surrounding coral reef. That reef would provide more cover when the tsunami came, but it was a danger too.
     Fishing boats were straggling in through the reef. Probably doomed. There was nothing Villars could do for them.
     It was just dusk. Clouds covered the sky. It would be difficult to see anything coming. Four thousand megatons. Bigger than any bomb we ever dreamed of, much less built.
     The crew waited tensely. John Antony, the Exec, stood close by.
     "About time," Antony said.
     "If their estimate was on."
     "If their time was off, so were their coordinates."
     I know that. I had the same instructor at Annapolis as you did.
     Somebody laughed and choked it off. The news had filtered through the ship, as news like that always did.
     The cameras were working. Villars wondered how many would survive. He peered through the darkest filter available. Four thousand megatons . . .
     Suddenly the clouds were blazing like the sun. "First flash at 1854 hours 20 seconds," he called. "Log that." Where? Where would it fall?
     All in an instant, a hole formed in the clouds to the northeast, the glare became God's own flashbulb, and the cameras were gone. "Get those other cameras up," Villars bellowed at men who were already doing that. His right eye saw nothing but afterimage. He put his left to the periscope.
     He saw light. He squinted and saw light glaring out of a hole in the ocean. A widening hole in the ocean, with smoothly curved edges; wisps of mist streaming outward, and a conical floodlight beam pointing straight up. The beam grew wider: the pit was expanding. Clouds formed and vanished around a smoothly curved wall of water sweeping smoothly toward the sub.
     The rim of a sun peeped over the edge.
     "I make it about forty miles east northeast of present position. Okay, that's it." Villars straightened. "Bring in the cameras. Down periscope. Take us to ninety feet." How deep? The further down, the less likely we'll get munched by surface phenomena, but if those tsunamis are really big they might pile enough water on top of Ethan Allen to crush us. "Flank speed. Your course is 135 degrees." That leaves us in deep water and puts Rodriguez between us and that thing, for whatever good it'll do.
     So we've seen it. A sight nobody ever saw—well, nobody who wrote it down, anyway. Now all I have to do is save the ship.
     Ethan Allen was about to fight the biggest tsunami in human history—and just now he was broad on to it. He glanced at his watch. Tsunamis traveled at speeds from two hundred to four hundred miles an hour. Call this one four. Six minutes . . .
     "Left standard rudder. Bring her to 85 degrees."
     "Bring her to 85, aye, aye," the quartermaster answered.
     "Warn 'em," Villars said.
     "Now hear this. Now hear this. Damage control stations. Stand by for depth charges."
     Might as well be depth charges . . .
     The ship turned.
     It surged backward. Villars felt the blood rushing into his face. Somewhere aft, a shrill scream was instantly cut off, and the Captain heard a thud.
     Minutes later: "There's a current. Captain, we're being pulled northeast."
     "Steady as she goes." Goddam. We lived through it!

     The contorted moonlet dropped away, dwindled, vanished. Earth grew huge. A flashbulb popped above the Indian Ocean, and was replaced at once by a swelling, darkening fireball. Ring-shaped shadows formed and faded in and around it. Far from the central explosion, new lights blinked confusingly in points and radial streaks.
     The Earth's face streamed past, terrifyingly close but receding now. A wave in the cloud cover above the Indian Ocean raced outward, losing its circular shape as it traveled. Northward, it took on a triangular indentation, as if the edge of a blanket had snagged on a nail.
     "India," Dawson said. "How fast are you running this tape?"
     "Thirty-two times normal," Tashayamp answered.
     "What is . . . that?" Alice asked.
     "Land masses. The tsunami distorts the clouds," Arvid said.
     "So does the ocean floor," Dawson amplified, "but not as much. That's India going under. Those flashes would have been secondary meteors, debris, even water from the explosion thrown out to space and reentering the atmosphere."
     That's India going under. Good-bye, Krishna, and Vishnu the elephant god. Jeri shuddered. "Dave took me to India once. So many people. Half a billion."
     Arvid stood near. She felt his warmth and wanted to be closer to him.
     Tashayamp said, "Number?"
     Arvid said, "Eight to the eighth times eight times three."

(ed note: 402,653,184)

     "Human fithp in India? Where the wave goes now?"
     "Yes." ...
     ...The distortion in the clouds swept against Africa, then south. Here was clear air, and a ripple barely visible in the ocean . . . but the outline of the continent was changing, bowing inward.
     "Cape of Good Hope," Jeri muttered. She watched the waves spread into the Atlantic. Recorded hours must be passing. She found herself gasping and suspected she had been holding her breath. The waves were marching across the Atlantic, moving on Argentina and Brazil with deceptive slowness and a terrible inevitability.
     Cloud cover followed, boiling across the oceans, reaching toward the land masses. "My God," Jeri said. "How could you do this?"
     "It is not our choice," Raztupisp-minz said. "We would gladly have sent the Foot safely beyond your atmosphere, but your fithp would not have it so."
     "Look what you made me do," Alice said in a thick, self-pitying whine. Her voice became a lash. "All the sickies say that—the rogues say that when they've done something they're ashamed of. It was somebody else's fault."
     "They can say all they like," Carrie Woodward said. "We know. They came all the way from the stars to ruin the land."
     "You should not say such things," said Takpusseh. "You do not want this to happen again. You will help us."
     "Help? How?" Dawson demanded.
     "You, Wes Dawson, you tell them. More come."
     Dmitri spoke again in Russian. Arvid shuddered.
     The screen changed again. Clouds moved so unnaturally fast that Jeri thought they were still watching a tape until Takpusseh said. "That is now. Winterhome."
     Earth was white. The cloud cover was unbroken.
     "Rain. Everywhere," Nikolai said. "The dams are gone. There will be floods."

From FOOTFALL by Larry Niven and Jerry Pournelle (1985)

Relativistic attack

If the invaders are attacking the planet using relativistic weapons, it is more or less game over. There really is no realistic defense, unless the defenders are a Kardashev type II civilization. The problem is light-speed lag. Since the r-bombs are traveling so near the speed of light, they are only a little bit behind the wave of photons announcing their presence. In other words, you only see where they were, not where they are now. From the target's point of view, they would suffer from the optical illusion of the r-bomb apparently moving faster than light. Before you had time to react, the r-bombs would hit with all their devastating effects.

The thing to keep in mind is that all the energy the r-bomb releases has to be put into it in the first place. It takes an astronomical amount of energy to accelerate an object up to 92% lightspeed. If your civilization has managed to anger another civilization who has access to that much energy, you already know you are in deep trouble.

Bombardment in Fiction


Shus and his staff watched the screens and instruments, but nothing happened. Finally the moment came and the tension ebbed away. The fleets were in position—it was time to begin the last battle of the Terra-Sparta War.

"So be it," said Shus, staring at the screens devoid of the enemy. He no longer felt any sympathy for these people; they had had their chance to come out and die fighting. Now they would be slaughtered like cowering animals. "Commence bombardment!"

The order flashed nearly instantaneously through the fleets in close orbits around the planet. Knobs were spun and buttons pushed. Smoothly, on near-frictionless hinges, the bomb-bay doors of the battle wagons opened out like the petals of a flower seeking the sun—or in this case, the planet Earth. Then, as naturally as plants releasing millions of spores, the cargoes of bombs were carried away as if by an unseen wind.

The number of bombs grew, increased incredibly. It seemed that they would never cease spewing forth from the bomb-bays. Swarm upon swarm fell toward Earth in the grip of gravity.

Shus shook his head as he watched the bombs fall away from his ships. He had conducted many a siege against hostile planets, but he had never undertaken anything like this before. Short of using outlawed megatons, this was the ultimate. It was against the rules of space warfare to reduce a planet to ashes with one bomb. He was circumventing those rules by using a million bombs. Never before had it been tried, and he guessed it would never be again, for there was no other race quite like Man. Never had a race been so dreaded.

The bombs touched the Earth's atmosphere like drops of rain on a roof. The whole sky was darkened with their sinister forms. And even as those in the vanguard struck the atmosphere, still more issued forth from the bellies of the battleships.

The bombs hurtled down like a thundering deluge, their heat shields glowing red from the friction of the air. Here and there flaws in the heatshields of the hastily mass-produced weapons caused them to burn up. These appeared as colorfully spectacular shooting stars to awed viewers on the Earth.

The defenders had long ago picked up the falling bombs. Instruments and computers had tracked them diligently. Suddenly the Earth erupted with scintillating light. It was the flickering of the energy beams of Earth filling the heavens.

Shus smiled grimly. They were trying to shield the planet. The attempt was doomed to failure. There were too many bombs. The Terran defensive fire was unbelievable, incredible. An ordinary bombardment would have been nearly neutralized. Dane Barclay had prepared well, but not even he could defend against a bombardment on this scale.

Thousands of bombs were hit and exploded high in the atmosphere, turning night into day and day into eye-searing brightness. But for every bomb hit, ten others fell unmolested. And still more were coming from the bomb-bays of the ships above. The laser batteries stabbed desperately and with clever efficiency, but they could not stop all the bombs or even a tenth of them.

The bombs struck.

They smashed down everywhere, wiping out laser batteries and cities, homes and roads. Mushrooms sprouted on the Earth as if the planet were one huge mushroom farm. The atmosphere and the surface of the planet became a virtual hell. All the things Man had worked for and built were vaporized, smashed flat, obliterated within seconds.

The tall buildings were flattened, shattered or blown apart. Like falling dominoes they toppled to lie in ruin. Flames leapt up and the once proud structures, despite their supposed fireproofness, burned like logs casually thrown upon a fire.

Blinding flashes, coming almost continuously, seared earth and rock. Metals melted and stone bubbled. Lava flowed where no volcanos existed. Land quaked and trembled. Forests burned everywhere, huge seas of flame. Small lakes and rivers dried up instantly, leaving only barren beds to mark their former presence.

Mountains gained dazzling halos as the bombs sought to reduce them.

Even underground, no one was safe, for the bombs fell and blasted their way through with terrible violence. Hastily constructed bunkers caved in, burying the occupants dead and alive.

The bombs spared nothing. They fell everywhere.

The seas boiled. Fish died by the billions. Mighty tidal waves raced across the land, battling each other and attempting blindly to batter whatever got in the way. Shus had been right; never in galactic history, since the rules of civilized warfare had been set down, had such destruction been visited upon a planet. One bomb was against the rules—a million were not.

Doomsday was upon the Earth.

As the scout ships moved through the death-shroud atmosphere of Earth, their instruments and equipment sent data and pictures back to the command ship. In the control room Shus and his staff studied the data and viewscreens intently.

There was no doubt that Earth had been ravaged as no other planet in the annals of space war. The cities were gone. They had been leveled, pulverized, vaporized. In some places mounds of rubble were visible. In others, where the bomb concentration had been greater, only deep craters remained. The entire planet bore the pock marks of the bombardment. The atmosphere was a nightmare of storm and hurricane, of thunder and lightning, of typhoons and tornados, of monsoons and gigantic fires that turned rain to steam. Upon the heels of the cataclysmic bombs had come condensation of moisture and the creation of huge rainfalls. Drops of rain large as fists drenched the land. The tormented atmosphere reacted by sending killer winds hundreds of miles per hour over the leveled land, uprooting, sweeping all with it.

While the wind blew and the rain pelted heavily, the world burned. Forests, buildings, parks, vehicles, all burned. Anything that could burn, did. Smoke filled the air. Never had there been anything like it before. Even where the noon-day sun shone fiercely upon the Earth, all was in darkest gloom.

Shus watched the angered sea batter and beat the land, watched it send towering waves to submerge the land. Tens of thousands of miles of seacoast were overrun and claimed by the water.

The huge planetary lasers that might have cost him dearly in ships and Spartans were gone—blasted out of existence by the saturation bombardment. Of the battle fleets of Terra there was no sign.

As he looked at the overwhelming devastation visited upon the planet below, he wanted to believe that those fleets had perished in the bombardment, that the planet was defenseless, that the battle had already been won, but he could not. He knew otherwise. Bitter experience told him that beneath that ravaged outer crust there were fleets, laser batteries, soldiers and cities. But the bombardment, beyond anything they could possibly have expected, must surely have dealt their remaining forces and plan of defense a staggering—if not fatal—blow.

"There is little point in continuing the bombardment," he said to his officers. It would be a case of diminishing returns, and the remaining bombs would best be of service against specific targets as they were uncovered.

"Phase One, I would say, is an overwhelming success. We have destroyed the static defenses, leveled the cities and no doubt killed most of the population.

"We will move to Phase Two now: the removal of surviving defensive forces. Let's go down and get this over with."

The Spartan fleets descended into the dark gloom, seeking the surviving forces of once proud Earth.

From SIEGE OF EARTH by John Faucett (1971)

Londo Mollari: Refa, any force attempting to invade Narn would be up to its neck in blood—its own!

Lord Refa: We have no intention of invading Narn. Flattening it, yes—but invading it? We will be using mass drivers. By the time we are done their cities will be in ruins, we can move in at our leisure!

Londo Mollari: Mass drivers? They have been outlawed by every civilized planet!

Lord Refa: These are uncivilized times.

Londo Mollari: We have treaties!

Lord Refa: Ink on a page!

Capt. John Sheridan: Any news?

Cmdr. Susan Ivanova: Just rumors. They say the Centauri are using mass drivers. I can't believe they'd resort to planetary bombardment!

Capt. John Sheridan: Right now I'd believe just about anything.

From Babylon 5: The Long, Twilight Struggle by J. Michael Straczynski (1995)

The Moon Is A Harsh Mistress

In the novel, the lunar colony is fed up with the yoke of Terran oppression and stages their very own war of independence. Among their assets is a large mass driver (called a "catapult") ordinarily used to send shipments of grain back to Terra. The colonists weaponize it, firing cannisters of steel-belted solid rock as orbital bombardment weapons.

(ed note: "Mike" is the supercomputer that controls and maneuvers the cannisters from the catapult. The text is the internal dialog of the main character, who is a native Russian speaker. This explains his broken English. "F.N." is the Federated Nations, the world government and "owner" of the lunar colony and all the inhabitants. TANSTAAFL is an acronym for There Ain't No Such Thing As A Free Lunch)

     LuNoHoCo was an engineering and exploitation firm, engaged in many ventures, mostly legitimate. But prime purpose was to build a second catapult, secretly.

(ed note: colonists know that once they start lobbing cannisters as weapons, their catapult will become the priority military target)

     Operation could not be secret. You can’t buy or build a hydrogen-fusion power plant for such and not have it noticed. (Sunpower was rejected for obvious reasons.) Parts were ordered from Pittsburgh, standard UnivCalif equipment, and we happily paid their royalties to get top quality. Can’t build a stator for a kilometers-long induction field without having it noticed, either. But most important you cannot do major construction hiring many people and not have it show. Sure, catapults are mostly vacuum; stator rings aren’t even close together at ejection end. But Authority’s 3-g catapult was almost one hundred kilometers long. It was not only an astrogation landmark, on every Luna-jump chart, but was so big it could be photographed or seen by eye from Terra with not-large telescope. It showed up beautifully on a radar screen.
     We were building a shorter catapult, a 10-g job, but even that was thirty kilometers long, too big to hide.

(ed note: the shorter the catapult, the higher the gravities of acceleration required)

     So we hid it by Purloined Letter method.

(ed note: workers know catapult is at the end of the subway line, but have no idea where exactly that is.)

     What we needed was something else. Needed steel at new catapult and plenty—Prof asked, if really necessary to put steel around rock missiles; I had to point out that an induction field won’t grab bare rock. We needed to relocate Mike’s ballistic radars at old site and install doppler radar at new site—both jobs because we could expect attacks from space at old site.

     “A maximum of instructive shrecklichkeit with minimum loss of life. None, if possible”—was how Prof summed up doctrine for Operation Hard Rock and was way Mike and I carried it out. Idea was to hit earthworms so hard would convince them —while hitting so gently as not to hurt. Sounds impossible, but wait.
     Would necessarily be a delay while rocks fell from Luna to Terra; could be as little as around ten hours to as long as we dared to make it. Departure speed from a catapult is highly critical and a variation on order of one percent could double or halve trajectory time, Luna to Terra. This Mike could do with extreme accuracy—was equally at home with a slow ball, many sorts of curves, or burn it right over plate—and I wish he had pitched for Yankees. But no matter how he threw them, final velocity at Terra would be close to Terra’s escape speed, near enough eleven kilometers per second as to make no difference. That terrible speed results from gravity well shaped by Terra’s mass, eighty times that of Luna, and made no real difference whether Mike pushed a missile gently over well curb or flipped it briskly. Was not muscle that counted but great depth of that (gravity) well.
     So Mike could program rock-throwing to suit time needed for propaganda. He and Prof had settled on three days plus not more than one apparent rotation of Terra—24hrs-50min-28.32sec—to allow our first target to reach initial point of program. You see, while Mike was capable of hooking a missile around Terra and hitting a target on its far side, he could be much more accurate if he could see his target, follow it down by radar during last minutes and nudge it a little for pinpoint accuracy.
     We needed this extreme accuracy to achieve maximum frightfulness with minimum-to-zero killing. Call our shots, tell them exactly where they would be hit and at what second—and give them three days to get off that spot.
     North America had struck me as horribly crowded, but her billion people are clumped—is still wasteland, mountain and desert. We laid down a grid on North America to show how precisely we could hit—Mike felt that fifty meters would be a large error. We had examined maps and Mike had checked by radar all even intersections, say 105° W by 50° N—if no town there, might wind up on target grid … especially if a town was close enough to provide spectators to be shocked and frightened.
     We warned that our bombs would be as destructive as H-bombs but emphasized that there would be no radioactive fallout, no killing radiation—just a terrible explosion, shock wave in air, ground wave of concussion. We warned that these might knock down buildings far outside of explosion and then left it to their judgments how far to run. If they clogged their roads, fleeing from panic rather than real danger—well, that was fine, just fine!
     But we emphasized that nobody would get hurt who heeded our warnings, that every target first time around would be uninhabited—we even offered to skip any target if a nation would inform us that our data were out-of-date. (Empty offer; Mike’s radar vision was a cosmic 20/20.)
     But by not saying what would happen second time around, we hinted that our patience could be exhausted.
     In North America, grid was parallels 35, 40, 45, 50 degrees north crossed by meridians 110, 115, 120 west, twelve targets. For each we added a folksy message to natives, such as:
     “Target 115 west by 35 north—impact will be displaced forty-five kilometers northwest to exact top of New York Peak. Citizens of Goffs, Cima, Kelso, and Nipton please note.
     “Target 100 west by 40 north is north 30° west of Norton, Kansas, at twenty kilometers or thirteen English miles. Residents of Norton, Kansas, and of Beaver City and Wilsonville, Nebraska, are cautioned. Stay away from glass windows. It is best to wait indoors at least thirty minutes after impact because of possibility of long, high splashes of rock. Flash should not be looked at with bare eyes. Impact will be exactly 0300 your local zone time Friday 16 October, or 0900 Greenwich time—good luck!

     But our attitude was conciliatory—“Look, people of Terra, we don’t want to kill you. In this necessary retaliation we are making every effort to avoid killing you… but if you can’t or won’t get your governments to leave us in peace, then we shall be forced to kill you. We’re up here, you’re down there; you can’t stop us. So please be sensible!”
     We explained over and over how easy it was for us to hit them, how hard for them to reach us. Nor was this exaggeration. It’s barely possible to launch missiles from Terra to Luna; it’s easier to launch from Earth parking orbit—but very expensive. Their practical way to bomb us was from ships.

     Came Friday with no answer from F.N. News up from Earthside seemed equal parts unwillingness to believe we had destroyed seven ships and two regiments (F.N. had not even confirmed that a battle had taken place) and complete disbelief that we could bomb Terra, or could matter if we did—they still called it “throwing rice.” More time was given to World Series.

     My worries had to do with Mike. Sure, Mike was used to having many loads in trajectory at once—but had never had to astrogate more than one at a time. Now he had hundreds and had promised to deliver twenty-nine of them simultaneously to the exact second at twenty-nine pinpointed targets.
     More than that— For many targets he had backup missiles, to smear that target a second time, a third, or even a sixth, from a few minutes up to three hours after first strike.
     Four great Peace Powers, and some smaller ones, had antimissile defenses; those of North America were supposed to be best. But was subject where even F.N. might not know. All attack weapons were held by Peace Forces but defense weapons were each nation’s own pidgin and could be secret. Guesses ranged from India, believed to have no missile interceptors, to North America, believed to be able to do a good job. She had done fairly well in stopping intercontinental H-missiles in Wet Firecracker War past century.
     Probably most of our rocks to North America would reach target simply because aimed where was nothing to protect. But they couldn’t afford to ignore missile for Long Island Sound, or rock for 87° W x 42° 30’ N—Lake Michigan, center of triangle formed by Chicago, Grand Rapids, Milwaukee. But that heavy gravity makes interception a tough job and very costly; they would try to stop us only where worth it.
     But we couldn’t afford to let them stop us. So some rocks were backed up with more rocks. What H-tipped interceptors would do to them even Mike did not know—not enough data. Mike assumed that interceptors would be triggered by radar —but at what distance? Sure, close enough and a steelcased rock is incandescent gas a microsecond later. But is world of difference between a multi-tonne rock and touchy circuitry of an H-missile; what would “kill” latter would simply shove one of our brutes violently aside, cause to miss.
     We needed to prove to them that we could go on throwing cheap rocks long after they ran out of expensive (milliondollar? hundred-thousand-dollar?) H-tipped interceptor rockets. If not proved first time, then next time Terra turned North America toward us, we would go after targets we had been unable to hit first time—backup rocks for second pass, and for third, were already in space, to be nudged where needed.
     If three bombings on three rotations of Terra did not do it, we might still be throwing rocks in ‘77—till they ran out of interceptors… or till they destroyed us (far more likely).
     For a century North American Space Defense Command had been buried in a mountain south of Colorado Springs, Colorado, a city of no other importance. During Wet Firecracker War the Cheyenne Mountain took a direct hit; space defense command post survived—but not sundry deer, trees, most of city and some of top of mountain. What we were about to do should not kill anybody unless they stayed outside on that mountain despite three days’ steady warnings. But North American Space Defense Command was to receive full Lunar treatment: twelve rock missiles on first pass, then all we could spare on second rotation, and on third—and so on, until we ran out of steel casings, or were put out of action… or North American Directorate hollered quits.
     This was one target where we would not be satisfied to get just one missile to target. We meant to smash that mountain and keep on smashing. To hurt their morale. To let them know we were still around. Disrupt their communications and bash in ommand post if pounding could do it. Or at least give them splitting headaches and no rest. If we could prove to all Terra that we could drive home a sustained attack on strongest Gibraltar of their space defense, it would save having to prove it by smashing Manhattan or San Francisco.
     Which we would not do even if losing. Why? Hard sense. If we used our last strength to destroy a major city, they would not punish us; they would destroy us. As Prof put it, “If possible, leave room for your enemy to become your friend.”
     But any military target is fair game.

     Got into shade of shed and peeked around edge at Terra.
     She was hanging as usual halfway up western sky, in crescent big and gaudy, three-plus days past new. Sun had dropped toward western horizon but its glare kept me from seeing Terra clearly. Chin visor wasn’t enough so moved back behind shed and away from it till could see Terra over shed while still shielded from Sun—was better. Sunrise chopped through bulge of Africa so dazzle point was on land, not too bad—but south pole cap was so blinding white could not see North America too well, lighted only by moonlight.
     Twisted neck and got helmet binoculars on it—good ones, Zeiss 7 x 50s that had once belonged to Warden.
     North America spread like a ghostly map before me. Was unusually free of cloud; could see cities, glowing spots with no edges. 0837—
     At 0850 Mike gave me a voice countdown—didn’t need his attention; he could have programmed it full automatic any time earlier.
     0851—0852—0853… . one minute—59—58—57 … . half minute—29–28—27 … . ten seconds—nine—eight— seven— six—five—four—three—two—one—
     And suddenly that grid burst out in diamond pinpoints!

     Prof looked puzzled. “I am confused by that, too. This dispatch so alleged. But the thing that puzzled me is that we could actually see, by video, what certainly seemed to be atomic explosions.”
     “Oh.” I turned to Wright. “Did your brainy friends tell you what happens when you release a few billion calories in a split second all at one spot? What temperature? How much radiance?”
     “Then you admit that you did use atomic weapons!”
     “Oh, Bog!” Head was aching. “Said nothing of sort. Hit anything hard enough, strike sparks. Elementary physics, known to everybody but intelligentsia. We just struck damnedest big sparks ever made by human agency, is all. Big flash. Heat, light, ultraviolet. Might even produce X-rays, couldn’t say. Gamma radiation I strongly doubt. Alpha and beta, impossible. Was sudden release of mechanical energy. But nuclear? Nonsense!”

From The Moon is a Harsh Mistress by Robert Heinlein (1965)

Orbital Fortress

The defenders remaining spaceborn assets will be in orbit around the planet. If the defender is fortunate enough to have a moon or two these can also be armed with defensive bases and weapons.

Orbital fortresses have far more punch than the equivalent combat spacecraft, kilogram for kilogram. This is because the spacecraft has to use part of its mass for propulsion, while the orbital fortress can use that mass allocation for more weapons instead. However orbital fortresses do have problems with heat radiators and supply.

Supporting the fortresses, the planet's orbit will probably be full of defensive assets such as small but deadly weapons designed to mission-kill invading spacecraft and any ortillery they drop. In the Strategic Defense Initiative, two concepts looked into were "Space-Based Interceptor" and "Brilliant Pebbles" (the latter were the heirs to "smart rocks")

The indispensable Future War Stories blog makes the point that there is a big difference between a Battle Station (orbital fortress) and a Military Space Station.

A battle station, mobile assault platform, or orbital fortress is basically a huge warship armed to the teeth that has no engine. It has lots of offensive weapons. Much like the Death Star from Star Wars, but used more to defend planets instead of blowing them up.

A military space station is a military base that just happens to be in orbit instead of on the ground. It is used to support troops, house spacecraft, administer logistical aid, and the like. Generally it only has defensive weapons, but may be protected by a space navy task force. They are much like the U.S. military bases located in the continental United States.


We who have grown up with the bomb can hardly imagine a world without the Sword of Damocles hanging over our heads by a thread. Strategic warfare has been dominated by offense for over 30 years.

Even though it might take 20-50 years, advances in space might swing the balance back toward defense. Here are some wild speculations, hopefully based on engineering realities.

Don’t be surprised if the Department of Defense picks up the slack in NASA funding of mass drivers and solar sails. A mass driver is just what you need to bring most of an asteroid to the vicinity of the Earth by throwing away part of the asteroid for reaction mass. Solar sails would bring them back piecemeal. A million-ton asteroid in high Earth orbit would solve a number of problems such as providing hardening for certain advanced weapons systems and their heat sinks.

"Hardening" is the capacity to take a beating and remain functional.

Heat sink" is an engineering term which can mean anything from a tiny clip on a transistor to the Mississippi river. It's whatever is used to get rid of waste heat. On Earth, waste heat is mostly carried off by water or air and eventually radiated from the vast area of the planet into the cold (three degrees above absolute zero) universe. Some of you may remember the story by Poul Anderson about a rogue planet (Satan) that was thawed out by a close pass near a star and then kept warm as it sailed back into the dark by industrial waste heat on a grand scale.

Getting rid of waste heat without a planet isn’t hard, but it isn't cheap either. Waste heat radiators are a major factor in the design of space industrial facilities, habitats, farms and military bases. For all of these, including, in the long run, military bases, the Stefan-Boltzman law relating temperature and radiation rate and the fact that people and their machines function best around “room temperature" implies that the radiator surface area will be about four square meters for every kilowatt of waste heat.

Military fact #1: in the size we need, waste heat radiators will be very large. Radiators must be filled with something (substitutes for wind and water) to carry the heat. For both physical and economic reasons radiators should have walls no thicker than required to contain the filling material.

Military fact #2: radiators are unavoidably fragile. Something both large and fragile would make a lousy military heat sink. Nobody can cheat on physical laws, but with an asteroid, you would be able (for a while) to use the "Alice's Restaurant" method of waste-heat disposal. (Alice lived in the belfry of a deserted church and put the garbage downstairs.) Two weapon systems, particle beams and lasers, have the potential to end the current offense-dominated Mexican stand-off known as "Mutual Assured Destruction," or MAD. Lasers are getting about $200 million per year development money in this country, and particle beams are believed to be better supported in the U.S.S.R.

Both particle beams and lasers are line of sight, speed of light weapons. This could make for some mighty short wars! They are very similar in needing millions of kilowatts of power and large heat sinks (because they are not very efficient) and both work better in space. Either method, with enough power behind it and a good enough aiming system, could make short work of ICBMs, submarine launched ballistic missiles and perhaps even bombers and cruise missiles, thereby eliminating all three of the U.S. "triad" at one stroke.

Skip for a moment the moral and geopolitical implications: how does an asteroid fit into this picture?

First, it's by far the easiest way to get a hardened site into space. Hardening is absolutely essential if the opposition has a similar installation. Otherwise, all the advantages go to "he who shoots first," a much worse situation than MAD. An actively defended fort could most likely stop missiles, but there is no way to shoot down a laser beam. However, you need not worry about lasers if you are inside a multimillion-ton asteroid. An MIT study some years ago concluded that even to slightly deflect an asteroid (e.g. Icarus) would take a lot of the very largest hydrogen bombs people make. A laser that could wipe out missiles would just blow little pock marks in the surface of an asteroid.

Second, to keep the laser cool you need a monster heat sink that a hostile laser won't cut to confetti. Radiators are just too vulnerable, (see above) so the waste heat will have to be stored till the war is over and that means an asteroidal sized mass to store the heat. Even if the laser gasses only make one pass through the laser and then are discharged into space, a substantial heat sink would be needed for the auxilliary equipment and such things as cooling the laser mirrors. For the same reason, all the energy to fight a war will have to be stored inside.

How much energy storage and heat sink capacity would be needed to fight a hypothetical war between the major powers with space-based lasers zapping all the missiles? Unless you complicate things by having the forts try to fight each other to the finish, a few gigawatt hours of beam energy is sufficient to wipe out the warhead delivery systems inventory of the entire world. Altogether there are less than 5000 ICBM's and submarine-launched ballistic missiles. Five gigawatt hours of beam energy would give a little less than a ton of explosive effect for each one. Because lasers are only about 20% efficient,* and allowing for some safety margin, energy storage might be ten times the beam energy and heat sink capacity about eight times the beam energy. To get a feel for this amount of energy in standard military terms, one gigawatt hour is equivalent to about 900 tons of TNT.

(*If you believe in higher efficiency, plug in your own numbers. Free electron lasers might reach 50%.)

The next question is how big an asteroid do you need in order to absorb, say, 40 gigawatt hours? A simple rule of thumb is that a kilowatt second will heat a kilogram of rock about one degree C. Forty gigawatt hours is 14.4 x 109 kilowatts, which means this much energy would heat a million ton asteroid 14.4 degrees C. Thermal stress rather than absolute temperature rise may turn out to be a determining factor. To keep a fort ready, you keep it cold.

How would an asteroid fort be constructed that could take considerable pounding from lasers and missiles and still be able to zap ICBM's. The best type, to start, would be the solid nickel-iron variety found in science fiction stories. Unfortunately, that may be the only place to find them. The processes (hotly argued over) that formed these objects may have left fracture-prone weak zones of silicate material between large blocks of solid metal.

For iron asteroids with fracture zones of stony iron (lumps of iron mixed with rock) the first job will be some outside shaping followed by drilling a lot of holes through the asteroid and stringing it together with steel cables. This would probably work with any asteroid that had as much compressive strength as concrete.

Next, a maze of coolant channels would be drilled through the rock or iron. Iron would provide an advantage here because of its much better conductivity. Either rock or iron would be fairly easy to drill through, but a mixture would be more difficult. The laser, control system and power storage would be installed in cavities dug out of the center of the asteroid. My guess is that energy would be stored in flywheels or fuel cells. Primary power could be nuclear reactors or solar cells. Either the solar cells or heat radiators for the reactor would hang outside and you could expect them to be shot off right at the start of any action.

Lastly, the surface would be covered many meters deep with foamed metal to soak up energy from a close nuclear blast or a short laser pulse. Much of the energy from a nuclear blast in space arrives in the form of X-rays which heat the outside surface so fast that a shock wave causes pieces to fly off the inside wall (spallation). A substantial layer of something crushable takes care of this problem.

To track targets and control the aiming of the laser would require a dispersed phased array radar too spread out to knock out with missiles and too hard to take out quickly with a laser. Verification of target destruction and some tracking would be done optically or with infrared. The radar information would be transmitted over redundant channels to a very large, fast computer in the fort. This part is within the capacity of present day electronics.

Like the Death Star in Star Wars, a space fort would have a vulnerable spot. It could be knocked out by a beam that went in where its beam went out. To protect its “Achilles Heel" each one might be surrounded with a flotilla of actively controlled mirrors: a fort could take bank shots with a diffuse beam at the other forts, while avoiding looking directly at them. (A fully focused beam would be so energetic that it would not be reflected, but would just vaporize the bank-shot mirror. The bank-shot mirror would refocus the beam more tightly.)

There are many counter and counter-counter strategies including shooting lasers at the forts from the ground, trying to disable all the enemy's reflector flotilla, hardening the bank shot reflectors, and slinging rocks at the forts. None look very promising. Attacking from the ground with lasers looks like it would bankrupt the country that tried it.

Why? Missiles can be destroyed by the energy equivalent of a few kilograms of TNT. A bomber can be wrecked by the equivalent of a few hundred kilograms. But an asteroid would take hundreds of megatons of TNT or millions of gigawatt hours. Not counting laser inefficiency, or the cost of the laser, a million gigawatt hours at one cent per kWh is $100 billion. The lasers would cost a thousand to a million times this much. Hitting a fort with another asteroid would be effective, but would take years due to celestial mechanics considerations. Also, it isn't easy to do secretly. Even the slightest ability of a fort to dodge would make it vastly more difficult to hit.

And a shoot-out between forts of similar size looks to be a real idiot's delight: “this hurts me more than you" really applies to space forts because four times as much energy as is in the laser beam must be dumped internally, and the vast majority of energy delivered by the laser beam would be reradiated; the remainder would do very little heating. Even with limitless energy available, an attacker using lasers would cook itself long before doing much damage to a target fort.

For the same reason, a small amount of hardening would protect a ground installation from attack by a space-based laser. The total energy available within a fort due to laser energy storage is equivalent to only a few hundred to a few thousand tons of TNT.

Schemes to put forts out of action would be less attractive if many countries owned several forts each. If only two countries owned one fort each, a fort being put out of action would leave the owner of that fort in a very bad fix, exposed to ICBM's without any way to retaliate. If a dozen countries owned several forts each, there would be very little point in keeping ICBM's active at all. Of course, some countries would still keep ICBM's around just to force others to spend money on defense. (A major effect of the U.S. bomber fleet is to force the U.S.S.R. to spend a bundle on air defense—money that would otherwise be spent on other military projects.)

Whether or not asteroid forts and very large lasers in space would have a major effect on ground warfare is a good question. I am sure tanks would be much more difficult to take out of action than cruise missiles or bombers. However, if the problems of shooting down through the atmosphere can be solved, it might accelerate the current trend, started by precision guided munitions, to quickly remove large, expensive objects from the battlefield. I don't think the troops will go back to swords and horses, but automatic rifles, hand-held rocket launchers and motorcycles might be the most expensive items practical on a year 2000 battlefield.

May the force be with you!

STARGUARD (brev'tal bir, against the cold) The opposite of a Starcruiser, since the diffusion of thousands of discrete members initially lacks any significant target. A Starcruiser might cut for hours through this jungle of tiny ships, mines, etc., until suddenly there were more targets than fire control could handle. Once such a ship began to take damage, the Starguard units quickly stung it to death. Total spherical complement about 4 million; mass 40 million tons; length about 100 km.

From Metagaming microgame HOLY WAR by Lynn Willis (1979)

The record began with a close up of one of the foldpoint fortresses. This was obviously library footage tacked on by the Sandarians for Altan benefit. The screen showed a great sphere that bristled with weapons and sensors. The dark snouts of several hundred laser ports dotted the surface of the sphere, as did a like number of missile launchers. Other features included thick layers of ablative shielding, power plant exhaust ports, and vast radiators to rid the fortresses of internal heat. A destroyer cruised past the fortress in the foreground of the picture, giving a sense of scale to the scene. The battle station was as large as a small asteroid, and brimming with destructive power.

“My God, what a behemoth!” Bethany said.

Drake nodded. “It’s even more impressive when you realize how much ship volume is normally taken up by photon engines and foldspace generators. That monster was designed to deliver its punch to the target without worrying about maneuvering. I’d estimate its power at about five blast ships, maybe more!”

From ANTARES DAWN by Michael McCollum (1986)

Space Superiority Platform

A variant on the orbital fortress is the Space Superiority Platform. Instead of defending the planet from invading spacefleets, this is an armed military station keeping an eye on the planet it is orbiting.

If a planet is balkanized, the platform will watch military ground units belonging to hostile nations, and bombard them if required. Militarily they have the high ground.

If the planet is a conquered one, or the government is oppressing the inhabitants, the platform will try to maintain government control and deal with revolts. By bombarding them if required.

In the real world, such platforms are currently limited to spy satellites. Orbiting nuclear bombing satellites are frowned upon. Or even things like the Strategic Defense Initiative.


      "It‘s getting bad, isn‘t it?" he asked.
     Heinemann sighed. "Worse than you might think, Hauptmann. Even the ranks of the Peace Enforcers are not immune to these internecine squabbles that have broken out all over the face of the Earth. If it is not the North Americans against the South, then it is the Australians versus Indonesia, or Japan against China and West Russia. I tell you the whole world is going to Satan in a hand trolley."…

     …Wing Commander Livingston was on detached service from the RAF. His powder blue uniform looked out of place next to Stassel‘s silver and black. Stassel sat in an aluminum chair and took notes as Livingston reeled off figures in his clipped, Oxford accent.
     " … Your area of responsibility will include Longitudes 100 West to 120 West, Captain. Your satellite will be in an alternating synchronous orbit with Beta-Nine, of course, and you will have prime responsibility in the Northern Hemisphere during even watch periods and Southern Hemisphere during the odd. Luckily, south of the equator there is only empty ocean between 100 and 120 West, so you‘ll be able to get some rest.
     "You are hereby directed to pay especially close attention to the situation around the US-Mexican border..." Livingston looked up, the podium light casting shadows on his face. "Watch your a** on that one, Fred. It is a tinderbox. The Mexicans are bound to try a raid between now and the Security Council vote on Friday."…

     …The second development was the formation of the UN Peace Enforcers following the twenty-day scare of the Misfire War. The Peace Enforcers were a multinational force with a single mission: To stop any aggressor who struck against any UN member state. Their unofficial motto was, "You start the war and we‘ll finish it!"
     In theory, any act of aggression by one nation against another would be met instantly by the orbital lasers and Peace Enforcer fusion rockets. However, in practice there was a threshold level of violence, a tripwire effect, below which the cumbersome Security Council machinery would fail to respond.…

     …The lift whooshed him upward toward the station axis. The familiar, ever changing Coriolus force as he approached the axis clamped his stomach muscles in a familiar vise. At the zero gravity axis, Stassel kicked off and floated to the docking port at the north pole of the station and through a flexible tube to the shuttle.
     The shuttle was a standard orbit-to-orbit supply bus — three spherical sections assembled as though they had been skewered onto a shish-kabob sword with a hydrogen-fueled rocket at one end and the personnel cabin at the other. The shuttle was used to transfer personnel and consumables from the mid-Atlantic Space Station (and her mid-Pacific counterpart) to the orbiting Peace Control Satellites.
     The station was in synchronous orbit 37,000 kilometers above the equator so that it hung perpetually over thirty degrees west longitude. The Peace Control Satellites also orbited 37,000 kilometers out, but in two separate orbits, each inclined sixty degrees from the plane of the equator and from each other. Each satellite thus described a figure eight over a stationary strip of land, taking one day for the full traverse across the face of the planet. The satellites climbed to the latitude of Hudson‘s Bay in the north and dropped to the northern tip of Antarctica in the south. Spaced every ten degrees of longitude — or 7500 kilometers apart — in their orbits, the satellites passed over every industrialized and developing nation on Earth four times daily. The seventy satellites and two space stations in orbit gave the UN‘s hundred gigawatt lasers overlapping fields of fire against any conceivable opponent. War was impossible.
     At least, that was the theory.…

     …The pilot cleared his throat. "Uh, get your things ready for transfer. We‘ll be coming up on Alpha-Nine in about twenty minutes."
     Peace Control Satellite Alpha-Nine floated into view fifteen minutes later. Like all such, it was constructed in two pieces. The thirty-meter long cylinder that housed the hydrogen-fluorine gas dynamic laser and its fuel tanks was attached by a hundred meter long umbilical to a sphere painted in a haphazard pattern of light and dark checks. The ten-meter sphere was festooned with antennas, telescopes, and the more arcane paraphernalia of a dozen different kinds of information sensors and communications devices. The doghouse, as the sphere was called, was crammed solid with hardware that acted as the satellite‘s eyes and ears and brains. The umbilical — floating limply in space as the shuttle moved in slowly for a hard dock — connected the two halves of the satellite together and isolated the laser module with its sensitive aiming mechanisms from extraneous perturbations. For instance, the force of a hundred-ton shuttle coming to rest in the doghouse‘s docking collar, or the effect of the satellite commander doing his morning calisthenics.
     The satellite living quarters were located at the end of the doghouse arbitrarily labeled 'top'. They were tiny, consisting of a control center, shower bath, and combination galley and recreation-bunk room. The crew quarters of a PCS did not have to be large. The satellite commander was the only crewmember. Even so, the UN had a perennial problem keeping seventy satellites manned with reliable people on a one-week rotation schedule. What the satellite commander lacked in numbers, he more than made up for in firepower. At his fingertips were the controls to a hundred-gigawatt laser, powerful enough to strike down any opponent. Moreover, if needed, he would be backed up by the power of the space fleet.

Interdiction Platform

This is a sort of combination of Space Superiority Platform and Planetary Defense. The idea is that the station is to prevent anything unauthorized from entering or leaving the planet it is orbiting.

  • A planet might be invested, meaning that the planet is under siege from whoever owns the space station. The station does not want planetary inhabitants escaping, nor does it want blockade runners entering.
  • A planet might be interdicted because they contain something very dangerous (Xenomorphs, thionite, the City on the Edge of Forever, replicators, or 100% lethal plagues).
  • A planet might be interdicted because it has something very valuable and the station owner does not want poachers sneaking in and stealing any.

Planetary Fortress

After the invaders have neutralized the defenders orbital fortresses, the only thing left stopping the invaders from carpet-bombing the vulnerable planet are the defending planetary fortresses. Orbital fortresses do have problems with heat radiators and supply. Planetary fortresses on the other hand have practically no radiator or supply problems, since they have an entire planet for support. In the Strategic Defense Initiative, concepts looked into included "Extended Range Interceptor", "Homing Overlay Experiment ", and "Exoatmospheric Reentry-vehicle Interception System"

In space opera, "force fields" are generally spherical. So a planetary fortress (or civilian city) protected by such a field will have a circular boarder. Anything outside of the circle will also be outside of the force field, and thus vulnerable to bombardment. If the force field prevents defending weapons from firing out along with preventing attacking weapons from firing in, the fortress might have weapon emplacements outside of the boundary of the force field.

In Larry Niven and Jerry Pournelle's classic The Mote In God's Eye, some times Imperial task forces would find the Langston Field defense over the cities on a rebel planet too difficult to crack. If the task force was under a severe time limit, they would be forced into the draconian option of using nuclear weapons to take out all the agriculture on the planet, then leaving. The rebels would then mostly starve to death, since it is impossible to ship food for millions of people over insterstellar distances. The imperials would have fullfilled their mission, since the rebels would cease to be a threat, eventually.

Rod Blaine scowled at the words flowing across the screen of his pocket computer. The physical data were current, but everything else was obsolete. The rebels had changed even the name of their world, from New Chicago to Dame Liberty. Her government would have to be built all over again. Certainly she'd lose her delegates; she might even lose the right to an elected assembly.

He put the instrument away and looked down. They were over mountainous country, and he saw no signs of war. There hadn't been any area bombardments, thank God.

It happened sometimes: a city fortress would hold out with the aid of satellite-based planetary defenses. The Navy had no time for prolonged sieges. Imperial policy was to finish rebellions at the lowest possible cost in lives—but to finish them. A holdout rebel planet might be reduced to glittering lava fields, with nothing surviving but a few cities lidded by the black domes of Langston Fields; and what then? There weren't enough ships to transport food across interstellar distances. Plague and famine would follow.

From The Mote In God's Eye by Larry Niven and Jerry Pournelle (1974)

And Alistair Young points out that if the spaceports of the planet use laser launchers they are also planetary fortresses.


It is a truism of celestial warfare that among the most valuable targets to seize in the course of a major planetary assault operation is the primary planetary starport or local starports close to the intended target(s) of the operation. Starports, for all the obvious reasons, make perfect orbitheads, offering existing facilities eminently suitable for the landing and disembarkation of troops and materiel in quantity. (Orbital elevators, by contrast, are usually considered too fragile and susceptible to sabotage for this purpose, if the enemy are willing to absorb the ensuing damage to their own planet, until the orbitals and the continental area surrounding the elevator have been entirely secured.)

Why, then, are combat drops rarely, if ever, targeted at the vicinity of starports?

Again, it is important to remember that which is unseen. The popular image of starports is heavily biased towards the facilities for ground-landing starships — understandably, since the giant launch/landing pads built to handle nucleonic-thermal ships, with their blast-deflecting berms, “hot” shafts, and motile structures are some of the most impressive structures ever built — and towards the shuttleport terminals used by commuters and starship passengers alike. Nonetheless, the majority of cargo in the developed Worlds is carried by dedicated spacecraft incapable of atmospheric landing, to and from which cargo is transported in high volumes using suitably cheap methods: either laser-launch/deceleration facilities, mass drivers, or both, in which case the former handles light or delicate cargo and the latter hardbulk.

What this means in military terms is that, any other defense grid aside, the majority of starports in the developed Worlds have at their disposal a multi-gigawatt-range phased-array laser system, and/or a pair of mass drivers capable of accelerating a solid slug the size of a shipping container (or, equally effective, a shipping container packed with rubble or cheap heavy-metal ingots) to orbital velocities — both, admittedly, equipped with safety systems designed to prevent them from being used in exactly the manner which is desirable for military purposes, but that is something usually corrected readily enough by a software change — along with all the high-resolution traffic-control sensor equipment needed to target them effectively.

It is also a truism of warfare in general that one shouldn’t stab a heavily-armed man in the front. That is doubly relevant when the things they’re using as weapons are also the value that you want to capture.

— Elementary Principles of Orbit-to-Ground Maneuver Plans, pub. INI Press


However, it should not be assumed that any space colony will be totally defenseless.  Depending on the configuration and environment of the colony, it is possible that the colony will have some form of anti-meteor system in place, which could easily be repurposed to serve as a ground-based defense laser.  The problem with the idea of such a system is that it’s relatively easy to put a colony underground, and being underground also provides protection from radiation.  Meteors large enough to be a threat to the buried colony are likely to be too large for a practical laser system to deal with, but should also give enough warning to be deflected by other methods.

A much more plausible source for some sort of fixed ground-based defense is a laser propulsion system.  This would either be used to boost spacecraft into orbit, or to propel them between planets.  In either case, the biggest issue is likely to be setting up the system to target non-cooperative spacecraft.  On the other hand, the fact that such a system is fixed removes its value as an offensive weapon, and such capability might be standard.  In either case, the existence of such a system will make the possibility of a direct attack even more remote, limiting warfare to skirmishing between the colonies involved.  A ground invasion is remotely plausible, but the defender must either be taken totally by surprise or be faced with overwhelming force, both of which are unlikely in the sort of conflict described here.  Farther problems are the requirement for overland movement, and the difficulty posed by the defender’s orbital bombardment.

by Byron Coffey (2016)

“We’ve had time to map the planet rather extensively over the last thirty hours. In the process, we have spotted numerous large installations built on the polar ice caps. Our experts have tentatively identified them as a network of planetary defense centers. To judge by their number, this may well be the best defended planet in human space!”

Sandarian Military Headquarters was a truncated pyramid that had been built on an island of bare rock in the middle of an ocean of ice. At first, the structure was a tiny patch of brown on the horizon amid an endless panorama of white. Then, as the aircar carrying Duke Bardak, Richard Drake, and Argos Cristobal closed the distance, the true size of the building became increasingly apparent with each passing second.

“Big building,” Drake said to Bardak.

“Big enough,” the Sandarian nobleman replied. “It measures one kilometer to a side and rises half-a-kilometer above bedrock. The sides are armor plated to a depth of two meters, and screened by anti-rad fields.”

“Wouldn’t it have been more convenient to build it near Capital?” Drake asked. It had taken the aircar some three hours to fly from Capital to Military Headquarters, and for virtually the whole of that time, the car had droned above a seemingly endless expanse of ice.

“More convenient, yes. Safer, no. Military Headquarters doubles as a planetary defense center. You no doubt saw many of our PDCs from orbit.”

Drake nodded.

“An operational planetary defense center requires a lot of power and generates a considerable quantity of waste heat. In a prolonged engagement, the waste heat from a single fixed mount laser can raise the temperature of even a medium size river by several degrees. Were we to build our PDCs in the temperate zone, we would trigger a massive fish kill every time we tested the weapons. The polar ice cap, on the other hand, has a heat carrying capacity that is virtually infinite. You cannot see it, but the ice fields around Military Headquarters are honeycombed with heat rejection piping. Theoretically, we could fire every fixed mount laser in the battery continuously for days before we would have to worry about overheating problems.

“Then, of course, there are the strategic considerations. By spreading our installations evenly over Sandar’s surface, we avoid blind spots while also dispersing our military assets. Lastly, should the Ryall ever get this far — highly unlikely considering the power of our foldpoint fortresses — we hope to draw their fire to the PDCs and away from the cities.”

As Bardak talked, the aircar circled Military Headquarters to give the Altans a good look at the manmade mountain below. The sides of the pyramid were studded with phased array radar elements, as well as a variety of less identifiable sensors. Around the base, the business ends of several dozen fixed mount lasers poked skyward.

From Antares Dawn by Michael McCollum (1986)

(ed note: this was written before the invention of the laser. The fortress is called Project Thor, no relation to Rods From God, and is located on the Moon near Pico crater. It is about to be attacked by three spacecraft. Dr. Steffanson has invented a technobabble "ray screen" that reflects electromagnetic radiation. Jamieson and Wheeler are hiding in a crevasse several miles away near their crippled tractor. The description is so cinematic that it is just begging for an SF artist to depict it.)

EVEN TODAY, little has ever been revealed concerning the weapons used in the Battle of Pico. It is known that missiles played only a minor part in the engagement. In space warfare, anything short of a direct hit is almost useless, since there is nothing to transmit the energy of a shock wave. An atom bomb exploding a few hundred meters away can cause no blast damage and even its radiation can do little harm to well-protected structures. Moreover, both Earth and the Federation had effective means of diverting ordinary projectiles.

Purely non-material weapons would have to play the greatest role. The simplest of these were the ion-beams, developed directly from the drive-units of spaceships. Since the invention of the first radio tubes, almost three centuries before, men had been learning how to produce and focus ever more concentrated streams of charged particles. The climax had been reached in spaceship propulsion with the so-called "ion rocket," generating its thrust from the emission of intense beams of electrically charged particles. The deadliness of these beams had caused many accidents in space, even though they were deliberately defocused to limit their effective range.

There was, of course, an obvious answer to such weapons. The electric and magnetic fields which produced them could also be used for their dispersion, converting them from annihilating beams into a harmless, scattered spray.

More effective, but more difficult to build, were the weapons using pure radiation. Yet even here, both Earth and the Federation had succeeded. It remained to be seen which had done the better job—the superior science of the Federation, or the greater productive capacity of Earth.

Less than a million kilometers away, Carl Steffanson sat at a control desk and watched the image of the sun, picked up by one of the many cameras that were the eyes of Project Thor. The group of tired technicians standing around him had almost completed the equipment before his arrival; now the discriminating units he had brought from Earth in such desperate haste had been wired into the circuit.

Steffanson turned a knob, and the sun went out. He flicked from one camera position to another, but all the eyes of the fortress were equally blind. The coverage was complete.

Too weary to feel any exhilaration, he leaned back in his seat and gestured toward the controls.

"It's up to you now. Set it to pass enough light for vision, but to give total rejection from the ultra-violet upward. We're sure none of their beams carry any effective power much beyond a thousand Ångström. They'll be very surprised when all their stuff bounces off. I only wish we could send it back the way it came."

"Wonder what we look like from outside when the screen's on?" said one of the engineers.

"Just like a perfectly reflecting mirror. As long as it keeps reflecting, we're safe against pure radiation. That's all I can promise you."

(ed note: 1000 Ångströms = 100 nanometers = extreme ultraviolet)

There was no warning of any kind. Suddenly the gray, dusty rocks of the Sea of Rains were scorched by a light they had never known before in all their history. Wheeler's first impression was that someone had turned a giant searchlight full upon the tractor; then he realized that this sun-eclipsing explosion was many kilometers away. High above the horizon was a ball of violet flame, perfectly spherical, and rapidly losing brilliance as it expanded. Within seconds, it had faded to a great cloud of luminous gas. It was dropping down toward the edge of the Moon, and almost at once had sunk below the skyline like some fantastic sun.

"We were fools," said Jamieson gravely. "That was an atomic warhead—we may be dead men already."

"Nonsense," retorted Wheeler, though without much confidence. "That was fifty kilometers away. The gammas would be pretty weak by the time they reached us—and these walls aren't bad shielding."

"I can just see the dome," he said with some satisfaction. "It's quite unchanged, as far as I can tell."

"It would be," Jamieson replied. "They must have managed to explode that bomb somehow while it was miles away."

"Perhaps it was only a warning shot."

"Not likely! No one wastes plutonium for firework displays. That meant business. I wonder when the next move is going to be?"

It did not come for another five minutes. Then, almost simultaneously, three more of the dazzling atomic suns burst against the sky. They were all moving on trajectories that took them toward the dome, but long before they reached it they had dispersed into tenuous clouds of vapor.

From somewhere beyond Pico, six sheaves of flame shot up into the sky at an enormous acceleration. The dome was launching its first missiles, straight into the face of the sun. The Lethe and the Eridanus were using a trick as old as warfare itself; they were approaching from a direction in which their opponent would be partly blinded. Even radar could be distracted by the background of solar interference, and Commodore Brennan had enlisted two large sunspots as minor allies.

Then he saw that something was happening to the dome. It was no longer a gleaming spherical mirror reflecting only the single image of the sun. Light was splashing from it in all directions, and its brilliance was increasing second by second. From somewhere out in space, power was being poured into the fortress. That could only mean that the ships of the Federation were floating up there against the stars, beaming countless millions of kilowatts down upon the Moon. But there was still no sign of them, for there was nothing to reveal the track of the river of energy pouring invisibly through space.

The dome was now far too bright to look upon directly, and Wheeler readjusted his filters. He wondered when it was going to reply to the attack, or indeed if it could do so while it was under this bombardment. Then he saw that the hemisphere was surrounded by a wavering corona, like some kind of brush discharge. Almost at the same moment, Jamieson's voice rang in his ears.

"Look, Con—right overhead!"

He glanced away from the mirror and looked directly into the sky. For the first time, he saw one of the Federation ships. Though he did not know it, he was seeing the Acheron, the only spaceship ever to be built specifically for war. It was clearly visible, and seemed remarkably close. Between it and the fortress, like an impalpable shield, flared a disk of light which as he watched turned cherry-red, then blue-white, then the deadly searing violet seen only in the hottest of the stars. The shield wavered back and forth, giving the impression of being balanced by tremendous and opposing energies. As Wheeler stared, oblivious to his peril, he saw that the whole ship was surrounded by a faint halo of light, brought to incandescence only where the weapons of the fortress tore against it.

It was some time before he realized that there were two other ships in the sky, each shielded by its own flaming nimbus. Now the battle was beginning to take shape; each side had cautiously tested its defenses and its weapons, and only now had the real trial of strength begun.

The two astronomers stared in wonder at the moving fireballs of the ships. Here was something totally new—something far more important than any mere weapon. These vessels possessed a means of propulsion which must make the rocket obsolete. They could hover motionless at will, then move off in any direction at a high acceleration. They needed this mobility; the fortress, with all its fixed equipment, far out-powered them and much of their defense lay in their speed.

In utter silence, the battle was rising to its climax. Millions of years ago the molten rock had frozen to form the Sea of Rains, and now the weapons of the ships were turning it once more to lava. Out by the fortress, clouds of incandescent vapor were being blasted into the sky as the beams of the attackers spent their fury against the unprotected rocks. It was impossible to tell which side was inflicting the greater damage. Now and again a screen would flare up, as a flicker of heat passes over white-hot steel. When that happened to one of the battleships, it would move away with that incredible acceleration, and it would be several seconds before the focusing devices of the fort had located it again.

Both Wheeler and Jamieson were surprised that the battle was being fought at such short ranges. There was probably never more than a hundred kilometers between the antagonists, and usually it was much less than this. When one fought with weapons that traveled at the speed of light—indeed, when one fought with light itself—such distances were trivial.

The explanation did not occur to them until the end of the engagement. All radiation weapons have one limitation: they must obey the law of inverse squares. Only explosive missiles are equally effective from whatever range they have been projected: if one is hit by an atomic bomb, it makes no difference whether it has traveled ten kilometers or a thousand.

But double the distance of any kind of radiation weapon, and you divide its power by four owing to the spreading of the beam. No wonder, therefore, that the Federal commander was coming as close to his objective as he dared.

The fort, lacking mobility, had to accept any punishment the ships could give it. After the battle had been on for a few minutes, it was impossible for the unshielded eye to look anywhere toward the south. Ever and again the clouds of rock vapor would go sailing up into the sky, falling back on the ground like luminous steam. And presently, as he peered through his darkened goggles and maneuvered his clumsy periscope, Wheeler saw something he could scarcely believe. Around the base of the fortress was a slowly spreading circle of lava, melting down ridges and even small hillocks like lumps of wax.

That awe-inspiring sight brought home to him, as nothing else had done, the frightful power of the weapons that were being wielded only a few kilometers away. If even the merest stray reflection of those energies reached them here, they would be snuffed out of existence as swiftly as moths in an oxy-hydrogen flame.

The three ships appeared to be moving in some complex tactical pattern, so that they could maintain the maximum bombardment of the fort while reducing its opportunity of striking back. Several times one of the ships passed vertically overhead, and Wheeler retreated as far into the crack as he could in case any of the radiation scattered from the screens splashed down upon them. Jamieson, who had given up trying to persuade his colleague to take fewer risks, had now crawled some distance along the crevasse, looking for a deeper part, preferably with a good overhang. He was not so far away, however, that the rock was shielding the suit-radios, and Wheeler gave him a continuous commentary on the battle.

It was hard to believe that the entire engagement had not yet lasted ten minutes. As Wheeler cautiously surveyed the inferno to the south, he noticed that the hemisphere seemed to have lost some of its symmetry. At first he thought that one of the generators might have failed, so that the protective field could no longer be maintained. Then he saw that the lake of lava was at least a kilometer across, and he guessed that the whole fort had floated off its foundations. Probably the defenders were not even aware of the fact. Their insulation must be taking care of solar heats, and would hardly notice the modest warmth of molten rock.

And now a strange thing was beginning to happen. The rays with which the battle was being fought were no longer quite invisible, for the fortress was no longer in a vacuum. Around it the boiling rock was releasing enormous volumes of gas, through which the paths of the rays were as clearly visible as searchlights in a misty night on Earth. At the same time Wheeler began to notice a continual hail of tiny particles around him. For a moment he was puzzled; then he realized that the rock vapor was condensing after it had been blasted up into the sky. It seemed too light to be dangerous, and he did not mention it to Jamieson—it would only give him something else to worry about. As long as the dust fall was not too heavy, the normal insulation of the suits could deal with it. In any case, it would probably be quite cold by the time it got back to the surface.

The tenuous and temporary atmosphere round the dome was producing another unexpected effect. Occasional flashes of lightning darted between ground and sky, draining off the enormous static charges that must be accumulating around the fort. Some those flashes would have been spectacular by themselves—but they were scarcely visible against the incandescent clouds that generated them.

Wheeler never knew why the fortress waited so long before it used its main weapon. Perhaps Steffanson—or whoever was in charge—was waiting for the attack to slacken so that he could risk lowering the defenses of the dome for the millisecond that he needed to launch his stiletto.

Wheeler saw it strike upward, a solid bar of light stabbing it the stars. He remembered the rumors that had gone round tie Observatory. So this was what had been seen, flashing above the mountains. He did not have time to reflect on the staggering violation of the laws of optics which this phenomenon implied, for he was staring at the ruined ship above his head. The beam had gone through the Lethe as if she did not exist; the fortress had speared her as an entomologist pierces a butterfly with a pin.

Whatever one's loyalties, it was a terrible thing to see ho the screens of that great ship suddenly vanished as her generators dies, leaving her helpless and unprotected in the sky. The secondary weapons of the fort were at her instantly, tearing out great gashes of metal and boiling away her armor layer by layer. Then, quite slowly, she began to settle toward the Moon, still on an even keel. No one will ever know what stopped her, probably some short-circuit in her controls, since none of her crew could have been left alive. For suddenly she went off to the east in a long, flat trajectory. By that time most of her hull had been boiled away and the skeleton of her framework was almost completely exposed. The crash came, minutes later, as she plunged out of sight beyond the Teneriffe Mountains. A blue-white aurora flickered for a moment below the horizon, and Wheeler waited for the shock to reach him.

And then, as he stared into the east, he saw a line of dust rising from the plain, sweeping toward him as if driven by a mighty wind. The concussion was racing through the rock, hurling the surface dust high into the sky as it passed. The swift, inexorable approach of that silently moving wall, advancing at the rate of several kilometers a second, was enough to strike terror into anyone who did not know its cause. But it was quite harmless; when the wave-front reached him, it was as if a minor earthquake had passed. The veil of dust reduced visibility to zero for a few seconds, then subsided as swiftly as it had come. When Wheeler looked again for the remaining ships, they were so far away that their screens had shrunk to little balls of fire against the zenith. At first he thought they were retreating; then, abruptly, the screens began to expand as they came down into the attack under a terrific vertical acceleration. Over by the fortress the lava, like some tortured living creature, was throwing itself madly into the sky as the beams tore into it.

The Acheron and Eridanus came out of their dives about a kilometer above the fort. For an instant, they were motionless; then they went back into the sky together. But the Eridanus had been mortally wounded, though Wheeler knew only that one of the screens was shrinking much more slowly than the other.

With a feeling of helpless fascination, he watched the stricken ship fall back toward the Moon. He wondered if the fort would use its enigmatic weapon again, or whether the defenders realized that it was unnecessary.

About ten kilometers up, the screens of the Eridanus seemed to explode and she hung unprotected, a blunt torpedo of black metal, almost invisible against the sky. Instantly her light-absorbing paint, and the armor beneath, were torn off by the beams of the fortress. The great ship turned cherry-red, then white. She swung over so that her prow turned toward the Moon, and began her last dive. At first it seemed to Wheeler that she was heading straight toward him; then he saw that she was aimed at the fort. She was obeying her captain's last command.

It was almost a direct hit. The dying ship smashed into the lake of lava and exploded instantly, engulfing the fortress in an expanding hemisphere of flame. This, thought Wheeler, must surely be the end. He waited for the shock wave to reach him, and again watched the wall of dust sweep by—this time into the north. The concussion was so violent that it jerked him off his feet, and he did not see how anyone in the fort could have survived. Cautiously, he put down the mirror which had given him almost all his view of the battle, and peered over the edge of his trench. He did not know that the final paroxysm was yet to come.

Incredibly, the dome was still there, though now it seemed that part of it had been sheared away. And it was inert and lifeless: its screens were down, its energies exhausted, its garrison, surely, already dead. If so, they had done their work. Of the remaining Federal ship, there was no sign. She was already retreating toward Mars, her main armament completely useless and her drive units on the point of failure. She would never fight again

"What puzzles me most of all," Wheeler concluded, "is the weapon the fort used to destroy the battleship. It looked like a beam of some kind, but of course that's impossible. No beam can be visible in a vacuum. And I wonder why they only used it once? Do you know anything about it?"

"I'm afraid not," replied Sadler, which was quite untrue. He still knew very little about the weapons in the fort, but this was the only one he now fully understood. He could well appreciate why a jet of molten metal, hurled through space at several hundred kilometers a second by the most powerful electromagnets ever built, might have looked like a beam of light flashing a for an instant. And he knew that it was a short-range weapon designed to pierce the fields which would deflect ordinary projectiles. It could he used only under ideal conditions, and it took many minutes to recharge the gigantic condensers which powered the magnets.

(ed note: there is no reason for the metal to be molten. It is better to be solid, which would make the weapon a species of mass-driver)

From Earthlight by Arthur C. Clarke (1955)

(ed note: This is pure space opera with zero scientific accuracy. This is probably where Sir Arthur C. Clarke got the idea of the sea of lava around Project Thor. In the following, they use a handwaving method to turn copper into pure energy with all the might of e=mc2, for electricity and explosives. The transparent material pure handwavium that is several thousand times as strong as steel.)

"There, we can see what they're doing now," and DuQuesne anchored the vessel with an attractor. "I want to see if they've got many of those space-ships in action, and you will want to see what war is like, when it is fought by people, who have been making war steadily for ten thousand years."

Poised at the limit of clear visibility, the two men studied the incessant battle being waged beneath them. They saw not one, but fully a thousand of the globular craft high in the air and grouped in a great circle around an immense fortification upon the ground below. They saw no airships in the line of battle, but noticed that many such vessels were flying to and from the front, apparently carrying supplies. The fortress was an immense dome of some glassy, transparent material, partially covered with slag, through which they saw that the central space was occupied by orderly groups of barracks, and that round the circumference were arranged gigantic generators, projectors, and other machinery at whose purposes they could not even guess. From the base of the dome a twenty-mile-wide apron of the same glassy substance spread over the ground, and above this apron and around the dome were thrown the mighty defensive ray-screens, visible now and then in scintillating violet splendor as one of the copper-driven Kondalian projectors sought in vain for an opening. But the Earth-men saw with surprise that the main attack was not being directed at the dome; that only an occasional ray was thrown against it in order to make the defenders keep their screens up continuously. The edge of the apron was bearing the brunt of that vicious and never-ceasing attack, and most concerned the desperate defense.

For miles beyond that edge, and as deep under it as frightful rays and enormous charges of explosive copper could penetrate, the ground was one seething, flaming volcano of molten and incandescent lava; lava constantly being volatilized by the unimaginable heat of those rays and being hurled for miles in all directions by the inconceivable power of those explosive copper projectiles—the heaviest projectiles that could be used without endangering the planet itself—being directed under the exposed edge of that unbreakable apron, which was in actuality anchored to the solid core of the planet itself; lava flowing into and filling up the vast craters caused by the explosions. The attack seemed fiercest at certain points, perhaps a quarter of a mile apart around the circle, and after a time the watchers perceived that at those points, under the edge of the apron, in that indescribable inferno of boiling lava, destructive rays, and disintegrating copper, there were enemy machines at work. These machines were strengthening the protecting apron and extending it, very slowly, but ever wider and ever deeper as the ground under it and before it was volatilized or hurled away by the awful forces of the Kondalian attack. So much destruction had already been wrought that the edge of the apron and its molten moat were already fully a mile below the normal level of that cratered, torn, and tortured plain.

Now and then one of the mechanical moles would cease its labors, overcome by the concentrated fury of destruction centered upon it. Its shattered remnants would be withdrawn and shortly, repaired or replaced, it would be back at work. But it was not the defenders who had suffered most heavily. The fortress was literally ringed about with the shattered remnants of airships, and the riddled hulls of more than a few of those mighty globular cruisers of the void bore mute testimony to the deadliness and efficiency of the warfare of the invaders.

From Skylark Three by E. E. "Doc" Smith (1930)

(ed note: again, pure space opera. Again lava is a factor)

To such good purpose did every Valeronian do his part that at dawn of The Day everything was in readiness for the Chloran visitation. The immense fortress was complete and had been tested in every part, from the ranked batteries of gigantic converters and generators down to the most distant outlying visiray viewpoint. It was powered, armed, equipped, provisioned, garrisoned. Every once-populated city was devoid of life, its inhabitants having dispersed over the face of the globe, to live in isolated groups until it had been decided whether the proud civilization of Valeron was to triumph or to perish.

Promptly as that sunrise the Chloran explorer appeared at the lifeless mine, and when he found the loading hoppers empty he calmly proceeded to the nearest city and began to beam it down. Finding it deserted he cut off, and felt a powerful spy ray, upon which he set a tracer. This time the ray held up and he saw the immense fortress which had been erected during his absence; a fortress which he forthwith attacked viciously, carelessly, and with the loftily arrogant contempt which seemed to characterize his breed.

But was that innate contemptuousness the real reason for that suicidal attempt? Or had that vessel's commander been ordered by the Great Ones to sacrifice himself and his command so that they could measure Valeron's defensive power? If so, why did he visit the mine at all and why did he not know beforehand the location of the fortress? Camouflage? In view of what the Great Ones of Chlora must have known, why that commander did what he did that morning no one of Valeron ever knew.

The explorer launched a beam—just one. Then Quedrin Radnor pressed a contact and out against the invader there flamed a beam of such violence that the amoebus had no time to touch his controls, that even the automatic trips of his zone of force—if he had such trips—did not have time in which to react. The defensive screens scarcely flashed, so rapidly did that terrific beam drive through them, and the vessel itself disappeared almost instantly—molten, vaporized, consumed utterly. But there was no exultation beneath Valeron's mighty dome. From the Bardyle down, the defenders of their planet knew full well that the real attack was yet to come, and knew that it would not be long delayed.

Nor was it. Nor did those which came to reduce Valeron's far-flung stronghold in any way resemble any form of space ship with which humanity was familiar. Two stupendous structures of metal appeared, plunging stolidly along, veritable flying fortresses, of such enormous bulk and mass that it seemed scarcely conceivable for them actually to support themselves in air.

Simultaneously the two floating castles launched against the towering dome of defense the heaviest beams they could generate and project. Under that awful thrust Valeron's mighty generators shrieked a mad crescendo and her imponderable shield radiated a fierce, eye-tearing violet, but it held. Not for nothing had the mightiest minds of Valeron wrought to convert their mechanisms and forces of peace into engines of war; not for nothing had her people labored with all their mental and physical might for almost twoscore days and nights, smoothly and efficiently as one mind in one body. Not easily did even Valeron's Titanic defensive installation carry that frightful load, but they carried it.

Then, like mythical Jove hurling his bolt—like, that is, save that beside that Valeronian beam any possible bolt of lightning would have been as sweetly innocuous a caress as young love's first kiss—Radnor drove against the nearer structure a beam of concentrated fury; a beam behind which was every volt and every ampere that his stupendous offensive generators could yield.

The Chloran defenses in turn were loaded grievously, but in turn they also held; and for hours then there raged a furiously spectacular struggle. Beams, rods, planes, and needles of every known kind and of every usable frequency of vibratory energy were driven against impenetrable neutralizing screens. Monstrous cannon, hurling shells with a velocity and of an explosive violence far beyond anything known to us of Earth, radio-beam-dirigible torpedoes, robot manned drill planes, and many other lethal agencies of ultrascientific war—all these were put to use by both sides in those first few frantic hours, but neither side was able to make any impression upon the other. Then, each realizing the other's defenses had been designed to withstand his every force, the intensive combat settled down to a war of sheer attrition.

Radnor and his scientists devoted themselves exclusively to the development of new and ever more powerful weapons of offense; the Chlorans ceased their fruitless attacks upon the central dome and concentrated all their offensive power into two semicircular arcs, which they directed vertically downward upon the outer ring of the Valeronian works in an incessant and methodical flood of energy.

They could not pierce the defensive shields against Valeron's massed power, but they could and did bring into being a vast annular lake of furiously boiling lava, into which the outer ring of fortresses began slowly to crumble and dissolve. This method of destruction, while slow, was certain; and grimly, pertinaciously, implacably, the Chlorans went about the business of reducing Valeron's only citadel.

The Bardyle wondered audibly how the enemy could possibly maintain indefinitely an attack so profligate of energy, but he soon learned that there were at least four of the floating fortresses engaged in the undertaking. Occasionally the two creations then attacking were replaced by two precisely similar structures, presumably to return to Chlora in order to renew their supplies of the substance, whatever it was, from the atomic disintegration of which they derived their incomprehensible power.

And slowly, contesting stubbornly and bitterly every foot of ground lost, the forces of Valeron were beaten back under the relentless, never-ceasing attack of the Chloran monstrosities—back and ever back toward their central dome as ring after ring of the outlying fortifications slagged down into that turbulently seething, that incandescently flaming lake of boiling lava.

Valeron was making her last stand. Her back was against the wall. The steadily contracting ring of Chloran force had been driven inward until only one thin line of fortified works lay between it and the great dome covering the city itself. Within a week at most, perhaps within days, that voracious flood of lava would lick into and would dissolve that last line of defense. The what of Valeron?

All the scientists of the planet had toiled and had studied, day and night, but to no avail. Each new device developed to halt the march of the encroaching constricting band of destruction had been nullified in the instant of its first trial.

"They must know every move we make, to block us so promptly," Quedrin Radnor had mused one day. "Since they certainly have no visiray viewpoints of material substance within our dome, they must be able to operate a spy ray using only the narrow gravity band, a thing we have never been able to accomplish. If they can project such viewpoints of pure force through such a narrow band, may they not be able to project a full materialization and thus destroy us? But, no, that band is—must be—altogether too narrow for that."

Stirred by these thoughts he had built detectors to announce the appearance of any nongravitational forces in the gravity band and had learned that his fears were only too well founded. While the enemy could not project through the open band any forces sufficiently powerful to do any material damage, they were thus in position to forestall any move which the men of Valeron made to ward off their inexorably approaching doom.

Far beneath the surface of the ground, in a room which was not only sealed but was surrounded with every possible safeguard, nine men sat at a long table, the Bardyle at its head.

" ... and nothing can be done?" the coordinator was asking. "There is no possible way of protecting the edges of the screens?"

"None." Radnor's voice was flat, his face and body alike were eloquent of utter fatigue. He had driven himself to the point of collapse, and all his labor had proved useless. "Without solid anchorages we cannot hold them—as the ground is fused they give way. When the fused area reaches the dome the end will come. The outlets of our absorbers will also be fused, and with no possible method of dissipating the energy being continuously radiated into the dome we shall all die, practically instantaneously."

From Skylark of Valeron by E. E. "Doc" Smith (1934)

(ed. note: still more space opera)

A WAILING signal interrupted the conversation and every Vorkul in the vast fleet coiled even more tightly about his bars, for the real battle was about to begin. The city of the hexans lay before them, all her gigantic forces mustered to repel the first real invasion of her long and warlike history. Mile after mile it extended, an orderly labyrinth of spherical buildings arranged in vast interlocking series of concentric circles—a city of such size that only a small part of it was visible, even to the infra-red vision of the Vorkulians. Apparently the city was unprotected, having not even a wall. Outward from the low, rounded houses of the city's edge there reached a wide and verdant plain, which was separated from the jungle by a narrow moat of shimmering liquid—a liquid of such dire potency that across it, even those frightful growths could neither leap nor creep.

But as the Vorkulian phalanx approached—now shooting forward and upward with maximum acceleration, screaming bolts of energy flaming out for miles behind each heptagon as the full power of its generators was unleashed—it was made clear that the homeland of the hexans was far from unprotected. The verdant plain disappeared in a blast of radiance, revealing a transparent surface, through which could be seen masses of machinery filling level below level, deep into the ground as far as the eye could reach; and from the bright liquid of the girdling moat there shot vertically upward a coruscantly refulgent band of intense yellow luminescence. These were the hexan defences, heretofore invulnerable and invincible. Against them any ordinary warcraft, equipped with ordinary weapons of offense, would have been as pitifully impotent as a naked baby attacking a battleship. But now those defenses were being challenged by no ordinary craft; it had taken the mightiest intellects of Vorkulia two long lifetimes to evolve the awful engine of destruction which was hurling itself forward and upward with an already terrific and constantly increasing speed.

Onward and upward flashed the gigantic duplex cone, its entire whirling mass laced and latticed together—into one mammoth unit by green tractor beams and red pressors. These tension and compression members, of unheard-of power, made of the whole fleet of three hundred forty-three fortresses a single stupendous structure—a structure with all the strength and symmetry of a cantilever truss! Straight through that wall of yellow vibrations the vast truss drove, green walls flaming blue defiance as the absorbers overloaded; its doubly braced tip rearing upward, into and beyond the vertical as it shot through that searing yellow wall. Simultaneously from each heptagon there flamed downward a green shaft of radiance, so that the whole immense circle of the cone's mouth was one solid tractor beam, fastening upon and holding in an unbreakable grip mile upon mile of the hexan earthworks.

Practically irresistible force and supposedly immovable object! Every loose article in every heptagon had long since been stored in its individual shockproof compartment, and now every Vorkul coiled his entire body in fierce clasp about mighty horizontal bars: for the entire kinetic energy of the untold millions of tons of mass comprising the cone, at the terrific measure of its highest possible velocity, was to be hurled upon those unbreakable linkages of force which bound the trussed aggregation of Vorkulian fortresses to the deeply buried intrenchments of the hexans. The gigantic composite tractor beam snapped on and held. Inconceivably powerful as that beam was, it stretched a trifle under the incomprehensible momentum of those prodigious masses of metal, almost halted in their terrific flight. But the war-cone was not quite halted; the calculations of the Vorkulian scientists had been accurate. No possible artificial structure, and but few natural ones—in practice maneuvers entire mountains had been lifted and hurled for miles through the air—could have withstood the incredible violence of that lunging, twisting, upheaving impact. Lifted bodily by that impalpable hawser of force and cruelly wrenched and twisted by its enormous couple of angular momentum, the hexan works came up out of the ground as a waterpipe comes up in the teeth of a power shovel. The ground trembled and rocked and boulders, fragments of concrete masonry, and masses of metal flew in all directions as that city-encircling conduit of diabolical machinery was torn from its bed.

A PORTION of that conduit fully thirty miles in length was in the air, a twisted, flaming inferno of wrecked generators, exploding ammunition, and broken and short-circuited high-tension leads before the hexans could themselves cut it and thus save the remainder of their fortifications. With resounding crashes, the structure parted at the weakened points, the furious upheaval stopped and, the tractor beams shut off, the shattered, smoking, erupting mass of wreckage fell in clashing, grinding ruin upon the city.

The enormous duplex cone of the Vorkuls did not attempt to repeat the maneuver, but divided into two single cones, one of which darted toward each point of rupture. There, upon the broken and unprotected ends of the hexan cordon, their points of attack lay: theirs the task to eat along that annular fortress, no matter what the opposition might bring to bear—to channel in its place a furrow of devastation until the two cones, their work complete, should meet at the opposite edge of the city. Then what was left of the cones would separate into individual heptagons, which would so systematically blast every hexan thing into nothingness as to make certain that never again would they resume their insensate attacks upon the Vorkuls. Having counted the cost and being grimly ready to pay it, the implacable attackers hurled themselves upon their objectives.

Here were no feeble spheres of space, commanding only the limited energies transmitted to their small receptors through the ether. Instead there were all the offensive and defensive weapons developed by hundreds of generations of warrior-scientists; wielding all the incalculable power capable of being produced by the massed generators of a mighty nation. But for the breach opened in the circle by the irresistible surprise attack, they would have been invulnerable, and, hampered as they were by the defenseless ends of what should have been an endless ring, the hexans took heavy toll.

The heptagons, massive and solidly braced as they were, and anchored by tractor rays as well, shuddered and trembled throughout their mighty frames under the impact of fiercely driven pressor beams. Sullenly radiant green wall-screens flared brighter and brighter as the Vorkulian absorbers and dissipators, mighty as they were, continued more and more to overload; for there were being directed against them beams from the entire remaining circumference of the stronghold. Every deadly frequency and emanation known to the fiendish hexan intellect, backed by the full power of the city, was poured out against the invaders in sizzling shrieking bars, bands, and planes of frenzied incandescence. Nor was vibratory destruction alone. Armor-piercing projectiles of enormous size and weight were hurled—diamond-hard, drill-headed projectiles which clung and bored upon impact. High-explosive shells, canisters of gas, and the frightful aerial bombs and radio-dirigible torpedoes of highly scientific war—all were thrown with lavish hand, as fast as the projectors could be served. But thrust for thrust, ray for ray, projectile for massive projectile, the Brobdingnagian creations of the Vorkuls gave back to the hexans.

The material lining of the ghastly moat was the only substance capable of resisting the action of its contents, and now, that lining destroyed by the uprooting of the fortress, that corrosive, brilliantly mobile liquid cascaded down in to the trough and added its hellish contribution to the furious scene. For whatever that devouring fluid touched flared into yellow flame, gave off clouds of lurid, strangling vapor, and disappeared. But through yellow haze, through blasting frequencies, through clouds of poisonous gas, through rain of metal and through storm of explosive the two cones ground implacably onward, their every offensive weapon centered upon the fast-receding exposed ends of the hexan fortress. Their bombs and torpedoes ripped and tore into the structure beneath the invulnerable shield and exploded, demolishing and hurling aside like straws, the walls, projectors, hexads and vast mountains of earth. Their terrible rays bored in, softening, fusing, volatilizing metal, short-circuiting connections, destroying life far ahead of the point of attack; and, drawn along by the relentlessly creeping composite tractor beam, there progressed around the circumference of the hexan city two veritable Saturnalia of destruction—uninterrupted, cataclysmic detonations of sound and sizzling, shrieking, multi-colored displays of pyrotechnic incandescence combining to form a spectacle of violence incredible.

But the heptagons could not absorb nor radiate indefinitely those torrents of energy, and soon one greenishly incandescent screen went down. Giant shells pierced the green metal walls, giant beams of force fused and consumed them. Faster and faster the huge heptagon became a shapeless, flowing mass, its metal dripping away in flaming gouts of brilliance; then it disappeared utterly in one terrific blast as some probing enemy ray reached a vital part. The cone did not pause nor waver. Many of its component units would go down, but it would go on—and on and on until every hexan trace had disappeared or until the last Vorkulian heptagon had been annihilated.

From Spacehounds of IPC by E. E. "Doc" Smith (1931)


Planetary Attack may occur when one player has warships in a star hex which contains another player's colonized planet and that colony possesses Planetary Missile Bases (MB) and/or Advanced Missile Bases (AMB). Planetary Attack may occur only after all Ship/Ship Combat in a player's turn has ended. A colony is conquered when all of its defenses (missile bases) cease to exist and the attacker has a warship occupying that star hex. Warship barrage fire and missile base fire is considered to occur simultaneously. All warships and missile bases existing at the beginning of a Fire Turn may fire in that Fire Turn. A colony with a PFS is unconquerable. Any ship attacking a Planetary Force Screen (PFS) is automatically destroyed.

7.14 The colonist player may now announce the destruction of Industrial Units and/or Robotic Industrial Units (RIU) that are in excess of the Population/Industry ratio of one million population to one IU. The colonist player does this to prevent industrial capacity from falling into enemy hands. It is a last resort if the colonist player does not expect to re-conquer the colony in a reasonable amount of time. The colonist player may declare excess industrial capacity destroyed only if some colony missile bases still exist at this point in the Fire Turn sequence. If the colony has been conquered during this Fire Turn, as defined next, he may not exercise this option.

7.1.5 Planetary Attack on the contested colony is over for this Fire Turn if all the attacking players' warships are destroyed, or if all colony missile bases have been destroyed, and the colony conquered, or the attacking player declares the attack ended. The colony is not conquered if all remaining warships and missile bases were destroyed during the same Fire Turn. If there is more than one colonized planet in the con tested star hex, the attacking player may attack them separately or together or alternate his attacks at will. Planetary Attack is ended in the contested star hex if the above stated conditions apply to all colonies and warships in that star hex.

7.2.8 The Conqueror may destroy the population and Industrial Units of a conquered colony on a one to one basis. The population and industry destruction occurs during the Planetary Attack step of a Player Turn. Each type of warship the conquering player has in the conquered planet's star hex may destroy on the following basis:

  • For each Escort warship (ESC), 1 million population and 1 IU.
  • For each Attack warship (ATK), 3 million population and 3 IU.
  • For each Dreadnought warship (DN), 5 million population and 5 IU.

Each warship may destroy the above amounts only once during the Planetary Attack step of a Player Turn.

7.2.9 If a colonized planet's population consisting of 10 million or more colonists has been destroyed by an opponent's warships, that planet is rendered uninhabitable by any means for the rest of the game and is so marked on the Player Record Sheet. This uninhabitable Planet does not count any points toward winning, regardless of its original habitability type.

From game Stellar Conquest by Howard Thompson (1975)

Surface Defenses

In addition to large planetary forts, there may be scattered anti-spacecraft weapons sited all over the planet. The main difference is these have no real protection except being very good at hiding. Instead of armor or magic force fields, they are either one-shot sacrificial weapons or capable of frantically scuttling away after they give away their position. Or they are weaponized spacecraft launching facilities that the enemy wants or needs to capture intact so they are loath to damage it.

Because as soon as a ground (or sea) based gun opens fire on an enemy ship in orbit, the enemy is going to plaster the entire area with ortillery.

If you have sufficient stealth technology, it might be a good idea to put some planetary defensive weapons inside submarines. This made good sense back in the days of Mutual Assured Destruction, but nowadays orbital observation satellites have made it much harder for submarines to hide. Be aware though that their stealth is destroyed the instant they fire their weapons, and the attackers in orbit will lob a nuclear depth-charge that will crush the submarine like an eggshell. US Navy Ohio-class submarines carry 24 missiles, a planetary defense submarine would probably be carrying a similar amount. The PD sub would be well-advised to launch all of its missiles at once, and preferably the sub should be a remotely controlled drone.

In the 1955 Operation Wigwam test, the US military discovered that a 30 kiloton nuclear depth charge could kill a modern submarine with a radius of a bit more than a mile.

Like planetary fortresses, surface defences are at a disadvantage with respect to hostile spacecraft in orbit due to the gravity gauge.

Rick Robinson is of the opinion that the gravity gauge is not quite as one-sided as it appears. In an essay entitled Space Warfare I - The Gravity Well he makes his case. The main point is that the orbiting invading spacecraft have nowhere to hide, while the defending ground units can hide in the underbrush.

Of course it is a bit easier to inflict damage on orbital person now that lasers have been invented. Keep in mind that if the planet in question has an atmosphere similar to Terra, laser beams with wavelengths shorter than 200 nanometers are worthless for either bombarding spacecraft or planetary defenders. Such frequencies are totally absorbed by the atmosphere, this is why they are nick-named "Vacuum frequencies". The frequencies include Ultraviolet C, Extreme Ultraviolet, X-rays, and Gamma-rays.

And keep in mind that the defender's anti-orbit rocket also does not need a warhead, a bursting charge surrounded by nails and other shrapnel will do. The relative velocity between the more or less stationary cloud of shrapnel and the orbital speed of orbital person will do the rest. Orbit person will be riddled by shrapnel traveling at about 27,500 kilometers per hour (7,640 m/s) relative.

Luke Campbell

The case of attackers in orbit and defenders on the ground is not nearly so one-sided as one might think.

Remember — the attackers are in orbit, so their missiles and gun shells are also in orbit. the attacker needs to cancel most of their orbital velocity to put them on a highly elliptical orbit that will intersect the desired point on the planet. This can use up quite a bit of delta-V.

On the other side, the defender does not have to put munitions in orbit. He merely needs to loft them up on a sub-orbital hop that just happens to have the attacker's spacecraft smacking into it at orbital speeds. This does not always require much delta-V.

The closer the attacker is orbiting the planet, the harder it is for him and the easier it is for the defender to use kinetics. Distant orbits will be harder for the defender to reach and allow the attacker to de-orbit munitions easier — but it also gives the defender a lot of time to respond and makes the munitions hit the atmosphere faster (which tends to badly ablate away the munitions, making it more difficult for them to cause damage at the ground). Close orbits are likely to be death traps for the attacker — easy for the defender to launch munitions on sub-orbital intercept trajectories, not much time for the attacker to react to them, and expensive for the attacker to bring his own munitions to bear.

Isaac Kuo

Luke Campbell, there's no orbit from which it costs more delta-v to send munitions planet-ward than the delta-v required to send munitions on an intercept path. The situation is roughly symmetric, but the atmosphere reduces delta-v to go "downward" while increasing delta-v to go "upward".

Luke Campbell


In close orbit, if the orbiting spacecraft sends its munitions on a minimum energy trajectory to intercept the planet, it will take much longer to reach its target than a surface-launched missile will take to reach the spacecraft. In addition, this minimum energy missile can only hit things on the other side of the planet from where it is launched, so you will not have the bombarding spacecraft doing its own spotting. One might argue that this is not likely to be the case anyway, but it does detract from the usual trope of the orbiting spacecraft seeing its target and launching missiles at it.

Here are some details, assuming a 200 km altitude circular orbit around an airless Earth. A minimum energy orbit will have an apoapsis at the point of launch and a periapsis at the opposite side of the planet. It will take 43 minutes to reach its target, and will require about 1 km/s of delta-V. This gives plenty of time for the target to either move out of the way or shoot down the missile.

A surface launched missile will take 3.4 minutes to reach the spacecraft at a 200 km altitude, on a minimum energy boost of just under 2 km/s delta-V. Evasion or point defense may be possible within this time, but will be more difficult.

To match the performance of the surface missile, the orbiting spacecraft must cancel its orbital velocity of 7.8 km/s, and thus requires 7.8 km/s of delta-V. This is also roughly what it will take if the spacecraft is doing its own spotting and targeting.

Since ground launched missiles could reasonably be expected to have 4 to 5 km/s of delta-V even with chemfuel propellant, you are looking at the ability to track the target at something like 1 to 1.5 g of evasion acceleration, or reduce the time to intercept by launching faster.

Isaac Kuo

Luke Campbell, it's not necessary for the LEO spacecraft to launch with minimum energy. Like I said, the situation is roughly symmetrical. It can shoot a missile "directly" downward—that is, with the thruster pointed straight up.

In the rotating reference frame of the orbit, the spacecraft is shooting a missile straight downward fighting against 1 gee of centrifugal force. In this rotating reference frame, it's like shooting a suborbital missile downward.

Of course, the atmosphere in this case helps rather than hurts.

Luke Campbell


I will compute the orbital parameters later when I have a bit more time, but I will note that the situation you describe will not work on a world with a significant atmosphere. On Earth, for example, the projectile will slice through the exosphere and hit the mesosphere at a steep angle, rapidly getting to regions of air dense enough for the shock heating to incinerate the projectile while the ram pressure disintegrates it. Here, the atmosphere does not help. To get the atmosphere to help you need to enter at a shallow angle, where you can stay in the upper reaches of the mesosphere for long enough to let drag do its work without incinerating you. This would be something like the minimum energy solution I described earlier - or more likely an orbit with a periapsis at an altitude of 100 to 150 km or something similar.

Alternately, you can kill off much of your orbital velocity so the projectile enters the atmosphere at a much lower speed - similar to the method I described earlier, with the projectile dropping straight down.

For what it is worth, a projectile given 2 km/s delta-V straight down from a spacecraft in a circular 200 km altitude orbit above airless Earth will have a surface track distance of 781 km before impact, and will take 100.25 seconds for impact. It will hit with a speed of 8.275 km/s.

With an atmosphere, of course, it disintegrates long before reaching the ground.

Isaac Kuo

Luke Campbell, ICBM warheads reenter the atmosphere at steep angles at orbital speeds. This lets them reach their targets more quickly, which was considered important for trying to take out enemy launch sites before the enemy had time to react (by launching their nukes).


(ed note: Heinlein made the original prediction in 1950, revisited it in 1965, and revisited it again in 1980)

1950 The most important military fact of this century is that there is no way to repel an attack from outer space.

1965 I flatly stand by this one. True, we are now working on Nike-Zeus and Nike-X and related systems and plan to spend billions on such systems—and we know that others are doing the same thing. True, it is possible to hit an object in orbit or trajectory. Nevertheless this prediction is as safe as predicting tomorrow's sunrise. Anti-aircraft fire never stopped air attacks; it simply made them expensive. The disadvantage in being at the bottom of a deep "gravity well" is very great; gravity gauge will be as crucial in the coming years as wind gauge was in the days when sailing ships controlled empires. The nation that controls the Moon will control the Earth—but no one seems willing these days to speak that nasty fact out loud.

1980 I have just heard a convincing report that the USSR has developed lasers far better than ours that can blind our eyes-in-the-sky satellites and, presumably, destroy our ICBMs in flight. Stipulate that this rumor is true: It does not change my 1950 assertion one iota. Missiles tossed from the Moon to the Earth need not be H-bombs or any sort of bomb—or even missile-shaped. All they need be is massive . . . because they arrive at approximately seven miles per second, A laser capable of blinding a satellite and of disabling an ICBM to the point where it can't explode would need to be orders of magnitude more powerful in order to volatilize a house-size chunk of Luna. For further details see my THE MOON IS A HARSH MISTRESS.

From WHERE TO? by Robert Heinlein (1980)

Twenty-three hundred kilometers to the west at Allansport, Sergeant Sherman White slapped the keys to launch three small solid rockets. They weren't very powerful birds, but they could be set up quickly, and they had the ability to loft a hundred kilos of tiny steel cubes to a hundred and forty kilometers. White had very good information on the Confederate satellite's ephemeris; he'd observed it for its past twenty orbits.

The target was invisible over the horizon when Sergeant White launched his interceptors. As it came overhead the small rockets had climbed to meet it. Their radar fuses sought the precise moment, then they exploded in a cloud of shot that rose as it spread. It continued to climb, halted, and began to fall back toward the ground. The satellite detected the attack and beeped alarms to its masters. Then it passed through the cloud at fourteen hundred meters per second relative to the shot.

Four of the steel cubes were in its path.

From SWORD AND SCEPTER by Jerry Pournelle (1973)

Traditionally, spacecraft attacking targets on a planetary surface are assumed to have a high-ground advantage, referred to by Heinlein as the “gravity gauge”.  This assumption, like many about space warfare is wrong for several reasons.  Firstly, a spacecraft in orbit is very vulnerable to ground-launched kinetics, which only need to intercept it to do lethal damage, as described in Section 8.  Second, the ground-based defenders are able to use the clutter of the planetary surface to hide their actions, while the attackers are clearly visible.  Lastly, the planet itself offers advantages in the construction of defenses that serve as a very powerful force multiplier for the defender.

The thought experiment that underlies the gravity gauge is two men, one at the bottom of a well, the other at the top, having a fight with rocks.  The man at the top has an obvious advantage.  However, like many analogies, this one has deep flaws.  The largest is a misunderstanding of orbital mechanics.  Because of the motion of the orbital craft, any projectiles that it launches must slow down before they can leave orbit, and in low orbit, the delta-V requirement can be significantly higher than is required for a defender’s projectile to reach the attacker.  The requirement depends heavily on the geometry of the situation, but it is outside the scope of this section.  For more details, see Section 12 and Space Weapons, Earth Wars.  A warhead is unnecessary for the defender’s weapons, as the target’s orbital velocity provides all the kinetic energy required for the job.  Another issue is that the rocket necessary for this type of mission is quite small.  An R-17 Scud-B can reach a maximum altitude of approximately 150 km with a warhead of 985 kg and a launch weight of 5,900 kg, providing a marginal capability against targets in very low orbit.  Another version, the Scud-C, is capable of reaching about 275 km, with a warhead of 600 kg, and a total launch weight of 6,400 kg.  The MGM-31A Pershing has an apogee of about 370 km, a warhead of 190 kg, and a launch weight of 4,655 kg.  All of these missiles date back to the 1960s or before, but, with the proper seeker systems, should be capable of engaging targets in low orbit.  Their warheads are rather heavier than would be optimal for engaging orbiting vessels, and lighter warheads could result in somewhat higher altitudes.  For higher-orbit engagements, something like the Pershing II (altitude ~885 km, warhead 400 kg, launch weight 7,490 kg) is probably called for.  Above that, the various ICBM-type systems would take over, with apogees in the range of 5,000 km.  

Note that all of these missiles have warheads which are far heavier than are required for direct-hit kill on any practical spacecraft.  There are two ways this fact can be exploited.  First, the warhead could be replaced by another stage carrying a smaller warhead and achieving a greater altitude.  This should be good for another few hundred kilometers altitude, depending on the size of the warhead available and the previous burnout velocity.  Second, the unitary warhead could be replaced by a bursting warhead, as described in Section 8.  A detailed treatment of this concept with regards to planetary defense can be found in the Appendix to Section 12 of Physics of Space Security.

The two extant missiles that most closely approximate what would be required of a low-altitude surface-to-orbit missile (SOM) are the THAAD (Terminal High-Altitude Area Defense) and the SM-3.  The current model of THAAD, the block 4, has a launch weight of 640 kg, a warhead of approximately 40 kg, and a maximum altitude between 150 and 200 km.  Later (and presumably heavier) models could improve the maximum altitude to as much as 500 km.  The SM-3, which is currently ship-launched, has a launch weight of 1500 kg, a warhead of 23 kg, and a maximum altitude of as much as 500 km.  Later versions are reported to be capable of 1000 km, and have launch weights of approximately 2600 kg.  Both missiles use the same sensor system, which is reportedly able to acquire targets (presumably ballistic missile warheads) at ranges above 300 km.

 The above missiles are listed to demonstrate that the basic physical requirements for an SOM are quite simple, and well within the grasp of current technology.  All of the listed missiles are fired off of trucks of some sort or another (with the exception of the SM-3, which does have a fixed land-based version).  THAAD itself is launched from a vehicle the size of a semi.  If a system was designed explicitly for the SOM role, it should be very easy to conceal the missiles in trucks until the time of launch, preventing the attackers from detecting and destroying them.  Even if the attackers can see everything clearly, if the trailer is self-contained and built to look like an ordinary semi-trailer, the attacker won’t be able to tell it apart from the millions of others in use.  

Extensive tracking and control stations will be unnecessary, as the ship in question will be moving in a more-or-less predictable orbit, and the missile will have enough homing capability to compensate for the imprecision.  Orbit determination is a well-established science.  All that is needed are a few measurements of time, observer’s position, and target bearing.  These sensors are even easier to hide then the missiles themselves, as they could be as simple as a sextant at dusk or dawn.  At night, it should be possible to detect the vessel, probably through radiator glow.  During the day, it is somewhat more difficult.  This suggests that a sun-synchronous orbit might be ideal for an attacking spacecraft, as dawn and dusk occur over the poles which are (presumably) largely uninhabited.  However, the same could be said of any polar orbit, and other conditions are likely to play a large part in attack orbit selection. The advantage of a sun-synchronous orbit is that the illumination angle beneath the spacecraft is nearly constant, but for a long-period orbit, the inclination is likely to be fairly low, potentially placing dawn over an inhabited area.  Geographical conditions are as likely to dictate the orbit as astrodynamical conditions, although the astrodynamic effects of attacking a non-Earth planet should not be discounted.  In some cases a sun-synchronous orbit will place the attacker over territory that he would rather avoid.  For example, a 24-hour sun-synchronous attack orbit aimed at a target in North America will spend a large amount of time over South America, a situation that is hardly idea.  For optimal results, attacks would be made in daylight, which gives the best conditions for the attacker’s sensors and the worst for those of the defender.

The obvious counter to the visibility of spacecraft is for the spacecraft to maneuver regularly, hopefully spoiling any shots the defender may take.  A burn of approximately 3 m/s in the prograde or retrograde direction in a 150 km Earth orbit will change the period of the orbit by 2 seconds and the semi-major axis by about 5 km.  What this means is that the spacecraft will arrive on the opposite side of the orbit either a second late or early respectively, and will be either 10 km above or 10 km below where it was supposed to be depending on which direction the burn was made in.  However, this is unlikely to be enough to spoil the attack.  If the missile seeker locks on 30 seconds out, a 330 m/s delta-V would be sufficient to compensate for the divergence, and it is quite likely that SOMs will be designed to frustrate such tactics.  So long as the change remains relatively small, the results above can be linearized, with a 12 m/s delta-V producing a 4-second change in arrival time, and a 40 km change in altitude.  Note that the divergence in position only occurs in the orbital plane.  The plane itself can be changed by a burn half an orbit away, but the spacecraft will still pass through a point opposite the location of the burn.  For out-of-plane dodging, it is best to burn a quarter-orbit away (three-quarters of an orbit will produce identical results).  Moving the ground-track 10 km in our 150 km Earth reference orbit will require 12.25 m/s of delta-V, significantly more than an equivalent amount of in-plane dodging.  To a first approximation, the dodge delta-V for a quarter-orbit burn will be approximately twice that required for a half-orbit burn.  All of this assumes the initial orbit is circular, and the delta-V is fairly small.  Finding values for larger burns will require elaborate computations, which are beyond the scope of available data.  The requirements for a given amount of miss distance will be somewhat lower at higher altitudes, but this must be balanced against the fact that at higher altitudes, the missile will probably have significantly more time between lock-on and impact, reducing the delta-V required to compensate.

Of course, this does not address the practicality of using regular maneuvers to frustrate missile attacks.  For a ship with a high-thrust drive, the limiting factor is delta-V, and this dodging scheme will require something like 0.4 m/s/km/hr. for half-orbit burns, or 0.8 m/s/km/hr. for quarter-orbit burns.  Over the long term, this would add up, needing 10 to 20 m/s/km/day.  This is vaguely practical for small miss generation, but small miss generation is easily compensated for by the missile.  Even a minimal estimate of a required miss distance of 10 km would need 100 to 200 m/s/day, which will get expensive if the siege drags on more than a few days. Low-thrust systems might be more effective, although the achievable miss distance will be limited by the acceleration of the spacecraft.

The exact altitude requirements for an SOM are actually quite difficult to figure out.  A missile will only be able to attack a target at its maximum altitude if the target in question passes directly over the launch site.  All of the numbers posted above are estimated maximum altitudes, and in practice the maximum altitudes will be some fraction of those listed.  The one use of the SM-3 for ASAT purposes was at an intercept altitude of about 250 km, and used an early model, putting the interception at about half of the theoretical maximum.  The missile used in the Chinese ASAT test has a theoretical altitude of somewhere between 1350 km and 1500 km, and was used against a target at an altitude of approximately 860 km.  All of these indicate that the maximum practical altitude for a missile is probably not much more than half of the maximum theoretically achievable, though 75% might be possible for a battery positioned close to an important target, where the enemy will pass almost directly overhead.  Air-launched ASAT systems, such as the ASM-135, are theoretically capable of achieving much more nearly 100% due to better positioning of the launching platform, although the only known air-launched ASAT test, the ASM-135 shot at the Solwind P78-1, occurred at an altitude of 555 km out of an apogee altitude of 1000 km.  The question then becomes what sort of altitudes will be required of an SOM system.  The ISS orbits at approximately 400 km, while most recon satellites orbit between 250 and 600 km.  These put the requirements clearly into the SM-3 category.  Seapower and Space contained an interesting note on ASAT envelopes.  The Thor of Program 437 was apparently capable of engaging targets at 200 nm (370 km) at slant rages of up to 1,500 nm (2,778 km) (and higher targets at shorter ranges), while the Nike-Zeus was demonstrated up to 150 nm (278 km).  Encyclopedia Astronautica credits the Thor in question with an apogee of 500 km, and the Nike-Zeus with somewhere between 200 and 280 km, depending on the variant.  It therefore seems prudent to assume that the altitude given for Nike-Zeus was in fact the maximum altitude the weapon could reach.

Another factor controlling the altitude requirements of missiles is the necessity to hold down flight time.  Table 2 gives values for times of flight and view times for missiles fired at spacecraft at various altitudes, with the missiles having an apogee equal to the spacecraft altitude.  The missiles were assumed to be ballistic throughout, which is not a good assumption, but one that must be accepted for purposes of analysis.  Clear view was assumed to begin at 75 km, to account for the fact that defensive fire and sensors may not be fully effective through the atmosphere.  In this case, view time and rise time are very similar, and neither is likely to be strictly dominant.  The fact that view time is normally very close to rise time actually means that given the slowing a missile would experience during passage through the atmosphere, the target might not be able to see it during its burn, or would only be able to see it through a great deal of atmosphere.  If the missile is relatively stealthy during the unpowered portion of the ascent, the spacecraft might not have a good track until it is quite close.

Table 2
Altitude (km)1001502002503005007501000
Rise Time (sec)142.8174.9201.9225.8247.3319.3391.0451.5
View Range (km)1,122.11,369.91,576.81,757.31,919.02,447.02,952.13,359.4
View Time (sec)145.3179.4208.9235.5260.1346.6441.2528.7
Clear Rise Time (sec)71.4123.7159.6188.9214.2294.4371.0434.3
Clear View Range (km)567.1979.11,260.01,486.21,679.92,280.42,831.03,266.1
Clear View Time (sec)72.6126.8165.0196.8225.0319.3418.2508.1

It is obvious that even at the lowest altitudes, the missiles are vulnerable for a considerable period before impact. The obvious solution is to fire a missile that has an apogee considerably above the altitude of the target, minimizing this vulnerability. Table 3 shows the effects of apogee above that of the target.

Table 3
Altitude (km)100100100150150150200200
Missile Apogee (km)150200250200300500400800
Rise Time (sec)73.959.150.9101.072.452.283.654.1
View Range (km)1,122.11,122.11,122.11,369.91,369.91,369.91,576.81,576.8
View Time (sec)145.3145.3145.3179.4179.4179.4208.9208.9
Clear Rise Time (sec)22.716.914.058.739.327.255.534.7
Clear View Range (km)567.1567.1567.1979.1979.1979.11,260.01,260.0
Clear View Time (sec)72.372.372.3125.3125.3125.3161.9161.9
Vertical Velocity (km/s)0.9901.4011.7160.9901.7162.6201.9813.431
Impact Energy Factor1.
Altitude (km)30030050050075075010001000
Missile Apogee (km)40060075010001000150015002500
Rise Time (sec)142.8102.4165.3132.2225.8162.0233.7160.9
View Range (km)1,919.01,919.02,447.02,447.02,952.12,952.13,359.43,359.4
View Time (sec)260.1260.1346.6346.6441.2441.2528.7528.7
Clear Rise Time (sec)114.679.9145.2115.0208.5148.0219.7150.1
Clear View Range (km)1,679.91,679.92,280.42,280.42,831.02,831.03,266.13,266.1
Clear View Time (sec)217.4217.4299.6299.6378.6378.6444.4444.4
Vertical Velocity (km/s)1.4012.4262.2153.1322.2153.8363.1325.425
Impact Energy Factor1.

In most cases, the rise times and particularly clear rise times have been dramatically reduced, meaning shorter engagement times for the target.  The vertical velocity at impact will also increase the damage the warhead does (although the impact energy is not increased significantly unless the excess apogee is very large).  The impact energy factor is the KE of the warhead with the vertical velocity divided by the KE the warhead would have if it were stationary in front of the target.  The biggest drawback is that it is likely to make the missile and launch site easier for the target to locate.  This may not be a major concern if the attacker has a large number of deployed sensors, which could accurately locate the launch site and ascending missile no matter when it is fired.  Another potential problem is that it obviously requires a significantly larger missile to engage a given target with a given warhead.

ICBM-class weapons are less likely to be useful, due to the longer flight times involved.  This gives the target significantly more time to dodge the missile or shoot it down, moving the warhead into the realms described in Section 8.  The size of the weapon is also a serious hindrance to its operational use.  Even the Midgetman mobile ICBM’s launcher was an incredibly large vehicle, which would make it difficult to camouflage as a civilian vehicle.  Even if it could be successfully camouflaged, the number of vehicles of such size is relatively small, and it might be possible to simply destroy all of them.  A more plausible alternative would be to use immobile camouflaged silos.

Other launch platforms are possible as well.  THAAD is somewhat smaller than a BGM-109 Tomahawk cruise missile, which is launched from a variety of platforms, including submarines.  Early SM-3s are of a very similar size and shape to the Tomahawk.  Submarines have the advantage of being able to hide and maneuver in the sea, and are quite difficult to attack from orbit, even if an initial location is known.  The Ohio-class ballistic/guided missile submarines make excellent candidates for this analysis.  Originally built with 24 tubes for the Trident missile, four of them have been modified since the end of the Cold War to carry 7 Tomahawks each in 22 of those tubes, the other 2 being reserved for special operations equipment.  With a dedicated SOM submarine, it would likely be possible to switch out THAAD-class missiles, SM-3-class missiles, and ICBM-class missiles at the dock, giving the vessel capability against various types of targets.  

The single largest issue with submarine-based missiles is targeting.  A submerged submarine obviously cannot use most sensors, and it is unlikely that it will be capable of independently targeting, launching, submerging, and escaping, all before it is destroyed, either by nuclear depth charge or homing torpedo.  Transmissions to submerged submarines are usually made on the ELF (Extremely Low Frequency) and VLF (Very Low Frequency) bands.  The practical issues are the large size of the antennas required to transmit the signals, and the low bandwidth (a few minutes per character to a few characters per second).  The low bandwidth renders it virtually impossible to transmit the targeting data to a submerged submarine, while the size of the antenna sites makes them very vulnerable to attack from orbit.  It might be possible to harden one of these sites, as the US Navy proposed to do with Project Sanguine, or to use an airborne transmitter, such as the E-6B Mercury.  Both present practical difficulties.  The E-6 must orbit such that the trailing antenna is near vertical, while the expense of hardening is considerable, and can be defeated with a sufficiently large number of hits.  In both cases, the problems of bandwidth still remain.  The VLF/ELF systems are usually used to order the submarine to the surface for further orders.  That remains the most likely solution, but hardly the only one.  VLF communication might be able to provide rough orbit parameters, and a sufficiently advanced guidance/sensor system would be able to take that information and home in independently.  Another option is to make a burst transmission to the missiles as they clear the water.  This has the advantage of not requiring the submarine to come close to the surface. Coming to the surface (which is not the same thing as surfacing) is quite likely anyway, given that most submarine-launched missiles are fired at periscope depth, around 18 m (depth of keel).  The Tridents on the Ohio, however, can be fired from at least 40 m.

The effectiveness of the entire submarine-based system assumes that, as is the current situation, it is very difficult to detect submarines from orbit unless they are very close to or on the surface.  This may be changing, most likely due to blue-green lidar, which has been reported to have depth capabilities of 200 m.  The US has used similar systems to detect mines, starting with the Kaman Magic Lantern of the mid-90s, and continuing to the current AN/AES-1 Airborne Laser Mine Detection System (ALMDS).  A system of that type would significantly hinder if not defeat the operation of SOM-carrying submarines.  However, recent blue-green lidar systems have proven ineffective at finding submarines, due to the required dwell time.  They are excellent for searching a confined area for targets that do not move, but less effective as a wide-area search sensor.  

Nor is lidar the only option for orbital detection of submarines.  There have been rumors about programs involving the use of orbital radar platforms to detect submarines since the early demise of Seasat, which many allege was because it was detecting US submarines.  In theory, submarines produce several distinct features on the surface, including a Bernoulli Hump (a bulge in the sea surface) and a Kelvin Wake with a characteristic angle that distinguishes it from that of surface ships.  It also changes the surface wave spectrum, an effect the Soviets attempted to detect with a laser shortly before the end of the Cold War, along with other attempts involving detecting changes in the ocean structure as a result of the submarine’s passage.

A submarine should produce a detectable thermal wake, both because of the onboard heat and because of the disturbance in the ocean’s structure.  The Soviets attempted energetically to exploit this effect, but their IR detectors proved best suited to distinguishing between land and water.  Another possibility is the detection of the chemical wake, either the chemicals that come from the submarine’s hull or possible transmutation products produced by the radiation from the submarine’s reactor.  Attempts were even made to detect the electromagnetic effects caused by the submarine and its wake.  This involved using a laser to detect certain changes in atomic structure that should be caused by the submarine.  Bioluminescence was also investigated, but absorption of light by water appears to have frustrated this in most places.  There are, however, a few places where it is reportedly an effective means of submarine detection.

Unclassified accounts indicate that all of the concepts have been difficult to put into practice, because the signals are very weak unless the submarine is moving very fast very close to the surface, and because there are lots of objects that tend to produce signatures similar to submarines.  In theory, increased computational power and improved sensors should make detecting these features easier, but improved knowledge of the oceans will also be required.  This might be a problem when working with different planets.  The author is not an oceanographer, and does not know how much of the knowledge will be generalizable to other planets, and thus available to an invader, and how much will not. 1

If nonacoustic methods are infeasible, then the attacker must fall back on the old standby, sonar.  This would probably involve the use of what are essentially very large passive sonobuoys, which listen for submarines, and report back to the ships in orbit.  It might be wise to give them some mobility and the ability to submerge temporarily as well.  They would obviously have to run the gauntlet of the existing defenses to make it down, but once down, they would be extraordinarily difficult for the defender to deal with.  Provided that they landed a reasonable distance away from any defenders, they would have to be hunted like mines, and minesweeping in the open ocean is nearly impossible.  (Minelaying in the open ocean is nearly futile, so this is not something the Navy spends a lot of time worrying about).  How effective such a system would be is a matter of conjecture, made worse by the fact that anything to do with sonar performance is highly classified.

As depth increases, launching missiles becomes more difficult, and the communications problems increase.  A towed buoy would solve the communications problem, but it also runs the risk of revealing the submarine’s position.  There are several systems currently in service that use this principle, but all of them impose serious limitations on the depth and speed of the submarine, and most are intended to communicate with satellites, a possibility not available to the defender in this scenario.

In fact, the lack of satellite communications for the defender raises a serious problem.  Direction-finding on radio traffic was and is a major concern for military forces the world over, particularly navies.  One of the solutions to this has been the use of satellite communications, because the uplink from the ground to the satellite is very difficult to direction-find unless a satellite is directly in the uplink beam.  The downlink can be intercepted, but the satellite can be detected by other means as well, and a sufficiently wide beam means that the intercept gives no information on the location of the recipient.  With this capability denied, the defender would be forced to return to older means of communication, which are less reliable, slower, and vulnerable to direction-finding.  Obviously, the use of wired communications would eliminate this vulnerability, but that imposes restrictions on the location of the units, and is totally unsuitable for submarines.

One solution to the communications issues proposed today is a blue-green laser on a satellite.  The problem with that solution is twofold.  First, the defender can be assumed to no longer have any satellites.  Secondly, the defender must be tracking the submarine to a fair degree of accuracy, which is very difficult by definition, and any steps taken to make it easier would probably also make it easier for the attacker to detect the submarine.  It might be possible to avoid this problem by limiting the amount of information transmitted by the laser, and sweeping it over a vast area of the sea instead, to ensure that the target submarine receives it.  While the laser could be mounted on an airplane, communicating with a submarine by that method could give away the submarine’s general location.

Another option is the perennial darling of submarine communications, sonar.  There have been dozens of attempts over the years to use sonar to allow submarines to talk like surface ships.  All have failed for a variety of reasons, including limited range or bandwidth, and multipath scrambling, although the biggest problem has always been that a submarine is inherently stealthy, and announcing its presence to communicate defeats this.  It has been suggested that computers can deal with the multipath problem, and careful system design might allow adequate bandwidth.  The link can probably be made one-way, removing the problem of the submarine announcing its presence.  For that matter, if the attacker has not constructed a sonar net on the planet (as described above), the submarine could talk back without fear of being detected.  This alone might be a reason to deploy some form of sonar system, even if it is not capable of locating the submarines passively.

Attacking a submarine from orbit is likely to be just as difficult as finding it.  Proposed options for this task include homing torpedoes, nuclear depth charges, and dropping minisubmarines.  All of these weapons have issues.  Homing torpedoes suffer from short ranges, somewhere under 15 km for modern air-launched torpedoes.  At a submarine speed of 30 knots, from detection, it will take the vessel a little over 15 minutes to clear that radius.  The minimum time for a kinetic weapon drop, per Space Weapons, Earth Wars, is 12 minutes, although this requires between 40 and 150 satellites for constant worldwide coverage.  This is not as big of a problem as it seems at first.  Submarines are only likely to be detected when a ship is overhead, and the 12-minute time is for a projectile dropped straight down (which does require a large amount of delta-V).  The actual practical range of the homing torpedo is likely to be considerably shorter, as it has to acquire the target and chase it down.  This might also be less of a problem than it appears on the surface, as the projectile would probably be able to be steered after it is dropped.  While the projectile will be blinded by plasma for long periods during the drop (see Section 12), it must slow down to enter the water, giving a window during which it can receive commands.  The logical extension of this idea is fitting the torpedo into a miniature UAV, remotely steered onto the target in a manner similar to the Australian Ikara system.  This assumes, of course, that the target is still in sight, which depends on the altitude of the launching spacecraft and the technology used to detect the submarine.  While the physical range of the torpedo might be improved by advances in technology, the difficulty of the torpedo’s own seeker acquiring a target is unlikely to decrease by a significant amount.  Nuclear depth charges have radii that are likely to be on the order of 10 km, which means that the attacking spacecraft has to be in low orbit for them to reach the target in time to be effective, or the above-mentioned mini-UAV must be used.  Dropping a manned minisubmarine requires a fairly large gap in the defenses, and once it is in the water, it must deal with defensive submarines.  UUVs commanded by blue-green lasers are a better option, although they would likely suffer from limited armament and the possibility of being killed by the defender.  Both of these can be dealt with by making the UUV expendable, which would also eliminate the need for a nuclear power plant.  At the extreme, an expendable UUV would look quite similar to a long-range torpedo taking command guidance from orbit.  Some combination of those and orbital weapons would be the best way to deal with the submarine problem.

One practical issue with submarines is deployment time.  Modern US SSBNs patrol for 90 days at a time, and this seems to be a fairly hard limit based on human factors.  It might be stretched slightly in wartime, but submarine bases would be a priority target for any attacker.  On the other hand, it is also possible that the human factors issues will have been solved due to the demands of long-term spaceflight, which has many similarities to submarine operations.  Other operational issues would then limit the deployment time, such as food (although this could probably be resupplied by boat when there is cloud cover) and maintenance (which is the ultimate limiter in any case).

Another major option for planetary defense is lasers.  These lasers differ from those for deep-space use, both in the fact that they do not have to deal with the weight and heat restrictions of space-based systems, but they (and any bombardment lasers) must be of wavelengths that can penetrate the atmosphere.  This limits the range that said lasers can achieve due to diffraction.  Adaptive optics and other techniques can compensate for most of the various phenomena that occur when a high-powered laser is fired through an atmosphere, as can siting the laser at high altitude.  The largest weakness of ground-based lasers is that they are immobile, and thus can be targeted by high-velocity kinetics.  This is compounded by the fact that when a laser is fired it immediately reveals its position to the target.  The attacker can then pull back to an orbit out of reach of the laser and bombard it at his leisure.  

There are numerous factors involved in determining the viability of such an installation, including the vulnerability of such installations to bombardment, the effectiveness of the laser, the cost of the laser, and the difficulty of intercepting the bombardment projectiles.  The first is a difficult question to answer.  How effective is a deep bunker against kinetic bombardment?  While the projectile is unlikely to penetrate deep enough to be a threat to a Cheyenne Mountain-type installation (unless the projectile is very large), the shock wave from the impact could damage the laser machinery.  Shock mounting might mitigate this, although a full treatment of such matters is outside the scope of this discussion.  However, the main mirrors themselves must be located near the surface, and would be the points attacked anyway.  It would be entirely feasible to have one generator feeding multiple mirrors, but that tactic is unlikely to be used unless the mirror in question costs significantly less than the generator.  Such a ratio is significantly below the theoretically optimum ratio for mirrors and generators, as shown in Section 7.  The effectiveness of the laser is another question.  It has been suggested that a ground-based laser might be capable of attacking targets as far out as geosynchronous orbit, and could also be used to detect incoming kinetics, giving the laser as much as 12 hours to attack them.  If this is the case (which assumes a 10 meter mirror) the laser system might be intended for use in the defense of the higher orbits, the lower orbits being defended by missiles.

There is also the potential for submarine-based lasers.  It is theoretically possible to create a laser that can be mounted and fired from a submarine, probably using some combination of superhydrophobic surfaces and high-strength windows in front of the mirror that can take the shock of water on them being vaporized.  The problem is that the submarine itself does not make a good laser platform.  Modern submarines are optimized for underwater operation, which tends to mean poor stability on the surface, and mounting the mirror is not a trivial task when one remembers that the submarine as a whole has to be waterproof.  However, such a submarine is not entirely unprecedented.  The USS Triton (SSRN-586) was designed as a radar picket, and built to perform well on the surface.  This had significant drawbacks, most notably in making the submarine very noisy underwater.  On one hand, Triton was designed before the beginning of serious emphasis on submarine silencing.  On the other, a large portion of the noise problem is likely to be inherent in the hull form required for surfaced performance.  On the gripping hand, sonar detection is likely to be somewhat less important in planetary defense.

A laser launch system would also serve as an effective planetary defense station, provided with the proper targeting systems.  The drawback is that the laser itself is in a known location, denying it the element of surprise even for its first shot. Depending on the geometry of a planetary invasion (discussed in section 12) it might or might not be capable of firing on incoming enemies before it is destroyed.

Other means of intercepting the bombardment projectiles have been proposed, as well.  Most of these rely on the fact that a kinetic projectile is vulnerable to disruption during its entry into the atmosphere.  These proposals have ranged from nearby explosions to barrage balloons to some form of hit-to-kill CIWS.  All would disrupt the projectile enough for it to disintegrate, dumping almost all of its kinetic energy into the atmosphere.  The presence of effective defenses of some sort would greatly reduce the vulnerability of ground targets, particularly dug-in ones.  A similar concept was the ‘Dust Defense’ proposed during SDI, which involved using buried nuclear weapons to throw dust high into the atmosphere to destroy incoming warheads.  However, only limited information on the concept is available, precluding further analysis.  

A potential use for smaller, portable lasers is a dazzle system.  Smaller, lower-powered lasers are used to blind the attackers, allowing the defender to escape observation for a short time.  However, this is easily defeated by the use of multiple networked sensors, some of them on small, unmanned satellites that are essentially impossible to detect passively from the ground.  In some ways the best use of such lasers might be as a distraction from something important going on elsewhere.  Both optics and processing power make it impossible to monitor an entire hemisphere in high detail and in real time.

The last option the defender has is cannon of some kind.  When first proposed, this solution was questioned, as firing a cannon up a couple hundred kilometers runs into the problem of firing through the atmosphere.  It was later realized that Project HARP had done exactly that in the early 1960s.  Using a modified 16-inch gun, sub-caliber projectiles were fired to altitudes of up to 180 kilometers.  Obviously the HARP launcher would be unsuited to planetary defense roles, but it has been proved possible to fire ballistic projectiles from sea level (the HARP test site was on a beach in Bermuda) to significant altitudes.  However, these altitudes alone are insufficient to reach a target in most orbits.  The muzzle velocity for the high-altitude tests was approximately 2100 m/s.  This can be compared to 2500 m/s for the Navy’s railgun project.  For comparison, the early models of the SM-3 had a delta-V of about 4 km/s, while the later models are about 6 km/s.  If increases in velocity due to a switch to electromagnetic launching prove insufficient, then there is the option of using a rocket-boosted projectile.  This would require significantly less delta-V than a conventional rocket, preserving many of the advantages of the purely ballistic system.

Ballistic defense shares advantages and disadvantages with both lasers and missiles.  Any installation will almost certainly be fixed, as it requires a long barrel, though advanced coil/railguns might not have to be.  However, unlike lasers, a ballistic system does not by definition give away its position with each shot.  It is likely that the enemy could spot the muzzle flash if a chemical cannon is used, but railguns and coilguns do not have this problem.  The projectile would have to be guided, but it is possible to acceleration-harden a projectile, and aerodynamic effects could be used for minor course changes while low in the atmosphere, reducing required delta-V and preventing backtracking to the launch site.  At the same time, intense surveillance and intelligence efforts could probably locate the launch site eventually, and unlike lasers, all of the machinery must be close to the surface.

One advantage of cannons over missiles is that the projectile is much harder to detect during the climb.  The projectile lacks the exhaust signature of the missile, and is also smaller, both contributing to lower detection ranges and engagement times.  Also, it can be presumed that shells are cheaper than missiles. There have been some real-world investigations of electromagnetic suborbital launch systems, most notably by the ESA 2. Their investigation concluded that it would indeed be possible to use a railgun to replace sounding rockets, firing a 3 kg payload through a 22 m barrel at a velocity of 2,158 m/s.  The maximum altitude of the system was expected to be 120 km.  While this is a bit lower than would be necessary in a planetary-defense system, it does show the feasibility of such a system, and there is even the potential that it could be truck-mounted.  The largest problem with such a mounting would probably be power, although ultracapacitors could be used to store and transport power generated by deeply-buried reactors to the launch trucks.

In the absence of effective laser bombardment capability, aircraft become a viable defensive platform.  They are nearly impossible to target with kinetics, although some form of autonomous antiaircraft missile might be effective. The use of aircraft for planetary defense has some precedent.  The US ASM-135 ASAT missile was air launched, and had a ceiling of approximately 560 km.  The greatest advantage of air launch is that the launch platform can rapidly move to cover a vulnerable area.  The greatest disadvantage is the facilities required to base a conventional aircraft, which are immobile and vulnerable to bombardment.  VTOL aircraft would make this more practical, but the support facilities (and landed aircraft) would still be capable of being targeted.  However, it might be possible to use point defenses to secure an aircraft base, and deploy the aircraft as mobile missile platforms at need.

Lasers could also be mounted on aircraft, much like the YAL-1.  Aerodynamic limitations on the size of the mirror make it doubtful that an aircraft could successfully duel a spacecraft, and it is hard to see a set of technical assumptions under which aircraft-mounted lasers are practical but spacecraft-mounted ones are not.  Among other things, the physical environment of an aircraft is rather less well-suited to precise control of a laser than is a properly-designed spacecraft.  The aerodynamic forces on the aircraft will tend to produce vibration, which is undesirable when using lasers, and absent in spacecraft.  Crew, fluids, and thrust will also contribute, and are likely to be larger in magnitude than those found on spacecraft.  The atmosphere does provide a slight advantage in terms of cooling, and the fact that an aircraft can be presumed to be operating near a base increases the practicality of chemical lasers.  On the other hand, aircraft can successfully use clouds to protect themselves against lasers, which require gigawatt levels of power to burn through fast enough to track an aircraft.  

        While not technically surface defenses in the conventional sense, fortifications on moons could be vital for planetary defense.  Luna is a bit far out from earth for it to make a really effective fortress, but Phobos and Deimos would make excellent bases for large lasers.  The mass of the moon gives lots of places to dump vibration and heat to, and Phobos orbits in 7 hours 40 minutes, while Deimos takes 30.3 hours.  Even Luna could be strategically important, depending on the scenario.  Ignoring possible infrastructure present on Luna that would make it worth defending in its own right, there are several reasons that a defender would desire to deny it to an attacker.

        The most likely reason to land on Luna would be remass, although the practicality of that depends on the remass used by the fleet.  That in turn depends on the type of thruster used.  The standard cases used throughout this paper are chemfuel, nuclear-thermal, and electric of some kind.  Availability of remass for chemfuel and nuclear-thermal engines obviously depends upon the type of remass.  Some chemful mixtures, like aluminum-oxygen, are readily available anywhere on the lunar surface.  Others require much scarcer and more valuable elements, particularly hydrogen.  While the LCROSS mission did confirm the presence of large amounts of water at the poles, this water is likely to be too valuable for life-support purposes to be used as remass feedstock during normal times.  A potential attacker, however, might not care.  An NTR can theoretically use just about anything as remass, with exhaust velocity varying based on temperature, it is incredibly difficult to build one that will run with both oxidizing remass, such as oxygen, carbon dioxide and water, and reducing remass, such as hydrogen, ammonia, and methane (See Section 14 for more details on this).  Of these, only oxygen is truly readily available from lunar sources.  While there is water, the quantity is limited enough that using it for remass is questionable.  Also, the high molecular mass of the water makes it a less-than-ideal candidate for NTR usage.

        Electric thrusters are less likely to be able to get remass from Lunar sources (due to lack of information about both thruster propellants and body composition, the author refuses to speculate about other celestial bodies).  On the other hand, electric thrusters have much higher exhaust velocities, so less remass in total is required for a campaign.  In fact, the availability of a given remass is likely to play a significant factor in its selection for use on a vessel.  Most modern Hall Thrusters and other ion thrusters use Xenon for remass.  While Xenon is basically ideal for use as remass, it is far too rare to support the level of interplanetary trade that would be a prerequisite for any sort of serious war.  Krypton is the next best choice, but it is also too rare.  Argon is less effective, but probably the best among the noble gasses from an operational and engineering standpoint.  Some early ion thrusters were tested with Cadmium and Mercury, but both of these have had serious operational issues during tests, and are not notably abundant on or off Earth.  Possibly the best option is colloidal thrusters.  These use some form of hydrocarbon fuel, which has the advantage of being no less abundant than the other options throughout the solar system, and significantly more abundant on Earth.  However, the technical advantages of one of the other designs might well outweigh the logistical ones of the colloidal thruster, and the author does not know enough about the issue to be sure one way or the other.

1 Seapower and Space by Norman Friedman provided most of the information on attempts to detect submarines from space, along with information on the importance of satellite communications. It also pointed out that some stories of US nonacoustic detection might have been the result of deceptions intended to trick the Soviets into spending money in an attempt to match them."

2 Electromagnetic Railgun Technology for the Deployment of Small Sub-Orbital Payloads.

by Byron Coffey (2016)

Laser-equipped nuclear-powered submarines are the perfect last line of defense against an attacking force in orbit.

The situation

You don't win every fight. Eventually, there will come a time in space warfare where a fleet of space warships has defeated all your mobile forces and your immobile defenses. They will bear down on you from above with lasers, missiles and kinetic projectiles and you will have to find a way to prevent their forcing of an unconditional surrender.

We will refer to the opponents as the 'attackers' and to you as the 'defenders'. The first step to devising an effective defense is to understand the situation.

So what is the situation?

An attacking warship will start out in high orbit. This is an altitude of 2000km or above. Whether it has just arrived from an interplanetary voyage or has recently destroyed your remaining warships, it will go to high orbit to maximize the effectiveness of its space superiority. Space superiority, borrowing from the term 'air superiority', is when a force has complete dominance over all the orbits around a planet. No space-borne forces can oppose this superiority and no reserve forces can challenge them without being quickly destroyed.

What does losing space superiority mean for defenders?

The most important consequence is that enemy warships have free reign to change orbits, maneuver into favorable positions and receive re-supplies. Their mobility is unconstrained.

Attackers in high orbit can make optimal use of their laser weaponry. They can get clear lines of sight onto any spot on the surface, and the long distances between objects forces travel times to length with the secondary effect of giving lasers plenty of time to shoot down targets. Laser effectiveness is generally dependent on how far they are from a target and how much 'dwell time' a beam can spend on a target.

De-orbiting objects from high altitude is inexpensive in terms of deltaV. This works in favour of missiles sent down from orbit by allowing them to use very little propellant to strike ground targets, which makes them lightweight and cheap to send by the hundreds. Additionally, falling towards Earth gives them a big boost to the velocities they achieve before impact.

The same applies to kinetic projectiles, a fact applied in the Rods From God concept of orbital bombardment.


So you want to shoot back at the attackers.

Missiles can do the job. They are currently our only method of delivering weapons into orbital space. Something like an ICBM with an additional stage can reach LEO. Reaching higher orbits will require either a very large rocket, a high Isp engine for the upper stage or a launch system such a laser launch or ram accelerator.

However, each of these solutions have major issues when trying to shoot down an opponent in high orbit.

Large rockets are easy to target and shoot down. A chemical-propellant rocket that needs to minimize its dry mass to achieve the necessary deltaV capacity will have very good acceleration but will end up being very fragile. Nuclear-thermal or nuclear-powered rockets can be much smaller and more durable, but getting sufficient acceleration out of them implies a very high power requirement, which might make them very expensive to throw at the enemy.

Regular launches take tens of minutes and cannot be disguised from the attackers. Shortening this window of vulnerability can be done by using a launch system that powers the rocket or accelerates it externally. However, the infrastructure for the launch systems will in turn become a priority target for the attackers. Massive, hard to hide and immobile, they will receive a lot of firepower. Some launch systems are practically impossible to shield from damage, such as a laser launch system that needs thousands of exposed laser optics, and others reveal their positions as soon as they fire a rocket. Building underground is also a very expensive endeavour when considering that all the work can be undone by a single 'bunker buster'-type weapon.

The logistics of launching missiles against attackers sitting in orbit works against the defenders. The attackers can de-orbit a missile by expending only a few tens to hundreds of meters per second of deltaV. A defender must equip each missile with tens of thousands of meters per second of deltaV. It might be easier to build more missiles and create rocket fuels on the ground at the start of the war, but after an orbital bombardment campaign by attackers with space superiority, it is unlikely to be the case.

Kinetics that can be shot all the way to high orbit need to handle hundreds to thousands of Gs of acceleration, traverse the lower atmosphere at dozens of kilometers per second, survive laser fire for several minutes with minimal capacity to dodge and take out a target with a very short window of interception. This is a tall order!

So, what are the defender's options?

They need to retaliate with something that cannot be shot down, from a platform that can avoid counter-fire and can maintain functionality after infrastructure and services have been disabled world-wide.

One option that fits the bill is laser submarines.

Lasers cannot be shot down and hit their target instantly. They can be used so long as electrical power is supplied. Submarines operate underwater, an environment that can hide them until they surface and protects them from high-velocity projectiles and lasers while submerged. The can protect themselves this way for months on end, and if they employ the same life support systems as on spaceships, then it can add up to years.

For the same reason that today's submarine fleets are considered an unbeatable means of retaliating against a foe after nuclear armageddon has wiped out the homeland, laser submarines will be able to operate and remain dangerous even after orbital attacks destroy all support infrastructure.

Let us now look at how a submarine can be used to retaliate against attackers in orbit.

The Challenges

Submarines are already equipped with a high electrical power generation. Large modern nuclear submarines are already able to produce over 100MW for years on end. In a futuristic setting with common space travel and space wars, power generation technology developed for interplanetary travel will allow submarines to produce gigawatts or more.

The most likely generators for space travel will be nuclear due to their high power density. The biggest limitation to generating power from nuclear reactors is waste heat capacity: It is easy to heat up the reactor core but much harder to remove the heat. Submarines will have an entire ocean as a heatsink so will be able to produce more watts compared to a spacecraft with a reactor of the same mass and volume.

All of this electrical power can be used to power a laser generator.

Three elements determine a laser's effectiveness: wavelength, radius of focusing optics and beam power.

We have already determined that laser submarines will likely be able to produce more electrical power than a similar laser space warship, so laser submarines will also have the advantage in beam power.

The radius of the focusing optics will depend on the specific arrangement of the laser weapon's components and how they are deployed. We will look into the possible designs down below.

The wavelength however is not a variable laser designs have much control over. Submarines operate in an aquatic environment, on top of which is a hundred kilometers of Earth's atmosphere. At the interface of the ocean's surface is sea mist and suspended droplets of water in fog or clouds. The ocean's surface is not flat either, with waves of a few centimeters to a few hundred meters rolling over it endlessly. A beam emitted by a submarine will have to penetrate all of this environment and still travel the hundreds of thousands of kilometers' distance separating it from a target in high orbit.

The optical properties of water are therefore the determinant factor for which wavelengths the laser should produce.

Here is the absorption spectrum of water:

The lower the 'Relative Absorption value', the less the wavelength's energy is absorbed. We can clearly see that the lowest values are for the 'optical window' that corresponds to the 400-700nm visual spectrum. The highest absorption is for 100nm ultraviolet wavelengths and 3000nm infrared wavelengths.

Here is the absorption spectrum for our atmosphere:

The atmospheric attenuation of electromagnetic radiation has similar features to that of water: short wavelengths such as X-rays cannot go through while long wavelengths such as radio penetrate easily.

It might be easier to consider the laser beam as being fired from space and coming down to the surface.

A 100nm ultraviolet laser beam will traverse the vacuum of space with ease, but will stop short of reaching the upper atmosphere. A 400nm blue laser will go through the atmosphere and through 460 meters of water before being reduced to less than 1% of its initial power. A 1000nm infrared laser will lose 20% of its power to the atmosphere and be completely absorbed by half a meter of water. A 100m radio wavelength just bounces off the ionosphere.

While shorter wavelengths are preferred for laser weapons, as they allow a beam to be focused to destructive intensities over longer distances, a laser submarine should use 400nm wavelength lasers to penetrate water and the atmosphere without losing a lot of beam power.

The equation for how much of a beam's energy is retained after traversing a medium is given by:

Percentage transmitted: e^( -1 * Attenuation coefficient * Depth) * 100

The attenuation coefficient is usually given in cm^-1, so the depth should be converted into cm units.

For example, near-infrared light at 800nm wavelength has an attenuation coefficient of 0.01cm^-1. We want to know what percent of a near-infrared laser's energy remains after passing through one meter of water.

One meter equals 100 cm. Per our equation, we find the percentage to be approximately e^(-0.01*100)*100: 36.8%. Just over a third of the beam gets through.

Here is a table of values for how much beam power is lost if the blue wavelength laser submarine fires its weapon at different depths, using an attenuation coefficient of 0.0001cm^-1:

We see that to maintain a good percentage of laser power getting through the water, a laser submarine would have to fire at rather low depths. According the the table above, a 10 meters depth using blue laser light looks like a good compromise: deep enough to escape orbital surveys and strikes, with at least 90% of the laser power going through.

So is a laser being fired at 10 meters depth a good idea?

The table gives an incomplete picture. While laser power being absorbed is an important factor to consider, there is a large number of other elements that affect how effective a laser is. One such element is thermal blooming. That 90% power transmission rate implies that 10% of the laser power is absorbed and goes into heating the water. If the laser power is rated in megawatts, the heat absorbed by the water becomes significant. Hot water has different optical properties compared to colder water - it will work as a lens in reverse, effectively de-focusing the laser.

Another significant issue is the water/gas interface. When light travels between two mediums of significantly different density, like seawater and atmospheric gasses, it is bent by refraction. Even worse, the sea's surface is constantly disturbed by waves, tides and other movements. Instead of a smooth surface, which angling the laser can compensate for, it is continuously changing and bending light in different and hard to predict direction. Here is a familiar example of the effect:

This effect is familiar to astronomers trying to gaze at stars through the moving atmosphere, and adaptive optics are used to compensate for the deviations in light traversing the atmosphere.

Adaptive optics cannot be employed as effectively when used underwater. While the atmosphere's movements are already difficult to detect and correct, trying to effectively measure how light moves through two mediums with a complex and moving interface is much harder. Guide lasers are used for measurement, with the light reflecting off the ionosphere creating an artificial 'guide star' for astronomers to calibrate their instruments. A guide laser placed underwater would submit to the same chaotic disturbances as the weapon laser and would be unusable. They would also have to work much harder. Refraction means that inaccuracies are multiplied once they pass through the water/air interface.

Finally, there's the problem of reflection. While water is decently able to let higher wavelength visible light penetrate, the massive difference in density between the water and the air (x1000) above it means that it is a good reflector. Laser light would travel from the submarine to the water/air interface, and just be bounced back below the surface. For sea water, less than 6% of the laser light would be reflected at angles below 30 degrees.

While the above table might not give any large figures, remember that the angle is measured against the rippling, swelling and rolling waves. A nominal angle of 10 degrees against the water's surface might transition between -20 and 100 degrees as a wave moves over a laser. This is the difference between 3% and 45% of the laser not going through the ocean's surface.

The principal advantage of staying underwater is that the submarine will be protected from high velocity strikes and retaliatory laser fire. However, if it cannot return fire from this position, then it cannot serve as a perfect last line of defense.

The Solutions

So, based on the previous section, we can affirm that attempting to shoot a laser while underwater provides unequalled benefits but also significant challenges.

We can either tackle the problems a laser submarine faces directly or attempt to circumvent them.

-Long wavelengths

Previously, we considered that blue wavelengths were optimal for shooting while underwater as they were absorbed the least. The reduced absorption would have allowed a submarine to transmit over 90% of the laser power to the ocean's surface from a depth of 10 meters. However, the distortion at the water/air interface rendered this option impractical.

How about using a wavelength that is less efficient at penetrating water, but is less affected by distortion?

Looking at the absorption spectrum of water, we notice that wavelengths longer than 100 micrometers are absorbed less and less as the wavelength increases. At 1m wavelength, the laser would traverse water as easily as blue light. This corresponds to a frequency of 300MHz. This is the radio band.

Not coincidentally, frequencies of lower than 300MHz are used to communicate with submarines. At 3 to 30kHz, which is wavelengths of 10 to 100km, can penetrate the seas to a depth of several hundred meters. Using even lower frequencies further increases penetration depth, but would require impractically large lens to focus onto a target in high orbit. Another factor working against longer wavelength radio is that the ionosphere can reflect signals back down to the surface at frequencies below 30MHz.

A 1m wavelength radio laser will be able to traverse the atmosphere mostly undisturbed and go through the ionosphere without refraction. The features of waves are too small to cause it to wobble chaotically at the sea/air interface. The beam would bend coming out of the sea, but it is a single predictable deviation that can be corrected.

A radio-wavelength coherent beam or 'raser' can be generated by a Free Electron laser or inductive output tubes with an efficiency exceeding 70%.

The advantages of a radio-based anti-orbital system is that it allows a submarine to fire upon targets while deep underwater. Even at 20 meters depth, the radio beam would transmit 82% of its power through water and lose less than 1% going through the atmosphere. It is much less affected by small waves and other turbulence in the water, and mostly immune to above-surface weather effects.

There are several downsides however. Such a large wavelength makes it impossible to focus the beam down to destructive intensities without a very large radio dish - this might get impractical when you also want the submarine to move quickly while underwater. Another issue is that the beam won't interact with the target in a consistent manner.

Lasers, for example, are absorbed by the outermost layers of the materials the target's surface is made of. The heating is concentrated in the 'skin' of the target. Sufficiently intense laser beams heat this skin layer to very high temperatures, causing the material to boil away or even explode.

Radio beams would use wavelengths a million times longer that do not interact with the target's materials at an atomic level. They are much more sensitive to the conductivity of the materials they are striking. Good conductors such as steel or aluminium efficiently reflect radio waves and are not heated. Good insulators such as ceramics or glass are mostly transparent to radio waves and do no absorb the beam's energy as it passes through them. Radio absorbing materials have to be neither good conductors or insulators, such as (sic)

This is bad news if the targets are space warships with an external metallic hull and an internal structure based on advanced carbon-composite and ceramic materials. Large propellant tanks will let the radio waves pass straight through. Small features of 10cm or smaller are completely invisible to the radio waves too.

However, there will still be ways to deal damage.

Openings in the metallic hull would allow radio waves to enter and then bounce around on the internal surface. Like a microwave oven, the trapped radiation will pass through radio transparent materials thousands to millions of times before being fully absorbed. A human is mostly composed of salted water. He or she would absorb about between 0.1 and 1% of a radio beam going through their body. If the radio beam stays inside a 10m diameter hull for just 76 microseconds, 2300 bounces are possible and the percentage of beam energy absorbed rises to 90%. When the beam power is measured in tens to hundreds of megawatts, this has dire consequences for a human crew.

Another effect is induced current. If even a few watts manage to circulate in microcircuitry, it is enough to short-circuit or even melt down computers, avionics and delicate sensors. RF Shock and Burn is a serious issue for electricians and engineers working on conductive structures near a high frequency radio source. At the power levels radio-laser submarines will pump into targets, induced current is enough to melt steel.

Modern submarines are not powerful enough to compensate for the diffraction of a 1m wavelength radio beam. With 100MW of available electrical power, 30% of which is lost in an inductive output tube and another 10% to seawater and atmospheric absorption, less than 63MW will reach space. Even the largest submarines, such as the Ohio-class SSBN, have a hull diameter (beam) of 13m. Mounting an internal dish to focus a radio beam up to this diameter will create a very low performance laser.

Targets at 10km distance will receive about 22W/m^2 - a great radio signal, but a terrible weapon. Targets in low orbit and high orbit will receive milliwatts of power.

What is needed is much more power and an externally mounted radio dish. Thankfully, a 300MHz beam can be focused by a dish with holes up to a tenth of the wavelength in size. A radio dish for this wavelength can is very lightweight and easily collapsible, with conductive spars spaced by 10cm lengths. The spars can be made hollow to have neutral bouyancy, allowing them to support themselves without many structural elements. At 10m depth and below, there are few disturbances in the surrounding water.

Dish diameters of 100 meters or more are envisageable, massing less than 1kg per m^2. Tension wires hold the shape and serve as the mechanisms for adaptive optics to act on the dish.

A group of submarines using space-grade nuclear reactors might be able to put together 500GW of power with a combined reactor mass of only 2500 tons. Between them, they can hold up a dish 1km in diameter, as follows:

This arrangement allows the submarines to focus a 315GW beam to an intensity of 67kW/m^2 at an altitude of 1000km. For low orbit targets, the intensity is 1.67MW/m^2. These intensities are far from enough to melt or physically damage the structure of a spaceship. However, the beam is large and entirely envelops the target. Any hole through a metallic exterior or any cavity lined by a radar reflector will turn the spaceship into a microwave oven receiving megawatts of heating over time. At high altitudes, the intensity is lower by the targets orbit much slower, giving the Rasar beam time to boil crews to death and melt components directly or indirectly.

-Interface lens

The biggest trouble with optical wavelengths is the chaotic distortions and reflections created by the sea/air interface. They would allow submarines to physically damage targets with relatively small lens and shoot on the move, but aiming though the interface seems impossible...

... unless an interface lens is used.

It is an optical array that floats on the ocean's surface, serving to handle the beam's transition from underwater to atmospheric mediums. Glass can be made to have a refractive index similar to that of water. A laser beam traversing a sea/glass interface would not suffer any distortion. This is the reason why some transparent objects disappear completely while underwater - our eyes cannot make out any distortions that reveal their presence.

The interface lens can also serve to focus the laser. It can be made much larger than whatever the submarine can carry, as it is not confined by hull dimensions or hydrodynamics.

One primary advantage of these floating structures is that they can be deployed before firing commences, and each is much cheaper than a submarine. When a target passes overhead in orbit, a laser submarine can rise to firing depth and start shooting. It only needs to equipment to focus the beam from the laser generator to the interface lens, a distance of ten meters or so. The floating lens receives the beam, corrects the angle and aims at the target overhead. If the target is not destroyed, it can trace the laser back to its origin and initiate a retaliatory strike.

An interface lens is not very mobile and needs to stay on the surface, so it cannot protect itself by diving. There is a good chance it will be destroyed... at which point the laser submarine switches to firing through another interface lens and so on.

Although the lens will have to be rather large and heavy to receive a laser beam from a wide variety of angles underwater, and re-focus it in a moving target hundreds of kilometers above, which makes it expensive, it must be considered as an expendable asset when compared to cost and size of a nuclear submarine. Also, the lens can be covered by an isothermal sheet and made out of materials transparent to radar, making it hard to detect from orbit until it starts firing.

Using a 10m diameter interface lens, even a modern submarine with 100MW of available output will be able to deal serious damage to targets in low orbit. About 40MW of the submarine's power will reach the target using a diode laser generator at 400nm wavelength, but at an intensity of 133GW/m^2 at 200km altitude. This is enough to rip through 14.8 meters (!) of aluminium per second, or even 6.5m/s of carbon armor. Any target caught by this beam for even a second will be cut in half. At 1000km, its performance is still a respectable 52mm/s through carbon.

More advanced submarines can get away with smaller lens that are harder to counter-attack and still deal devastating damage to high orbit targets. A 10GW laser beam focused through a 4m wide interface lens can blast away targets at a rate of 837mm/s at 1000km.

Disadvantages of this system is the cost of 'expendable' large adaptive mirrors and possibly the inability to use floating structures in severe storms with large waves rocking the optics.

-Towed Lens

This fixes the problem with floating optical arrays. The laser is focused by towed apparatus that can be held underwater until firing starts, and then moved around after an initial volley to avoid counter-fire.

The towed lens can be lighter and cheaper than a fully independent floating lens. Electrical power, computing operations and other functions can be provided by the submarine towing the lens, with only the actuators and suspension retained.

Lasers have been carried through optical fibres with nearly zero losses of beam energy over distance. This is because internal reflection of the beam is done at grazing angles within the optic fibre. In other words, laser beams of megawatts to gigawatts power levels can be transported by optic fibres without any significant losses and no special provisions against heating.

Optical fibres can carry a laser generated by a submarine to the towed lens, and they can run parallel to the load bearing cables attaching the lens to the submarine. This method of delivering the beam to the lens bypasses the losses and complications of having the beam penetrate several meters of water to reach the surface.

To these advantages come some downsides. The submarine becomes more vulnerable that if it relies upon fully independent floating lens. If an enemy target locates a floating lens, it might correctly assume that the much more valuable submarine is very close to the lens. When a target spaceship in orbit sends down counter-fire before the submarine is ready, the latter would be forced to cut loose the lens and dive... this breaks the optical fibre cable and renders the lens useless.

Tactics using towed lens might involve dragging along a fleet of lenses and rotating them to the surface and back. Longer fibre optic cables gives the submarine more freedom to move, while a mix of decoys and intermittent and random firing patterns reduces the disadvantages of the design.

-Optical phased array floater.

Do away with the submarine!

A specialized vehicle can be built solely for the purpose of hiding a laser weapon underwater and surfacing for short bursts of fire. Since this weapon only attacks from above the water and does not have to worry about hydrodynamics, nifty solutions such as an optical phased array can be used. The laser generator's size is equal to the lens diameter and if it received damage, it will only suffer reduced output.

The more delicate components such as the power generator can remain submerged, only transporting electricity to the phased array on the surface through cables. A reactor embedded in the sea floor can be a very difficult target to locate and destroy.

-Supercavitating platform.

If submarines have access to gigawatts of power, they can also use it for propulsion.

This level of power output can enable submarines to reach supersonic speeds through supercavitation through water. If they can rise to the surface, fire, dive and relocate in a matter of seconds, then they can evade counter-fire through sheer agility.

It might be best in this case to mount a set of optical phased array lasers tailored to trans-atmospheric wavelengths to be used once a submarine surfaces. Gigawatts of power means gigawatts of heating: the local atmosphere can be cleared of moisture and mist that distorts the beam most heavily.

A 1GW laser at 400nm focused by even a relatively small 4m diameter lens can blast past 23.7m/s of aluminium or 10.4m/s of carbon at 200km, and remains deadly at 1000km with 83mm/s of carbon penetration. This allows for short bursts of laser fire to take down any target.

Advanced techniques such as thermal lensing using the Kerr effect is being developed in programs such as the Laser Developed Atmospheric Lens (LDALs) by DARPA. LDALs can allow high-power submarines to extend their effective range to tens of thousands of kilometers while reducing the effectiveness of laser counter-fire.


Lasers and underwater environments don't mix well, but there are many solutions to gain the protection a submarine enjoys while attacking instantly and repeatedly with direct energy weapons.

Once these solutions are applied, a defending planet with deep oceans can hope to maintain an effective last line of defense against invading spaceships.

From ANTI-ORBIT LASER SUBMARINES by Matter Beam (2017)

One of Rand’s soldiers addressed him.

“I’m getting a yellow on Miss B*tch, Mirror 6,” said Private Tim Yancey. Miss B*tch was the affectionate name for one of Rand’s three cannons. (Alpha Dawg and California Girl were the others.) “Think it’s stuck, or at least moving slowly. Might be some gunk in the way.”

Some officers didn’t like to swear in front of their people. Rand was not one of those officers. When he was finished, he said, “All right, I’ll go look at it. Get Smith and Ekkers to meet me down there.”

Rand walked half a klick through an extinct underground lava tube to reach Miss B*tch, buried beneath 20 meters of earth to protect it from bombardment.

His laser cannon looked nothing like the compact guns mounted on warships; with the earth stripped away, the entire contraption would appear as a giant, overturned spider. The beams were generated in the spider’s head, powered by batteries that drew energy from a fission plant buried underneath Fort Patton. A rotating emitter fired the beam down the spider’s “legs,” eight long, hollow corridors that angled toward the surface. When the cannon was fired, the beam would be directed down a random tube.

Mirrors directed the beams at each turn; a final mirror, mounted on an armored cupola that rose above the surface, directed the beam toward any target in Sequoia’s sky. This last mirror was “deformable;” with every shot, thousands of tiny actuators inside the mirror would subtly alter its shape, preventing distortion from Kuan Yin’s atmosphere from defocusing Rand’s beam. (The level of distortion was gleaned from a second, low-power laser that fired from atop the cupola a fraction of a second before the primary blast.)

After a shot, the cupola would drop beneath the surface, and a camouflaged hatch would close above it, more or less concealing it, at least to a spaceship hundreds or thousands of klicks away.

The whack-a-mole rig was a design of necessity; ground-based laser cannon were too large and heavy to safely operate on a planet’s surface or in its atmosphere. A 480-megawatt laser capable of drilling a hole in a starship 5,000 klicks distant needed a lot of batteries, it was simply too big a target to mount on wheels or turbofans.

     One of the nice things about the artillery was you got the best telescopes. Rand had a terrific view of the battle, and early on, it seemed to be going well, despite China’s superior numbers. The Chinese ships had formed a two-dimensional diamond, with one flat face to the main body of the American squadron. The American commander had elected to split his fleet and was coming at the Chinese from two directions, about 90 degrees off from one another. He was gunning for the Han assault carriers, which were hanging well back from the battle. The Chinese responded by concentrating their fire on only one group of American ships, leaving a flank exposed.
     At least two small Han vessels had suffered catastrophic failures in their fusion candles; their explosions were visible to anyone looking up from Kuan Yin’s surface. At least one American destroyer was badly damaged, drifting away on a vector that would take it out of the battle.
     Then, all at once – it was impossible from Rand’s vantage to tell what exactly happened – the battle came apart for the Americans. The senior captain’s flagship, the Puerto Rico, vanished in a fusion-fueled fireball. The light cruiser Norfolk died moments later. The surviving American ships – two frigates and an escort – turned and fled for the only American keyhole (stargate) in the system. A final barrage of missiles aimed at the Han troopships was shot down, their destruction showing up as pinpoint flickers on Rand’s monitor. The Han fleet then quickly disposed of the American surveillance and communications satellites in all but the lowest orbits over the planet.
     Major Montaño was on the comms a few minutes later. “Space Force did what they could. We’re up next. The Han formation will be in range in four-zero minutes. Out.”
     Rand’s platoon was in four separate locations in the underground complex, one crew at each gun in addition to his command center staff. Rand could control all three lasers from here, but the teams were stationed near each cannon in case communications were cut, and to perform any repairs that might be needed during a battle.
     He switched over to his platoon’s network. Time to say something inspiring.
     “Space Force had its shot,” he said. “They made sure the battle was over our heads rather than the other side of the planet, so we owe them one for that. It will be our turn in a little more than half an hour. They’ll shoot at our guns first, so everybody get in your armor. Primary target is their assault carriers, secondary is their bombardment ships. And for god’s sake, protect your mirrors! Good luck. Time for the artillery to shine. Castillo out.”
     It actually took an hour; the Hans were careful in forming up for their bombardment runs on Sequoia. They could either bunch up their fleet and do a massive strike on the surface every three hours, or spread out and keep up continuous fire, never giving the Americans on the surface a breather.
     They chose to concentrate their force.
     “Han ships moving in range,” said the battalion sensor officer over the comms net.
     Half the Chinese ships, including the assault carriers, hung back above 10,000 kilometers. The rest moved to a 6,000-klick orbit and kept descending. At that range, the artillery’s firepower far exceeded that of the Hans; the Chinese needed to get closer to use their bombardment lasers effectively.
     “All guns, target Bandit-3 and fire at will,” Montaño transmitted. “Two second duration.” Bandit-3 was the Wuhan, a battlecruiser, likely the Chinese flagship. Rand relayed the command to his gun crews, heard acknowledgements from all four.
     Sixteen of the battalion’s 18 cannon fired (two had been down for service when the battle started); their invisible, infrared beams were joined by shots from the other laser cannon buried across the continent. Several had to burn through overhead clouds, and the thunder of scorched air rolled across the surface of Sequoia.
     Almost all of the beams struck the Wuhan in the nose; a few, betrayed by faults in their targeting software or mirror-control machinery, simply missed. The Wuhan was well-armored, but the beams drilled multiple holes in her. Giant sparks of plasma exploded outward from the point of impact; at the same time, a second set of sparks exploded on the ship’s starboard rear quarter – at least one laser had burned all the way through.
     Cheers resounded. One of Rand’s operators shouted over them.
     “Counterbattery fire coming in!” A nervous pause as weak beams played across the Sequoia, aiming for mirrors even as they descended beneath the surface. “All guns reporting nominal. No damage.”
     More cheers. Rand watched his screen as his guns cooled for the next shot. Ten seconds, and a second barrage punished the Wuhan again. One laser touched part of her cooling system; an explosion of superheated lithium punched outward through the hull. The ship’s nose looked like Swiss cheese. The ship tumbled end over end.
     A kill! We might actually win this, Rand thought. Somebody called out over the comms net that the ship was burning out of control and would likely break up in Kuan Yin’s atmosphere.
     Then, “Vampire! (military brevity code for "Hostile antiship missile") Radar shows multiple kinetics inbound to our position from orbit!”
     Montaño’s voice: “Target on the kinetics.”
     That’s their plan, Rand thought. They must have launched missiles from high up, and timed their assault so we’d have to deal with them. Their timing wasn’t perfect, or else we wouldn’t have nailed that battlecruiser as well as we did.
     Scores of missiles entered the atmosphere above Sequoia. Battle computers analyzed their trajectories and noted their likely targets: the spaceports, government offices, and the bases of the 33rd Brigade.
     The artillery made a good accounting of itself, wiping out most of the missiles high above the surface. Some debris rained to the planet below, crashing into houses and lighting vast fires. Three missiles got through and destroyed the remote railway junction connecting Cottonwood to the other cities on the continent.
     But the weapons had also provided the necessary cover for the Han fleet; its orbit took it to the far side of the planet, where the few surviving U.S. satellites spotted it lowering its altitude preparing for a bombardment pass.
     During the respite, Rand let his crews take breaks in turns, allowing them to stretch, go to the bathroom and score some food, but he did not rise from his seat, even as the Vampire call came yet again.
     The Hans had timed their second bombardment pass with another wave of descending missiles. The ships and missiles “rose” over the far side of Sequoia first, leaving Rand and his guns to wait until they were higher in his sky.
     Reports came in from the other side of the continent, and the news wasn’t good. The Hans now had a good track on the locations of the American artillery stations, and bombarded them relentlessly. Lasers pounded mirrors as soon as they rose from the ground; missiles with penetrators burrowed into surface, seeking and finding the cannons emitters and batteries. The fire from the surface of Sequoia slackened …
     … and suddenly Rand’s unit was targeted. The lasers came first, burning across the landscape, aiming at known points where mirrors had surfaced thus far in the battle. Several bombardment beams found their targets, digging through the cupola armor and wrecking the mirrors below. The battalion’s radar also took a hit; computer-run telescopes would have to manage targeting going forward. Miss B*tch lost three mirrors; California Girl lost two.
     Rand, angry, tapped his handheld, called Alpha Dawg’s commander. “I’m taking your gun off defensive fire,” he told the junior sergeant. “I want to kill some Hans.” He looked over the list of ships in his sky and targeted a vulnerable-looking frigate that was contributing to the fire above him.
     The gun got two one-second shots off in thirty seconds. Laser fire from the frigate stopped, but two other ships targeted Alpha Dawg. The gun lost half its mirrors in a minute.
     Rand’s boss, Captain Groves, was in another underground control center 20 klicks away, but her shout came through crystal clear over the comms system.
     “Castillo, put your guns back on the designated target matrix, now! And don’t deviate from it again!”
     Rand knew better than to argue. He’d gotten his lick in …
     A corner of his screen started flashing. A moment later, the battery targeting officer called him.
     “Rand, several inbound kinetics are changing course toward your location. I think you p*ssed them off.”
     Groves’ voice, low with frustration: “Updating target priority matrix.”
     The battery’s remaining lasers targeted the inbound missiles, but it was clear that the defenses were wearing down. Several missiles dropped decoys, further complicating the defenders’ efforts. Chinese lasers took out more and more mirrors; missiles were still dying, but they were getting closer to the surface before being shot down. After a few minutes, California Girl was completely offline, her octet of mirrors shattered. Alpha Dawg had three mirrors left; Miss B*tch had two – Rand was bizarrely pleased to note that the mirror at the just-repaired hatch was among them. Most of the other gun platoons were in the same shape or worse.
     Rand hoped the guns would hold on until the Han fleet set over the planet; then his crews would have time to replace some of the damaged mirrors.
     Then the call came in: “More missiles inbound!”
     There weren’t enough lasers to stop them all.
     A penetrator warhead slammed into the ground just above Alpha Dawg, which was about 50 meters from Rand’s control room. It dug into the earth below. The earth collapsed into the cannon’s emitter, burying it under tons of dirt and rock.
     A roar filled Rand’s ears. In the control room, computer screens died, and everything went dark, save for the small blue glow from self-powered handhelds. A cloud of dust washed over him.


Boots On The Ground

The final stage is usually landing your invading army on the ground to capture key targets and force the planet to surrender.


(ed note: The Terran empire had been conquering planets belonging to other empires until they managed to cheese off the alien Spartan Empire. Under Spartan Grand Admiral Shus Show the fleet is attempting to render the human race extinct. Former pacifist Dane Barclay commands the armed forces of Terra in a desperate defense. Because of a silly law (based on Mutual Assured Destruction) the Spartans are forbidden to use a single bomb to blow Terra into vapor. First the Spartans do a nuclear carpet bombing over the entire surface of Terra, then they send in the fleets. After losing half their fleet to the unexpectedly savage Terran defense, the remaining Spartans do a war of attrition. The goal is to poke a hole in Terra's defense so that a beachhead can be established on Terra's surface. From there an invading army can be staged.)

     The siege continued.
     Attack after attack was beaten off. Continual bombardment was endured. Casualties mounted, yet Earth fought on. The defense sphere weakened, was reinforced, held.
     Still the attacks went on. The strategy was simple: hammer and pound the planet—sooner or later the defenses must falter—then pounce.
     The fighting grew more and more savage as Spartans fought to take the planet and Terrans fought to hold it. But despite the determination of the defense, Spartan strategy had to succeed eventually. Finally the moment came.
     Terran forces in the vicinity of the island of Puerto Rico were overwhelmed. Immediately Spartan heavy ships poured in on the island’s defenses which continued to fight despite their hopeless position. An all-out assault was launched against that general area of the planet, aimed at freezing the defense globe, freeing forces in the area and preventing them from reacting to the sudden Spartan success on Puerto Rico.
     On the edges of the island, Spartan cruisers set down, dropped their ramps and disgorged their cargoes of Spartan marines. Overhead, battle wagons and destroyers raked the surviving defenses with energy beams and shattered them with bombs. Reserve fleets hurtled into the battle, adding their weight and numbers to the force near the island.
     Spartan mobile armor roared down ramps and off A into the night. Equipment of every imaginable sort—from computers and laser batteries to medical and boring machinery—came tumbling from ports and locks. As soon as a ship had given up its cargo, it lifted clear and was replaced. by another. The Spartans had come to stay. They had their beachhead.
     As the Spartan marines fought their way inland, others turned seaward and began erecting line upon line of laser and rocket defenses with incredible speed. Other units began building fortifications of quick-hardening super-concrete.
     Over them wheeled the fleets of Sparta, maintaining a shield which the Terran fleets could not penetrate. Minute by minute the defense perimeter grew. More and more Spartan marines lumbered down the ramps, stooped and bent in their space armor.
     All was confusion, yet each one knew what he had to do. Mines were planted in the sea and on the beaches. Bunkers were constructed. Huge planetary lasers were put together and set up to add then mighty beams to those of the Spartan ships above. Some marines set up generators for power, while others began drilling to tap the thermal energy of the planet. Hospitals, command centers, roads, supply dumps grew out of nothing in the blink of an eye.
     Spartan marines were good.

     In the command bunker a young communications oflicer turned from her console to Dane Barclay. There was grief in her tired eyes. “Puerto Rico has fallen, sir. They died fighting to the last man.”
     Dane nodded. He looked to the master battle tank with its huge three-d simulation of Earth. Already the island had gone from Terran red to Spartan blue. He pushed the fog from his brain. This was a critical moment. He knew the speed with which Spartan marines could operate.
     The beachhead had to be smashed at once. Delay would be fatal. A beachhead was the beginning of the end. The Spartans would consolidate and strengthen their position while building up forces within it. Then the break-out would come. There would be little hope of stopping Spartan starships in the skies and Spartan marines and mobile armor on the ground.
     He swung around to his staff. “They’ve got pressure on that part of the planet. We'll have to ignore it. Tell McCode to take everything he's got and get to Puerto Rico!” The oflicer addressed rushed away.
     “You. I want half starship defense of each command sector. Shift them around. I want those forces here yesterday!” That officer hurried away.
     “I want every available strato-fighter, missile and sea sub thrown into the battle for that island.” He swung to a communications officer.
     “Get me all marine divisions capable of assaulting that island one hour. I want them moving at once. And I want to address them.” He swung around to another officer.
     “I want the defense globe to collapse back on that Spartan force over Puerto Rico. Tell them to keep the ships moving. Tell them to spread themselves thin elsewhere—on my orders. I’m going to gamble that Shus Show isn’t going to sacrifice a force of that size. He’s not that kind of fighter. Go!”
     He closed his eyes, trying to think of any. thing else he could do.
     Someone came up to him. He opened his eyes. “The marines are moving out now. You can speak to them if you wish.”

     Sergeant James T. O’Ryan, 76th Terran Marines, was jammed with his gear and twenty-three other marines into the heavy-duty battle copter beating its way through the howling wind above the angry, storm-tossed sea.
     “Attention back there,” announced one of the pilots. “Dane Barclay wants to speak to you.”
     They fell silent as the viewscreen blinked and swirled with color. Then they were looking at the man who had vowed the planet would not fall, the man who had been shot out of the skies of Terra and lived to fight again, the man who had lost his whole family and an arm, the man who was the greatest fighter that would ever live. They knew they would do anything this man asked.
     “Marines—warriors of Terra—Puerto Rico has fallen. A piece of the Earth is in enemy hands. Spartan marines are down in force and consolidating their position. Spartan marines are good. Very good. But you’re better. You’ve proven it before. I want you to prove it again. Get that island back for me.”
     The violence intensified as they drew near. Spartan ships were standing firm, but Terrans howled about them like angry bees. High above, the defense globe had collapsed onto the Spartan forces, seeking to squeeze shut the funnel through which personnel and supplies were being sent down in such huge quantities.
     The death-shrouded sky was a display of fire-works such as O’Ryan had never seen before. He had been a marine for a long time and he had been in many an assault and witnessed more than his due share of battles between starships, but he had never seen anything like this. Starships wheeled. and spun, dove. and darted. Beams of destruction raced across the sky. Strato-fighters flung themselves dauntlessly to the attack. Missile salvoes streaked the sky. Dying ships fell broken into the sea. Water-sprouts dotted the water. Wind howled and screamed and whistled over the battling craft.
     He could see other battle copters, anti-grav troop carriers and the giant sea subs, awesome waves crashing mightily on their superstructures. From the shore came the probing beams of the laser batteries. Fortunately for him and his companions, mountainous waves, torrential sea spray, smoke, radioactive dust. and the electronic transmitters of his own people jammed the Spartan aiming devices; otherwise the battle copters and a-g carriers would already have been wiped out.
     There was an explosion behind. A sea sub, bow gone, wallowed in the heavy seas, victim of a mine her detectors had missed in the turbulence. At an» explosion on his left, he turned to see the shattered remnants of a battle copter fall into the water, victim of a pencil-straight beam of destruction.
     The island was closer. The sea boiled and sprouted around them. The word came: “Get ready!” Men checked over their equipment one last time. Suddenly O’Ryan found himself looking down the length of a beam that passed beneath the craft.
     The battle above still raged in all its fury, while on the waves another battle copter was out down by the shore batteries. The ocean was steaming and boiling where beams struck all over its surface, hunting for the enemy that refused to make its own death easier.
     The copter was setting down, flotation collar out. It hit the water with a slight jar, and crashing waves instantly began to batter and toss it about. The men swayed drunkenly to the rocking of the craft. O’Ryan made one last check of his armor. It was sealed and all systems were “go.” He watched the men going through the bottom hatch, bent nearly double by the huge equipment packs and racks attached to them. They jumped into a little patch of dark swirling water and sank out of sight. One by one the armored marines disappeared until it was his turn. He stepped carefully to the hatch, then jumped. His feet hit the center of the water patch. He went down quickly and darkness engulfed him.
     It was quiet below the waves. Only the occasional fiicker of weapons disturbed the scene. Dimly he could make out other shapes. He gave a blast of his rockets. He continued to sink, but at an angle that would reduce the chance of the marine behind falling atop him. He hit the sandy bottom, plowed into it to his ankles. He stumbled, caught himself with an armored hand and started staggering toward the shore. It was a long and lonely walk Through the dark water, he could see bear: stretching over the surface. They were fascinating to watch. A combination of death and art. Then he saw the bigger shapes of the sea subs’ underwater transports crawling up the sand like giant turtles, carrying their luckier marine passengers right up to the Spartans’ doorstep.
     He was near to breaking clear. There were only a few feet of water over his head. He readied his portable heavy-duty laser cannon, checked the power pack and made certain his other weapons were readily accessible. He plunged forward, knowing the moment of battle was almost upon him. The water broke from his helmet and armed shoulders, and he emerged from the quiet of the ocean into the din of hell.
     The Spartans were waiting. They opened up as if they had been waiting especially for his arrival. The sea boiled with beams, erupted with rockets. All along the beach, marines rushed forward into the waiting embrace of death. Further up the beach, a marine flopped to the ground, the front of his battle armor a bloody blackened ruin. The ground heaved under O’Ryan, hurling him forward. He looked back. A newly created crater smoked behind him.
     He picked himself up and scuttled forward, sweating despite all the ingenious devices within the suit that were supposed to eliminate that problem. He wished he could wipe the clammy wetness away, but that was impossible. Something burst yards away, staggering him, Metal fragments rang off his armor, many scarring it deeply. If not for the armor, he would now resemble shredded meat. He moved on. He passed another marine who had been hit. He lay curled in a fetal ball, the white sand red with his blood. O’Ryan stopped, retrieved the man’s rocket launcher, leveled it and sprayed the super-concrete walls facing him. The rocket shells burst over it with little visible effect. He hurriedly dropped the empty launcher and lumbered away. He was almost too slow. The defenders retaliated and the sand about him smoked and bubbled as beams searched for him. The dead marine was caught and transformed almost instantly to a residue of brunt flesh and smoldering metal.
     Ahead of him a crouching figured glared brightly, armor giving off white-hot sparks, then fell dead. The universe seemed to close in upon O’Ryan as the earth trembled beneath him and beams licked about. He dropped to the ground and crawled. He was an insect, hoping a foot wouldn’t descend and squash him into oblivion. He was close now. He could make out clearly the ghostly, forlorn, forbidding superconcrete battlements of the Spartans. There were an angry whine and a throb of mighty engines behind him. He glanced up. An air-mobile armored vehicle was moving up, its grim lines visible in the light of battle. The sky over-head brightened suddenly as the marine vehicle opened fire. The sinister weapon muzzles spat and spat. Others became visible. The flickering became brighter, the sounds of violence greater. The ground trembled and shook so that his teeth rattled. Looking back he could see that the sea subs had opened fire, adding their meager firepower to that of the air-mobile armor. And now a battle wagon—a big, beautiful, heavy ship—was low over the waves, mighty beams spitting forth from the turrets studding her sides. The starship was lit up like a Christmas tree as every available weapon attacked the Spartan defense works. Before O’Ryan the defenses dissolved in a dozen places, blasted out of existence. The once formidable, nearly impenetrable Spartan fortress was no longer so.
     “Let’s gol” he screamed, heaving himself from the ground and scuttling forward, thinking with mixed emotions of the armor, equipment and weapons that slowed his advance, yet which made him so deadly when at grips with the enemy. Around him other marines who had endured as he had also moved forward. An idiot used this jets and made a great jump. He was burned out of the air. The air-mobile armor crashed and flamed and died. The air over the beach was deadly, but other armored units rode their flickering lasers into the Spartan defenses. And behind them came O’Ryan and the Terran marines.
     The defensive fire was withering. Many fell, but the rest went on; they hadn’t been promised a cakewalk. The wounded and the whole and the near dead went on. The Spartan marines were waiting in the shattered defenses. At pointblank range they fired and still more marines flamed and died.
     O’Ryan fired a clip of his own rockets, threw a grenade that sent several Spartans to their version of heaven and covered the final yards to the ruined wall in a desperate run. He heaved himself over a mound of rubble and ducked just in time to have a beam scorch the top of his helmet. He snapped a reply and was rewarded by a shower of molten sparks as the Spartan marine crumbled, his head a fused ruin.
     It was a wild scene. The night was tortured with death. O’Ryan scanned the scene, sprayed the area and moved forward. Terrans and Spartans died and others took their places. Everywhere were bodies and fighting and rubble and the sound and sight of weapons large and small in use. Rockets were filling the air as they arced into the Spartan camp, causing havoc and disruption among the tight-packed marines and technicians. Missiles too were zooming in on the Spartan positions despite the frantic and valiant efforts of the Spartan Space Forces to protect the ground troops and supplies. But the Terran starships only increased the pressure, beams slashing and cutting ceaselessly into them, sending ships falling everywhere. Nor could the Spartans stop the devastating sweep of Terran lasers and blasters.
     The outer ring of super-concrete defenses had been penetrated. Fighting was hand-to-hand amid incredible piles of hastily unloaded and stacked supplies.‘The missiles, rockets and beams had done their work in the close, crowded confines of the Spartan beachhead. Confusion reigned. Fires and explosions filled the night. Unprepared, after having expected this to be the beginning of the end of the siege, the Spartan marines folded. Many fought on grimly but O’Ryan knew the outcome was decided.
     As if to confirm that, in the glare of fire on a battle-seared hill, he saw a battered group of marines raising the Terran banner. A beam came out of nowhere and ripped across his midsection. By reflex he located the Spartan and fired. His aim was true and the Spartan fell, one shoulder blasted away. O’Ryan crumbled to the ground. But even as he fell another marine hurtled his body and moved on into the night, weapon blazing. O’Ryan looked to the hill. The banner of the Terran Empire was streaming grandly and the marines who had planted it ringed it, their weapons making sure that it would remain. Puerto Rico Was Terran again. “We took it back, Dane,” he whispered. Then he died.

     With the loss of the admiral in the midst of battle, command had fallen upon Vice Admiral Vhus, a native of Anghu, a world overrun by the Terrans, for whom he therefore had no love. He had one overriding desire: kill Terrans. He found himself in a diificult situation: his ground support had been cut from under with the overrunning of the beachhead. The attack lane to the planet had been sealed off by Terran forces. He was in effect cut off and surrounded. He could fight his way out, but didn’t want to.
     “Get me the Grand Admiral,” he ordered. Shus Show came on the viewscreen immediately.
     “The beachhead is lost, Vhus. Bring your ships out.”
     “No, Admiral. The marines did not come out. I cannot with honor leave them.”
     “They’re finished, Vhus. There’s no hope for them. But they didn’t die in vain. I now know the exact limits of Terran strength. This was their last gasp. We have them now. I’m sure of that.”
     “I’m sorry, Admiral. My force will maintain its position over this island—till we are annihilated or till we have re-established the beachhead.”
     “You will place yourself under arrest, Mr. Vhus. Commodore Andelthus will assume command in your place.” Vhus looked to the Commodore.
     “We are in the midst of battle, Admiral. Vice Admiral Vhus is here—you are not. His judgment takes precedence over yourslf”
     “Damn you—damn you!” Shus screamed. “I’ll see that you’re both shot. I—.”
     “It would be better if you gave us support.”
     “For the last time, come out.”
     “No, Admiral. There will be no more retreats. We are here to stay. If you will support us, perhaps we can re-establish the beachhead.”
     “I’ll support you—but damn you both!”
     Vhus signaled and the link was cut. He looked at the Commodore and his oflicers. “Well, it’s done. It’s time we stood and fought instead of running every time these Terrans give us a bloody nose. Let’s fight!”
     And they did.

     Looking from battle tank to viewscreens to data screens to sensor-scopes to comm-men with messages, Dane could not quite believe his eyes or ears. The Spartans were going to attempt to re-establish the beachhead! They were pulling ships from other sectors to throw into the battle.
     Dane whirled, eyes gleaming, mind going, sensing the opportunity presented. The orders came. “I want every defense command stripped of their remaining forces. Every available sea sub is to be brought up. I want as many marines and as many mobile laser batteries on that island as you can get. Scrape up every missile you can find and get it to the defense of that island.”

     It was very quiet. Shus could hear the whirr of the blowers, the chatter of the computers, the many miscellaneous sounds present in the command room of a starship. Shus sighed, looked at his oficers. They avoided his gaze. In defeat they sat or stood dejectedly, letting him bear the burden of the defeat alone. He squeezed his eyes shut tightly to clear his mind and wipe away all obsolete images. The battle picture had changed entirely. The last of the beachhead task force had gone down in flames. There was nothing to support now.
     “Order the withdrawal. We shall regroup, tend our wounds and begin again.”
     It was fortunate for Vice Admiral Vhus that he had gone to glory with his ship. Shus would have enjoyed breaking him. But he was beyond the reach of vengeance or discipline. There was still a siege to be won, and Shus now had to worry about the morale of his officers and men.

From SIEGE OF EARTH by John Faucett (1971)

Last month, this New York Times article by Steven Johnson raised the alarming question: what if our efforts to find and communicate with other intelligent life in the Universe brings us to the attention of intellects vast and cool and unsympathetic who want to wipe us out? The author quoted Stephen Hawking, warning that the result of interstellar contact might resemble the conquest of the New World by the Spanish.

But there are worse things than a galleon-load of extraterrestrial Conquistadors dropping down out of the sky some afternoon. Instead of going to the bother of conquering the Earth, a sufficiently xenophobic and/or ruthless alien civilization might simply lob a few relativisitic planet-busting projectiles in our direction at 99.9 percent of the speed of light. A barrage of brick-sized warheads at that velocity would be an extinction-level event.

Scary stuff! Maybe we should try to hide. Maybe that's the reason we haven't yet detected any other civilizations: they're all either hiding or dead.

Except that I don't believe it, and this isn't just wishful thinking. Interstellar conquest or extermination are simply bad ideas. They're either literally impossible or so difficult and fraught with uncertainty that they might as well be impossible. (I am assuming real-world physics here, with the speed of light as an absolute speed limit.) I am not worried; let me explain why.

I'm going to consider the two cases — extermination and conquest — separately, although there's a lot of overlap among the reasons neither is practical. I will begin with interstellar conquest. There are three good reasons why nobody is likely to try conquest across interstellar distances.

I. Why? Seriously, why do it? What benefit would Earth get by sending out starships across tens or hundreds of light-years to conquer some other planet? If you're a Habsburg King of Spain, conquering Mexico makes perfect sense: you can ship home tons of gold, plant colonies which enrich the mother country through trade, and generally make out like a bandit.

But suppose Mexico was so far away that a galleon would have to sail for decades, if not centuries to reach it? And suppose sending a single galleon to Mexico would cost twice Spain's entire annual economic output? How keen on the project would King Charles be once you told him that?

If that's not enough to destroy His Most Catholic Majesty's enthusiasm for the conquering Mexico, suppose you told him that shipping anything back would cost just as much and require decades to build up the Mexican shipbuilding infrastructure? There is one bright spot for King Charles, though: it's not hard to send messages from Mexico to Spain, so you can mail back pictures, descriptions of the new land and its inhabitants, and maybe a few useful new ideas. But you could just as easily mail a letter to King Montezuma and ask him to do that, without those incredibly expensive galleons.

Conquering inhabited worlds across interstellar distances makes no economic sense. If the planet's technology is sufficiently primitive that a starship full of troops with AK-47s can overawe them and take over, then it's highly unlikely that world produces anything valuable enough to ship home. Certainly no non-biological substances are worth it; even diamonds, at $50,000 per gram, wouldn't be worth it — especially since one could probably find diamond deposits on uninhabited planets much nearer to Earth. (If anyone mentions "stealing our water" I'm going to drown him.) And if you like diamonds so much, developing ways to manufacture them would be a much more lucrative investment than sending starships out in search of some new Kimberley mine.

Bioproducts like heroin or truffles might be incredibly valuable, but the shipping cost is murder. It's far more practical to grab some of the plants or animals you want, take them home, and figure out how to raise them on your own planet.  

And that's assuming conquest is easy. I doubt that conquering the Earth would be easy. Past experience shows that even trying to conquer smallish chunks of the Earth is very difficult. There simply isn't anything on Earth valuable enough to make us worth conquering, especially given the fact that there's seven billion humans armed with everything from pointed sticks to hydrogen bombs who might object.

Well, what about ideology? The aliens could be fanatical Marxists, or devotees of some hyper-evangelical religion, or just really insistent about which way the toilet paper roll goes. Ideology was a good enough reason for 180 million deaths on Earth in the 20th Century.1 We certainly can't assume any hypothetical aliens wouldn't be just as bloodthirsty in the name of their preferred cause.

But it still seems unlikely. The more bloody-minded the ideology, the shorter the shelf life. Of the regimes responsible for that horrifying 180 million death toll, only one is still around (the Chinese Communist Party) and it has transformed itself into a considerably more housebroken form. Obviously we can't tell if this is a "rule" of history (most "rules" of history are bunk anyway), and we certainly can't tell if it would apply to an alien civilization.

However, there is another reason to doubt the existence of hyper-ideological alien civilizations, and that's our old friend the Fermi Paradox. Radiotelescopes are considerably cheaper than starships, so one would expect that any fanatical alien ideologues would be flooding the Galaxy's airwaves with propaganda broadcasts. After all, if they can make converts via radio it frees up ships from the Holy People's Armada to go after the stubborn ones. But we don't see any of that; just silence.

So I won't rule out ideology, but I assign it a very low probability.

II. How? How do you conquer a planet across interstellar distances, anyway? If you're deliberately sending out an invasion, the planet's inhabitants have to be capable of giving you some indication that they exist — which in practice means at least a 1950s-era technology.

Assume that's what you're going after. Assume also that a world with substantial industrial technology is going to have a pretty big population; at least a billion. And while it would be convenient if some or all of them joined your side when you land, there's no way to find out before you launch the fleet. So you have to assume it will be a contested invasion. Finally, we assume that you actually want to conquer the planet — which means most of it must still be habitable when the conquest is over. (We'll cover simple destruction in the next post.)

Let's use the British Interplanetary Society's Daedalus spacecraft as our model starship, and assume arbitrarily good suspended animation for the crew and the soldiers of our invasion force. We will use the American plan for "Operation Olympic" as the model for that invasion force. Olympic was the invasion of Japan which never happened because of the timely deployment of the atomic bomb.

Operation Olympic would have involved 14 Army and Marine divisions, 15 air forces, and the aircraft from 42 carrier ships. That's about 300,000 soldiers, 10,000 airplanes, and at least 20,000 other vehicles. That force, with its supplies, would have a mass on the order of 1 million tons (and that assumes hyper-efficient power supplies for the planes and vehicles so there's no need to bring fuel).

The Daedalus starship was designed for a payload of 500 tons. So you'd need 2,000 of them to carry that invasion force. Each one would cost about 200 trillion dollars (or about twice the annual GDP of the planet). We're ignoring the cost of the actual military force. So building the ships for this invasion would require the entire economic output of a planet like the Earth for 4,000 years! That fanatical ideology has to maintain its grip on its home civilization for a period as long as recorded history just to get the invasion force built.

Oh, and since they have to detect the target civilization before they start building this armada, the target planet has 4,000 years to invest all of its economic output into technology research, industrial expansion, off-planet colonies, and massive defenses. They also get the multi-decade flight time of the armada to prepare to resist (because launching that fleet will be noticed across interstellar distances).

Let's just say the odds don't favor the invaders.

III. Time Lag. This is an important point and bears repeating. Interstellar travel takes a lot of time. The fleet will spend decades or centuries in transit. All of which means that when the invaders arrive, the target planet will be a very different place from when they were planning the operation years earlier.

I can't imagine what the reaction from any strategic planner would be if you asked him to prepare an invasion force against an enemy, but all data about the foe is at least a hundred years out of date. You don't know what their population numbers are, you don't know anything about their economy, you don't know the size of their military forces, you don't know anything about their weapons . . . it would be insanity.

Now, it is possible that the invaders are incredibly advanced and powerful, so that launching such an armada would be easy for them, and their weapons could overcome any opposition. Their species might be a Kardashev Type II civilization, able to command the entire mass and energy resources of an entire star system . . .

 . . . except that such a civilization should be detectable across interstellar distances. Professor Fermi at the back of the classroom is holding up a sign: "WHERE ARE THEY?"

It seems pretty much impossible to conquer a planet across interstellar distances — at least, any planet which could possibly repay the effort of conquering it. If you want living space you can find worlds which aren't inhabited, or terraform lifeless worlds to suit yourself. If you want resources there's a vast number of lifeless bodies in the universe you can strip-mine. 

So I'm not afraid of aliens coming to take over.

But what if they don't want to take us over? What if they just want to exterminate us? I'll get to that next time.

1I didn't even count World War I.


If you want to see stories about aliens who aren't conquering the Earth, check out my new ebook, Outlaws and Aliens!

Deploying To Planet

Once your troop spacecraft have made the long journey from the staging base to the planet to be invaded, there must be a way to insert the troops into the combat zone, and get them out if need be. The landing boats will need armor and weapons if the landing zone is "hot" (i.e., full of hostile troops shooting at you).

ITHACUS Payload breakdown
(x1200 @ 180 lb each)
Troop equipment
(40 lb/man))
Troop provisions
(20 lb/man
seat, restraints)
Life support
(7.5 psi)
Cabin structure
(bulkheads and floors)
Acoustic dampening12,000
Nose Faring25,000
Crew module10,000

Shown above is the "Ithacus". This was a 1963 proposal by Douglas Aircraft, inspired by the ROMBUS plug-nozzle concept. This bold proposal was a semi-single-stage-to-orbit intercontinental troop transport capable of carrying 1,200 soldiers. General Wallace Greene thought that rocket commandos deployed by Ithacus would reduce the need for oversea US Army bases.

The concept was orginally called "ICARUS", but the Marines objected to that name. You do not want to name your flying transport after a mythological figure whose melting wings caused him fall to his death.

Ithacus had a range of 14,000 kilometers, with a maximum payload of 226 metric tons (500,000 pounds) in theory. But if it was launched with an easterly trajectory Terra's rotation gave enough bonus velocity that it could carry 281 metric tons (620,000 pounds). Conversely, launching it westward reduced the payload to 171 metric tons (380,000 pounds).

Ithacus had six troop decks with 200 acceleration couches per deck. A flight crew module carried a crew of four. The module could eject in case of emergencies, but this feature was only incorporated into cargo or flight testing models, not the troop-carriers. Airline passengers are unnerved by the sight of the flight crew wearing parachutes, and presumably so are rocket marines. Ithacus did have emergency floatation balloons so it could abort to a water landing. But if could only abort to solid ground, the results would be unfortunate.

The flight would be limited to a maximum of 3-g acceleration, so the troops would not be too damaged to deploy and fight. It would reach an apogee of 127 nautical miles. Flight time would be about 26 minutes for 3750 nautical miles. When it approached the ground it would have enough fuel to hover for about 30 seconds and "translate" (i.e., move sideways) about 300 meters to find a suitable landing spot. You never know when the planned touch-down spot might be full of hostile troops.

It also would be possible for Ithacus to launch into a low polar orbit and loiter there. This would make the range global, and wage psychological warfare on the enemy as they nervously watched the orbital Sword of Damocles jam packed with marines. Such an orbital launch would reduce the allowable payload.

The fly in the ointment was the unanswered question of how the heck do you get the rocket back home? Blasted thing was 64 meters tall. Even after it burnt all its fuel and unloaded the troops it still had a mass of 500 metric tons. Refueling it and having it rocket back home was out of the question. The monster had a thrust of 80,200 kiloNewtons. It can only safely take off from a custom build launch pad. In theory, if the landing site could be fully secured and if the landing site was reasonable close to a coastal port, Ithacus could be refueled with enough hydrogen to hover and translate to the port. There it could be loaded onto a special transport ship for the journey home.

For more details about Ithacus, check out Aerospace Projects Review vol 2 number 6.

Robert Heinlein's classic novel Starship Troopers took a slightly more practical approach. In each insertion there were only a few troopers deployed. Each trooper was wearing a powered armor suit making each one the functional equivalent of Iron Man armed with nuclear weapons, so you had quality over quantity (i.e., they were more like space marines than they were like space army).

Heinlein's Starship Troopers were deployed from orbit, riding one-man atmospheric reentry pods surrounded by lots of decoys and anti-radar chaff. Dougherty and Frier's term for this kind of insertion is "Meteoric Assault", the soldiers are called "Drop Troops." The reentry pods were only slightly larger than the individual trooper. After a battle the troops were retrieved by a landing boat. A "spike" was fired into a relatively safe location to act as a radio homing beacon. Both the troops and the landing boat would then head towards the beacon. Dougherty and Frier point out that troops must secure a landing zone for the spike otherwise the landing boat will be shot to pieces on the way down. Since there is no other way besides landing boat to extract the troops, the only alternatives are to fight to the death or surrender to the enemy.

But in science fiction, by far the most popular method of deploying troops from orbit to the planet's surface is the dropship (see section below).


Bump! and your capsule jerks ahead one place—bump! and it jerks again, precisely like cartridges feeding into the chamber of an old-style automatic weapon. Well, that's just what we were . . . only the barrels of the gun were twin launching tubes built into a spaceship troop carrier and each cartridge was a capsule big enough (just barely) to hold an infantryman with all field equipment.

And clang!—it's my turn as my capsule slams into the firing chamber—then WHAMBO! the explosion hits with a force that makes the Captain's braking maneuver feel like a love tap.

Then suddenly nothing.

Nothing at all. No sound, no pressure, no weight. Floating in darkness . . . free fall, maybe thirty miles up, above the effective atmosphere, falling weightlessly toward the surface of a planet you've never seen. But I'm not shaking now; it's the wait beforehand that wears. Once you unload, you can't get hurt—because if anything goes wrong it will happen so fast that you'll buy it without noticing that you're dead, hardly.

Almost at once I felt the capsule twist and sway, then steady down so that my weight was on my back . . . weight that built up quickly until I was at my full weight (0.87 gee, we had been told) for that planet as the capsule reached terminal velocity for the thin upper atmosphere. A pilot who is a real artist (and the Captain was) will approach and brake so that your launching speed as you shoot out of the tube places you just dead in space relative to the rotational speed of the planet at that latitude. The loaded capsules are heavy; they punch through the high, thin winds of the upper atmosphere without being blown too far out of position—but just the same a platoon is bound to disperse on the way down, lose some of the perfect formation in which it unloads. A sloppy pilot can make this still worse, scatter a strike group over so much terrain that it can't make rendezvous for retrieval, much less carry out its mission. An infantryman can fight only if somebody else delivers him to his zone; in a way I suppose pilots are just as essential as we are.

I could tell from the gentle way my capsule entered the atmosphere that the Captain had laid us down with as near zero lateral vector as you could ask for. I felt happy—not only a tight formation when we hit and no time wasted, but also a pilot who puts you down properly is a pilot who is smart and precise on retrieval.

The outer shell burned away and sloughed off—unevenly, for I tumbled. Then the rest of it went and I straightened out. The turbulence brakes of the second shell bit in and the ride got rough . . . and still rougher as they burned off one at a time and the second shell began to go to pieces. One of the things that helps a capsule trooper to live long enough to draw a pension is that the skins peeling off his capsule not only slow him down, they also fill the sky over the target area with so much junk that radar picks up reflections from dozens of targets for each man in the drop, any one of which could be a man, or a bomb, or anything. It's enough to give a ballistic computer nervous breakdowns—and does.

To add to the fun your ship lays a series of dummy eggs in the seconds immediately following your drop, dummies that will fall faster because they don't slough. They get under you, explode, throw out "window," even operate as transponders, rocket sideways, and do other things to add to the confusion of your reception committee on the ground.

In the meantime your ship is locked firmly on the directional beacon of your platoon leader, ignoring the radar "noise" it has created and following you in, computing your impact for future use.

When the second shell was gone, the third shell automatically opened my first ribbon chute. It didn't last long but it wasn't expected to; one good, hard jerk at several gee and it went its way and I went mine. The second chute lasted a little bit longer and the third chute lasted quite a while; it began to be rather too warm inside the capsule and I started thinking about landing.

The third shell peeled off when its last chute was gone and now I had nothing around me but my suit armor and a plastic egg. I was still strapped inside it, unable to move; it was time to decide how and where I was going to ground. Without moving my arms (I couldn't) I thumbed the switch for a proximity reading and read it when it flashed on in the instrument reflector inside my helmet in front of my forehead.

A mile and eight-tenths—A little closer than I liked, especially without company. The inner egg had reached steady speed, no more help to be gained by staying inside it, and its skin temperature indicated that it would not open automatically for a while yet—so I flipped a switch with my other thumb and got rid of it.

The first charge cut all the straps; the second charge exploded the plastic egg away from me in eight separate pieces—and I was outdoors, sitting on air, and could see! Better still, the eight discarded pieces were metal-coated (except for the small bit I had taken proximity reading through) and would give back the same reflection as an armored man. Any radar viewer, alive or cybernetic, would now have a sad time sorting me out from the junk nearest me, not to mention the thousands of other bits and pieces for miles on each side, above, and below me. Part of a mobile infantryman's training is to let him see, from the ground and both by eye and by radar, just how confusing a drop is to the forces on the ground—because you feel awful naked up there. It is easy to panic and either open a chute too soon and become a sitting duck (do ducks really sit?—if so, why?) or fail to open it and break your ankles, likewise backbone and skull.

So I stretched, getting the kinks out, and looked around . . . then doubled up again and straightened out in a swan dive face down and took a good look. It was night down there, as planned, but infrared snoopers let you size up terrain quite well after you are used to them. The river that cut diagonally through the city was almost below me and coming up fast, shining out clearly with a higher temperature than the land. I didn't care which side of it I landed on but I didn't want to land in it; it would slow me down.

I noticed a dash off to the right at about my altitude; some unfriendly native down below had burned what was probably a piece of my egg. So I fired my first chute at once, intending if possible to jerk myself right off his screen as he followed the targets down in closing range. I braced for the shock, rode it, then floated down for about twenty seconds before unloading the chute—not wishing to call attention to myself in still another way by not falling at the speed of the other stuff around me. It must have worked; I wasn't burned.

Right that moment I was feeling unusually expendable, almost expended, because I was hearing the sweetest sound in the universe, the beacon the retrieval boat would land on, sounding our recall. The beacon is a robot rocket, fired ahead of the retrieval boat, just a spike that buries itself in the ground and starts broadcasting that welcome, welcome music. The retrieval boat homes in on it automatically three minutes later and you had better be on hand, because the bus can't wait and there won't be another one along.

A suit isn't a space suit—although it can serve as one. It is not primarily armor—although the Knights of the Round Table were not armored as well as we are. It isn't a tank—but a single M.I. private could take on a squadron of those things and knock them off unassisted if anybody was silly enough to put tanks against M.I. A suit is not a ship but it can fly, a little; on the other hand neither spaceships nor atmosphere craft can fight against a man in a suit except by saturation bombing of the area he is in (like burning down a house to get one flea!). Contrariwise we can do many things that no ship—air, submersible, or space—can do.

"There are a dozen different ways of delivering destruction in impersonal wholesale, via ships and missiles of one sort or another, catastrophes so widespread, so unselective, that the war is over because that nation or planet has ceased to exist. What we do is entirely different. We make war as personal as a punch in the nose. We can be selective, applying precisely the required amount of pressure at the specified point at a designated time—we've never been told to go down and kill or capture all left-handed redheads in a particular area, but if they tell us to, we can. We will.

We are the boys who go to a particular place, at H-hour, occupy a designated terrain, stand on it, dig the enemy out of their holes, force them then and there to surrender or die. We're the bloody infantry, the doughboy, the duckfoot, the foot soldier who goes where the enemy is and takes him on in person. We've been doing it, with changes in weapons but very little change in our trade, at least since the time five thousand years ago when the foot sloggers of Sargon the Great forced the Sumerians to cry "Uncle!"

Maybe they'll be able to do without us someday. Maybe some mad genius with myopia, a bulging forehead, and a cybernetic mind will devise a weapon that can go down a hole, pick out the opposition, and force it to surrender or die—without killing that gang of your own people they've got imprisoned down there. I wouldn't know; I'm not a genius, I'm an M.I. In the meantime, until they build a machine to replace us, my mates can handle that job and I might be some help on it, too.

Maybe someday they'll get everything nice and tidy and we'll have that thing we sing about, when "we ain't a-gonna study war no more." Maybe. Maybe the same day the leopard will take off his spots and get a job as a Jersey cow, too. But again, I wouldn't know; I am not a professor of cosmo-politics; I'm an M.I. When the government sends me, I go. In between, I catch a lot of sack time.

Of course, a six-platoon transport is not big compared with a battle wagon or passenger liner; these things are compromises. The M.I. prefers speedy little one-platoon corvettes which give flexibility for any operation, while if it was left up to the Navy we would have nothing but regimental transports. It takes almost as many Navy files to run a corvette as it does to run a monster big enough for a regiment—more maintenance and housekeeping, of course, but soldiers can do that. After all, those lazy troopers do nothing but sleep and eat and polish buttons—do 'em good to have a little regular work. So says the Navy.

The real Navy opinion is even more extreme: The Army is obsolete and should be abolished.

The Navy doesn't say this officially—but talk to a Naval officer who is on R & R and feeling his oats; you'll get an earful. They think they can fight any war, win it, send a few of their own people down to hold the conquered planet until the Diplomatic Corps takes charge.

I admit that their newest toys can blow any planet right out of the sky—I've never seen it but I believe it. Maybe I'm as obsolete as Tyrannosaurus Rex. I don't feel obsolete and us apes can do things that the fanciest ship cannot. If the government doesn't want those things done, no doubt they'll tell us.

From STARSHIP TROOPERS by Robert Heinlein (1959)

Misconception #1: Orbital drops are “kickass.”

Reality: No, they f*****g aren’t. Yeah, let me just cram my up-armored ass into a tiny pod that may or may not combust in the atmosphere of the hostile planet I’m about to assault and faithfully rely on all of the sensitive electronic components that are supposed to slow me down before I pummel into the ground at terminal velocity. Contrary to popular belief, those pods aren’t designed to gently deliver you dirtside. They’re only designed to keep you alive long enough to survive skipping across the planet’s crust like a flaming pinball.

There’s no doubt in my military mind that any of the sadistic bastards who designed this drop coffin ever took a ride in one. Honestly, I think it’s purposely designed to keep us so pissed off that we’ll kill everything we see after a rough landing. The sight of enraged Marines emerging from shattered drop pods, venting puke and s*** from their exo-armor, is bound to send enemy troops fleeing.

Nexus Journal #1 (PDF) is for the tabletop wargame Attack Vector: Tactical. However, of general interest to military science fiction writers is a set of three articles by Claudio Bertinetto. The first is an in-depth look at the mechanics and tactics of spaceborne assault operations. This includes the logistics of transporting the army, scouting the drop zones, the D-Day drop, and advancing to the targets. The second is a detailed look at the fictional Xing Cheng Celestial Navy Marine Corps, and I mean detailed. It analyzes the various branches and missions. The third article is the Xing Cheng Table of Organization and Equipment (TO&E), which goes on for seven full pages. Any author planning an orbital drop of troops will find the information fascinating.


(ed note: this is from the first article mentioned above. The analysis is about the planet of Xing Cheng {Zeta Tucanae III} performing a spaceborne assault on the planet Novaya Rossiyan {Alpha Mensae II} )

Each vehicle is loaded in an individual capsule, and then onto its carrying transport ship, a mobilized civilian freighter. The Marines drop with their trucks and AFVs, and travel on the same ships in adapted passenger pods. Six 100-ton or ten 60-ton capsules fit onto a standard cargo pod dock. The capsules themselves are automated, allowing the vehicle commander to operate them by simple commands.

An orbital assault is accepted by the Xing Cheng General Staff to be a one-off, extremely expensive affair. The entire brigade, including enough supplies to sustain combat for about two months, would require a minimum of 300 to 350 cargo pod docks, a sizeable portion of Xing Cheng’s civilian cargo fleet.

Selection of the Drop Zones would start as soon as the Fleet secures Alpha Mensae II orbit, once orbital defences have been reduced. From this phase on to the very end, the main constraint on operations will be the Heavy Zone Defense (ZD) and orbital defences around the Novaya Rossiyan capital city of Krasnograd, and Krasnaya Zezda spaceport itself.

As the Spaceport is the main target, yet another constraint appears: care must be taken not to damage the main launching laser, as this facility will be required to regain orbit after the invasion.

As mentioned earlier, recent CNMC doctrine states that a combat drop is far too risky to undertake close to the primary target area. Heavy ZD fire would ensure that most of the assault force would never even reach the ground. The main priority for the invasion force would thus be to determine the range of the local ZD umbrella, to ensure the landings take place outside of it.

From SPACEBORNE ASSAULT OPERATIONS by Claudio Bertinetto, Nexus Journal #1 (PDF)

In the quote above they note that the invaders must take care not to damage the launching laser. But they must also keep in mind is that a laser-launch site is fuctionally equivalent to a planetary fortress. It can hurl projectiles and use laser beams directly at any invading spacecraft.


(ed note: the ships in this novel use some sort of technobabble antigravity, so they can magically make the trip from orbit to the ground and back again with no trouble.)

"Four 115-mm rifles, two fore and two aft. A pair of lift-and-drive missile launchers amidships. And a secondary gun battery of 70-mm's and 50-mm autocannon. I know the class; we captured a few of them. Good ships."

A week later, the ship arrived from Storisende; a hundred and sixty feet, three thousand tons, small enough to be berthed inside a hyperspace transport, and fast enough to get a load of ammunition to troops at the front, unload, and get out again before the enemy could zero in on her, and armed to fight off any Army Air Force combat craft.

The ship settled quickly and daintily, while Conn and Anse and Rodney Maxwell sat in the car and watched. Immediately, she began opening like a beetle bursting from its shell, large sections of armor swinging outward. Except for the bridge and the gun turrets, almost the whole ship could be opened; she had been designed to land in the middle of a battle and deliver ammunition when seconds could mean the difference between life and death.


(ed note: this is talking about assaults set in the science fictional universe of the Traveller RPG, but the author is a former member of a U.S. Army Airborne unit. So the general strategy is universal, the more detailed bits are specific to Traveller. One big difference is that in the Traveller universe, antigravity is common. There are zillions of antigravity vehicles capable of tranporting troops from surface into orbit, so extraction is usually not a problem. In other universes this is not the case, so the primary objective is to capture a planetary spaceport intact so the troops can get away.)

A blast from the past, this was originally written for Freelance Traveller in 2002! It is largely unchanged since then.


There are a number of references to planetary sieges and the taking/retaking of planets by opposing navies in the Traveller Canon, especially during the Frontier Wars. And while the Imperium mainly controls the space between the stars, there are times when the enemy isn't only in space. And while a hostile planet can be interdicted, bombed, and talked to from orbit, only troops on the ground can truly control it. This paper is my attempt to explain how I think a planetary assault would work and how one could be set up in a campaign as background, plot device, or adventure.

Assumptions and Givens

  1. This is my opinion and in your Traveller universe YMMV. Much of this is based upon my knowledge of airborne operation as a former member of a U.S. Army Airborne unit.
  2. I am using the Third Imperium from about the time of the Fifth Frontier War as a baseline for the assaulting force; this implies an average Tech Level (TL)-13 with a top TL-15. Switching this to other races should be relatively simple and I will include some notes.
  3. I am primarily a Classic Traveller game master, but I will include references to other milieux. I hope to keep this as generic as possible.
  4. I am assuming that the Imperial Army will undertake large-scale planetary actions. IMO, Imperial Marines are 'johnny-on-the-spot'; they are the visible might of the Imperium and deal with brush fires. In large-scale actions they will concentrate on 'traditional' marine roles — boarding actions and quick assaults. With 'organic' support (artillery, medical units, etc.) and heavy units the Imperial Army and its colonial units are going to be the major players in ground actions.
  5. The relative superiority of near-space by the navy of the attacking force is a given. Without close orbit superiority planetary assaults are effectively doomed. This does not mean that the attacker must absolutely control close orbit, just that they must be capable of projecting great force into near orbit at specific times.
  6. Specific tactics will vary based upon the tech level of the planetary forces. Against foes of TL-0 through TL-5 or so the Marines just set down in grav vehicles and move out. While a large TL-5 army with heavy support could actually mount a credible defense against TL-15 marines in battle dress, they will not prevail. At higher tech levels, however, you can face serious opposition as those large armies gain nuclear weapons and more sophisticated armor and aircraft. I have divided assault procedures into TL-6 through TL-10 and TL-10+.
  7. I am taking it as a given is that military forces will generally be smaller as tech level increases. This will, of course, vary based upon law level, political stability, war footing, etc. But just as many modern armies are smaller than they were in previous generations, I am assuming that the increased efficiency of higher tech levels will reduce the number of sophonts under arms.
  8. This all assumes that the attacking force actually wants to capture the planet mostly intact. If there is no interest in preserving the structures, resources, or population, I assume that a heavy orbital bombardment until the defenders were unable to resist would be sufficient.

Planetary Assaults

A clear military objective is the key to clear military success. The ultimate goal of a planetary assault is to control the planet. In order to do this, the military objectives should be (not necessarily in order):
  1. Render defending military forces unable to effectively resist ('combat ineffective').
  2. Control or neutralize the defender's governmental or administrative functions.
  3. Control or contain major population centers.
  4. Secure means of resupply/reinforcement of attacking/occupying force.
Initially naval forces will conduct ortillery attacks against strategic targets. Defensive emplacements, command and control centers, sensor clusters, military bases, and downports will be primary targets. It is also highly likely that general infrastructure will be targeted to reduce the enemy's will to resist. Civil engineering (dams, mass transit, etc.) will be targeted. Depending on the level of resistance and the volume of ortillery fire available it is possible to reduce a planetary population to using flashlights and shipping water in trucks in a week.
The initial phase of ground assault is usually the use of drop troops (also called jump troops). Inserted from orbit, drop troops rely upon surprise, speed, and violence to secure a landing zone ('LZ'). Once secured, the landing zone is used to land heavy weapons, grav vehicles, landing ships, etc., etc. A secured LZ is called an 'orbit head'. The orbit head(s) are the start points for ground attacks against defenders and can quickly transform into the equivalent of a class C starport.
The main ground assault is performed by a mix of light and heavy infantry, mechanized infantry (infantry and g-carriers), armor, artillery, and support units. Because of the mix of units the force as a whole is called a 'combined arms army' or just 'combarm'.
Assuming the ground assault is successful, there are follow-on units that help secure the planet. Ranging from psychological warfare units to military journalists, these units strive to replace the destroyed or removed infrastructure and government of the planet with the tools of the Imperium.
Although it may be unusual to think of an operation as large as attacking a planet as tactical, but to a military force capable of such an action it is. The most critical decision is; where to insert drop troops? While this should remain fluid to allow changes based upon the differences from one operation to the next, it is often very advantageous to insert an orbit head near a population center of the defenders. In addition to allowing the operation to immediately threaten defenders, it will reduce the ability of the defending military to respond with full force without endangering their own populace. The simultaneous insertion of multiple orbit heads is also preferred. This will force the defenders to split their forces and the attention of their command staff. The use of deadfall ordnance at the same time can add confusion since gravity bombs can easily be configured to 'look' like drop pods to sensors.
Drop Troop Insertion
The most critical period of planetary assault is the insertion of drop troops. Although supported by orbital fire the drop troops are very exposed to defenders and can suffer significant losses before reaching the ground.
To increase their chances of securing an orbit head they are accompanied by a number of tools configured to resemble troop pods to sensors.
The first such tools are 'Landing Zone Preparation Devices', also known as daisy cutters or Sylean scythes. These explosive devices are the first pods fired and are designed to mimic troop pods. About one third of these devices detonate about the LZ and use gravity lensed explosives to direct a concussive cone toward the surface. The massive overpressure is designed to detonate any mines in the LZ and knock down most plant life and structures. The remaining devices detonate on impact and are grav-focused to concentrate their force in a 3-meter high plane parallel to the surface, flattening any remaining foliage and obstacles.
The most common devices that drop amongst the troops are jammers. In addition to radio and radar jammers, there are also meaconers (devices that distort navigation signals, i.e., give false GPS results), repeaters (devices that record defenders' radio communications and repeat them over on over on a number of frequencies), and mimics (devices that send electronic and radar 'chatter' that resembles the defender's communications but give false data).
Also accompanying the drop pods on the outer fringes are defense pods. These grav-stabilized devices have radar/lidar sensors and a laser cannon, all powered by a fusion generator. These air defense systems are designed to shoot down enemy aerospace fighters, missiles, etc. Once they are on the planetary surface they will continue in this role until out of power or shut down by the drop troops.
Last but not least, each squad will have an equipment pod. The equipment will vary based upon each squad's particular mission, but will include heavy weapons, air defense systems, telecomm gear, and combat engineering tools.

Tech Levels 6 through 10

While never easy, planetary assaults against worlds at tech levels 6 through 10 are less difficult.
Defending forces do not have access to meson weapons or powered battle dress. Also, the heavier man-portable weapons are not found at these tech levels.
As mentioned above, however, a large force with the support of nuclear weapons can mount a stiff resistance. The attackers must be sure that orbiting ships can provide nuclear damper support (prevents nuclear weapons from exploding) until the drop troop can set up their own. The drop troops themselves will be optimized to repel a large number of attackers with little special attention to heavy weapons. The average trooper in battle dress with an FGMP (plasma weapon) can deal with a great many main battle tanks of a TL-8 army, after all.
The defenders will also have less sophisticated sensors, making deception more effective. Combined, these make it likely that there will be more deadfall ordnance attacks and fewer actual orbit heads (no more than one per continent, likely only one or two).

Tech Levels 11 and higher

When the defenders approach or equal the technical ability of the attacker the risks become greater.
The inherent advantage possessed by the defenders forces the attackers to take greater risks. The high mobility and concentrated firepower of high-tech forces almost compels the attacker to try and overwhelm defenses with the number of attacks.
The best option for the attacker is to release a near-flurry of troop drops and deadfall attacks combined with heavy ortillery barrages. Preparatory ortillery must especially focus on meson sites and aerospace fighter bases. The drop troops must be prepared to face a number of threats, including grav armor (antigravity tanks) and meson gun artillery.

Special Note

The use of nuclear weapons to generate an electromagnetic pulse (EMP) effect is very common during planetary assaults. Against TL-6 through 10 defenders this can be a devastating attack. And the effect against high tech opponents can be more severe than may be assumed. Although most TL-11+ electronics (especially military electronics) are shielded against EMP effects it will still temporarily overload most sensors, increasing the survivability of drop troops as they enter the atmosphere. Also, while civilian communications systems may be shielded, often their antennae are not. While the means of communication will remain intact after an EMP attack, large areas of communications blackout will exist until antennae are replaced. This will add to the fear and confusion of the defenders.

Support Operations

Intelligence preparation can be a critical force multiplier in planetary assaults, especially against high tech level defenses. In addition to the routine strategic intelligence gathered by Imperial Intelligence, a planetary assault requires an in depth analysis of tactical response measures, apparent willingness of defenders to endanger their own populace, and overall readiness of the defenders ground forces. Effective counter-intelligence operations can also increase the levels of tactical and strategic surprise of the attacking force.
Commando operations in support of a planetary assault are extremely dangerous and prone to failure. However, when they are successful they can have a considerable impact upon the defender's will and ability to fight. For these reasons, they are often popular with players. If strategic surprise can be obtained commandos can be infiltrated and supplied in a large number of ways.
Their initial targets will generally be command and control, telecommunications, and strategic defense systems. The following scenario is a demonstration of the potential impact of successful commando operations in support of planetary assault:
Three commando squads are infiltrated onto a TL-13 world in advance of a planetary assault. Arriving as workers, tourists, and ship crew, they are supplied with a full combat load, including battle dress, smuggled in by intelligence operatives. In a coordinated series of attacks, two major telecomm hubs are sabotaged by pre-set explosives, a similar attack damages the refueling facilities of the major aerospace defense center, and teams of commandos in battledress armed with FGMPs assault the members of the planetary government, planetary defense commanders, and a deep meson site (planetary fortress) that defends a section of the planet. During the resulting confusion reports are received that an enemy fleet has jumped in-system and is on vector for planetary orbit. In addition to potentially neutralizing the defender's civil and military commanders and seriously disrupting planetary defenses these actions could very well panic the defenders, degrading their ability to fight.
While the first step is getting troops on the ground, the key to winning is supplying and reinforcing those troops. As soon as the orbit head is secured the follow on forces must begin to arrive. Initially these forces will be as 'heavy' as possible, i.e., g-carriers, grav tanks, and artillery pieces, preferably in large landing ships. This will be followed by a mix of combat and support units.
The job of the Navy is not over once the troop pods are fired. Without continued naval support the ground offensive will almost certainly fail. In addition to continued ortillery, naval aerospace fighters can provide direct close support to ground troops and engage tactical targets in the enemy's rear areas. Marines can conduct assaults against orbital facilities and can even be deployed by drop ships in support of threatened ground forces. If done properly, combined Army/Navy operations can achieve true vertical envelopment.

N-Hour Sequence

The N-hour sequence is a planning tool for military commanders, logistics planners, and political leaders. It is a rough outline of what will happen and when during a particular type of attack. The initial letter may change to determine what type of attack the sequence is for (for example, a ground attack plan can be called a G-hour sequence while a boarding action against an orbital spaceport could be an M-hour sequence). And certain times can be very broad or based entirely upon the success or failure of a different operation. They key to using an N-hour sequence is to remember that it is a tool, not the plan.
This N-hour sequence is, by necessity, abbreviated. It does not include frontier refueling, naval actions on approach to the planet, or orbital combat and boarding actions. It also omits a great many logistical steps that would be included in a 'real' sequence, as well as the preparatory steps that occur before the assault fleet enters jumpspace. Again, this is a rough estimation to give an idea of the flow of battle:
  • N minus 2 weeks: Assault squadron enters jumpspace.
  • N minus 1 week: Assault squadron enters normal space in target system.
  • N minus 2 days: Ortillery bombardment begins.
  • N minus 16 hours: Decoy deadfall ordnance attacks begin.
  • N minus 8 hours: Naval aerospace fighters increase tempo of attacks against tactical surface targets.
  • N minus 6 hours: Decision phase - commanders determine if planetary defenses are suppressed enough to allow close orbit insertion of drop ships. If so, drop ships move into close orbit. Bombardment ships direct their fire to both overwhelm defenders and clear a number of possible landing zones.
  • N minus 2 hours: Drop troops finish insertion preparation.
  • N minus 30 minutes: Decision phase - commanders determine if landing zones are prepared and the drop troops are likely to secure an orbit head. If so, drop troops are secured for insertion and troop carriers prepare for drop.
  • N minus 15 minutes: Naval forces trigger EMP effects.
  • N minus 5 minutes: Secondary EMP effects are triggered to disable automated responses. Naval forces begin blanket jamming from close orbit.
  • N-Hour: Simultaneous insertion of drop troops begins, accompanied by numerous decoy insertions with deadfall ordnance accompanied by jammer pods. Naval aerospace fighters deploy for close air support.
  • N+1 minute: Naval bombardment shifted to cover approaches to landing zones.
  • N+4 minutes: Landing zone prepped by daisy cutters.
  • N+5 minutes: Drop troops begin reaching surface. Drop ships begin move to high orbit.
  • N+7 minutes: Drop troops begin deploying to secure orbit head.
  • N+10 minutes: Drop troops finish landing on surface. Drop troops begin deployment of heavy weapons and support equipment. Aerospace fighters initiate close air support.
  • N+15 minutes: Decision phase - commanders determine if orbit head is secure. If so, landing ships with armor and mechanized forces begin planetary insertion.
  • N+20 minutes: Drop troops complete deployment of heavy weapons and support equipment.
  • N+25 minutes: Drop troops complete initial defensive positions.
  • N+35 minutes: Landing ships begin to reach the planetary surface. Mechanized and armor forces begin to deploy.
  • N+45 minutes: Decision phase - commanders determine if orbit head is ready for deployment of support elements. If so, landing ships begin cycling support units and equipment to the orbit head.
  • N+1 hour: Combarm begins offensive operations.
It should be obvious that the N-Hour sequence needs to be flexible. Planets with dense atmospheres will require more time for drop troops to reach the surface than planets that have no atmosphere, for example. Deployment of follow on forces may be delayed if there is a threat of significant air defense by the defenders. The number of changes that may need to be made are almost infinite. Recognizing this uncertainty, called 'the fog of war', and being able to anticipate and react to change without panic is what separates good commanders from great generals.


This section is not really about space warfare per se, but the topic of invading a planet from space is relevant to any discussion of interplanetary strategy.

Fundamentally, there are two methods by which one can seize control of a hostile planet: conquest and capitulation.  The question is the relative costs and efficacy of the two methods.  Conquest is based upon landing troops and physically overcoming the defender, while capitulation involves bringing him to a point where he realizes that further resistance is useless.  The problem is that the conditions required for a successful landing of troops are also those required to force the enemy to capitulate through threat of orbital bombardment.  Surface defenses (see Section 4) are quite effective, and an entering drop pod is a target comparable to a modern ICBM RV.  Modern ABM weapons have proven quite successful, and there is no reason to believe that the planetary defenses of the future will do any worse.  

While the ABM debate today is outside the scope of this paper, the analogy comes up regularly, so a brief discussion is in order.  Some point to the high failure rates of current ABMs in testing as justification for describing kinetic intercepts as difficult.  Those failures are a sign of insufficient operational maturity, not of serious problems with the concept.  Other weapons systems, such as air-to-air missiles, have had similar failure rates during their early development.  India has built an ABM system using unguided missiles that fly to the predicted location of the target, and has achieved significant success, and the 1950’s era Nike Zeus achieved 59 hits during 64 tests (including a classified number of skin-to-skin hits).

When compared to an ICBM RV, a drop pod has several advantages, but also several disadvantages.  First, it is likely to be going faster than an RV at the beginning of its path through the atmosphere.  Second, unlike the RVs that most ABM systems are designed to target, the pod is likely to be headed to an area far away from the system.  Such a crossing target is significantly less vulnerable than is an approaching one.  On the other hand, anything of critical military value is likely to be protected by ABM systems, so taking advantage of this vulnerability means that the attacking force will have to move a significant distance overland to reach its objective.

The biggest disadvantage is that the pod must come to a stop before it reaches the ground, while an RV is designed to keep as much of its velocity as possible for as long as possible. A pod carrying people must also keep deceleration to a reasonable level, slowing down in the upper atmosphere.  Human tolerances for acceleration limit theoretical maximums to about 17 G if the humans in question are supine and not more than 10 G if they’re in another position.  Theoretically, these capabilities could be increased by immersing the humans in liquid, which could raise tolerances as high as 50 G, although this might require liquid breathing, familiar from several Sci-Fi works.  One issue with liquid breathing that often is ignored is the amount of stress it places on the support structure of the lungs, which are normally filled with air.  These, along with the aorta, structural failure of which is the usual cause of death under extremely high acceleration, could be surgically reinforced, but this is not a procedure that would likely be carried out on every member of an invasion force.  Even if such measures were taken, there are other problems with extremely high deceleration drops.  The biggest is probably heating, which tends to dominate atmospheric entry calculations at very high velocities, and high velocity low in the atmosphere is exactly what a high-deceleration capsule would be designed to achieve.  Equipment and structural loads are both likely to be limiting factors.  Equipment will have to be specially designed for such loads, and carefully packed for drops.  The structure of the capsules, and their heat shields, will be significantly heavier, raising transport costs significantly (discussed below).

The other significant disadvantage of a drop pod is that even a one-man drop pod will be significantly larger than an RV.  A current US RV, like the ones that carry the W87 or W88, is about 55 cm across and 175 cm long.  A human would probably need a pod at least a meter across and two meters long, or two meters across if the human is supine.  

The actual sizing of such pods is a task which deserves more study.  For an individual pod, the best analogue is probably a 1960s project called MOOSE (originally, the acronym stood for Man Out Of Space Easiest, but it was later changed to Manned Orbital Operations Safety Equipment), which was described by its originators as a lifejacket for spacecraft.  It was a nonlifting conical body, with an empty mass of 90 kg, a gross mass of 215 kg, a diameter of 1.83 m, and a drag coefficient of around 1.42.  While advances in technology might have made the system lighter since it was originally conceived, basic physical limits (and the need to drop equipment with the personnel) mean that this remains a good representative of the minimum possible drop pod.

Sadly, details on larger pods are lacking.  While there were numerous studies of emergency return pods for either single people or groups of three people, there have been only a few studies of dropping squad-sized groups, and not at all of dropping vehicles.  The solution to this is to scale from various different types of known systems.  MOOSE, for instance, has a dry mass equal to 72% of its payload, although a larger system could probably do somewhat better.  Another interesting data point is from the airdrop rigging manual for the M551 Sheridan tank.  It suggests that the equipment for a low-velocity low-altitude airdrop is a full 20% of the mass of the vehicle.  Because of the greater structural loads involved in an orbital entry, and the need to include a heat shield, a figure in the ballpark of 60% of payload mass does not seem unreasonable.  A simple ballistic capsule might be slightly lower than this, while a more complex lifting pod will require more mass.

The best examples of squad-sized pods are two NASA programs, the HL-20 and the X-38.  While the HL-20, with 10 occupants, was intended to be a general-purpose space vehicle, compromising its utility for comparison purposes, the seven-man X-38 was meant as a Crew Return Vehicle for the ISS, giving it a generally similar mission to the notional drop pod involved.  The HL-20 massed about 10,884 kg, with a payload of 1,815 kg, which means that the dry mass of the spacecraft is 500% of the payload mass.  The X-38 appears to have an even higher ratio, approximately 818%, although this is probably at least partially because the entire payload consisted of people, which are notoriously intensive in terms of packaging mass.  Other lifting bodies appear to have similar payload fractions, although data is quite limited.  This is a serious problem, given how important transport costs are, and the limited utility of lifting bodies in avoiding defenses.

Using code from the author’s orbits class, a number of pods were investigated (see Table 4 for characteristics).  The ballistic pods were based on pods like the MOOSE, while the numbers on the lifting pods broadly correspond to Apollo.  The characteristics of the winged pods are based on the X-38, except for payload mass fraction, which has been significantly reduced relative to said spacecraft.  No account was made of entry heating, although that would of course be a primary design driver for real-world drop pods.

Table 4
1-man ballistic20031031.4073.81
1-man lifting20035031.30.589.74
1-man winged2001,0005.50.250.3727.27
12-man ballistic2,5003,850181.40152.78
12-man lifting2,5004,350201.30.5167.31
12-man winged2,50011,000300.250.31466.67
HMMWV ballistic4,5006,800301.40161.90
HMMWV lifting4,5007,800321.30.5187.50
HMMWV winged4,50019,000560.250.31357.14
Stryker ballistic18,50027,750601.40330.36
Stryker lifting18,50032,000641.30.5384.62
Stryker winged18,50077,5001250.250.32480.00

The most obvious result of the investigation was that ballistic pods are far inferior to either lifting or winged pods.  They must be fired into the atmosphere at very shallow angles, which means that they have long and predictable ground-tracks.  The savings in mass (and both directly and indirectly in cost) would be erased by the need to sanitize a larger corridor for the pods.  A pod capable of generating lift can use a trajectory with a steeper entry angle, using its lift to keep the deceleration at a reasonable level for a longer time, cutting down on groundtrack.  Furthermore, G-loads for ballistic capsules are relatively insensitive to variations in entry angle, and must be controlled by lowering the ballistic coefficient. This is contrary to what the analytic reentry equations would suggest, but it is due to the fact that we are not assuming entry angle to be constant.

The choice between winged and lifting pods is less clear-cut.  There is no practical difference in trajectory between the two before they reach about 50 km, where air resistance begins to build up quickly.  Above this altitude, they are vulnerable to ABM/ASAT systems, and totally unable to dodge.  Thrusters could be added to give such capability, but they would add both mass and significant expense to capsules.  This is a situation that might benefit larger pods, as there are significant economies of scale in such systems.  Below it, both become surprisingly maneuverable, capable of turning though 90° or more.  The lift vector can be altered by rolling the pod, and the choice of this direction can significantly alter the course the pod takes.  This could range from an attempt to extend the glide in the line of entry to as great a distance as possible, to a sharp turn to get ‘behind’ a heavily-defended area, to using the lift to get as low in the atmosphere as possible and keep deceleration up to get on the ground as quickly as possible.  The winged lifting body obviously has a much greater cross-range, but simply cannot slow down as quickly as a lifting capsule, because of its much lower CD and thus higher ballistic coefficient.  Most attacks would probably use lifting capsules, unless they needed to be able to maneuver around defenses low in the atmosphere.  Even then, there are serious limitations on the capability of a winged pod to maneuver after its initial bounce.  The most obvious application would be a case in which there is a narrow corridor between two sets of defenses that is too short to send pods down directly, and another corridor clear of ASAT systems that joins it at an angle.  However, this is unlikely to occur in practice, and even if it did, the attacker would have to know about it before leaving home, and then have it still be there when he arrived.  All in all, winged pods are unlikely to see significant service.

The data in Table 5 was generated with an entry interface of 150 km, and an initial velocity of 6,500 m/s, for the 1-man capsules.  The G-load was held below 10Gs.  The first scenario for the lifting and winged capsule involved the roll angle being set constantly to 0° (straight up).  The second involved the roll angle being set to 90° except in response to high G-loads, which cause it to roll towards 0°.  The third involved a 90° turn, and then attempted to hold that heading.  The last involved a complex set of control laws that was an attempt to get the capsule on the ground as quickly as possible.  Downrange is the distance from the entry interface to the final point along the spacecraft’s initial line of flight, while crossrange is the distance traveled perpendicular to the initial line of flight.  Downrange, crossrange, and duration were measured from entry interface until the pod reached an altitude of 5 km.

Table 5
Entry Angle (deg)-1.9-8.2-13.5-8.2-13.5-8.2-13.5-8.1-13.4
Downrange (km)1460.61077.22825.9777.2796.2777.3805.1742.0645.9
Crossrange (km)00056.9226.965.3311.620.679.8
G-max (Gs)9.959.919.939.999.999.999.999.989.98
Duration (sec)5956691124472310500516414194

Various plots from all four scenarios are attached at the end of this section.  In all cases, the ballistic pod’s trajectory is in blue, the lifting pod’s in green, and the winged pod’s in red.  As can be seen for the simple lifting trajectory plot (figure 3), both the lifting and winged capsules bounce significantly, and the high lift to drag ratio of the winged pod carries it well downrange.  This was then countered by having the pod roll, causing the lift vector to push it to the side instead.  The resulting trajectory (figure 4) is rather interesting, as the lift of the winged body again carries it a significant distance from the initial point of impact with the atmosphere. Figure 5 shows the 3-D trajectory of the 90° turn scenario, although due to scaling of the graph, it is less obvious how much of a difference there is in crossrange.  However, the table makes it quite clear that most of the energy appears to be lost in the initial turn.  Figure 6 shows the pull-down trajectory. Table 4 clearly shows that this trajectory will get a pod on the ground fastest.  This trajectory involves the pod bleeding off most of its velocity while relatively high in the atmosphere (above 20 kilometers), then spiraling steeply down.  This could be helpful in avoiding defenses, or make the pod quite vulnerable to them, depending on type and configuration.  It might be possible, by tweaking entry angle or some other facet of pod dynamics, to get the pod deeper before it slows down, and research into trajectories has not been completed.  Due to coding limitations, the author was unable to test the effects of S-turns, but in theory there is no reason why the crossrange of an entry profile could not be reduced significantly.  

Table 6 was generated for the 12-man pods, using the same set of trajectory designs and the same constraints as used for the 1-man pods above.

Table 6
Entry Angle (deg)-1.45-8.1-13.4-8.1-13.4-8.1-13.4-8.1-13.4
Downrange (km)1646.41113.52854.4825.0805.3825.1823.1775.0689.2
Crossrange (km)00057.3266.463.5358.019.488.5
G-max (Gs)9.989.929.9510.009.9910.009.999.999.99
Duration (sec)5125771058394311412486331178

No figures are provided for the 12-man capsules, as the trajectories are broadly similar to those of the 1-man capsules.  One of the most interesting results is the much shorter entry durations for the 12-man lifting pods, as opposed to the 1-man pods.  This is likely because of the higher ballistic coefficient of the 12-man pods increasing terminal velocity during the final phases of flight.  The much smaller difference between the winged pods is probably attributable to the same mechanism, but the lift generated by the pod makes the difference much smaller.  The 12-man pods do tend to have slightly longer downrange footprints than their smaller brethren, probably because of the same mechanism described above, in that they spend more time at higher velocities during deceleration.

Table 7 is the equivalent table for the HMMWV pods, while Table 8 is for the Stryker pods.

Table 7
Entry Angle (deg)-1.45-8.1-13.4-8.1-13.4-8.1-13.4-8.1-13.4
Downrange (km)1651.81118.22853.0835.5802.7835.6820.5780.5673.9
Crossrange (km)00057.3265.863.1360.019.077.2
G-max (Gs)10.009.949.9310.009.999.999.999.999.99
Duration (sec)5055641064384313400493318172
Table 8
Entry Angle (deg)-0.85-8.0-13.35-8.0-13.35-8.0-13.35-8.0-13.35
Downrange (km)1898.41158.82871.8881.6825.6881.1842.8823.5694.9
Crossrange (km)00057.2268.760.6351.418.975.7
G-max (Gs)10.049.959.9610.0010.0010.0010.0010.0010.00
Duration (sec)4714921022324296331447257.4160

It should be noted that no trajectory could be found which would give the ballistic Stryker pod a trajectory that kept it under 10 G.  This appears to be a function of the very high ballistic coefficient.  The Stryker pods continue the pattern seen earlier.  Higher ballistic coefficient means shallower entry angle, longer downrange distances, and significantly shorter durations.  However, the values are similar enough that it is still probably feasible to mix them during a drop.

However, there are significant limitations to the analysis used.  While it is significantly more accurate than a simple analytic approximation, there are several causes of significant error.  First, any planetary invasion is unlikely to be of Earth.  Even if we assume that the planet is broadly Earth-like, details like local gravity and atmospheric density variations could skew the results.  Also, it assumes that aerodynamic characteristics are constant, which is false in two separate ways.  First, aerodynamic characteristics are not constant across an entire entry, although the variation with Mach number is smaller than might be expected, and can generally be ignored.  The largest variation occurs at subsonic speeds, and when an attempt was made at improved modeling, the differences were very minor.  Second, a pod could easily be designed to change its shape or angle of attack during entry to allow it to better optimize its trajectory.  An investigation of all the factors involved would require more time than the author has available.  

Note the variations in entry angle for the various trajectories.  This has a significant effect on the danger zone in which ASAT systems could attack the pod from below.  While exact orbits before entry interface were not computed, it is clear that the ballistic capsule will be vulnerable to even SM-3 and late-model THAAD-type systems from the time it is placed into its entry orbit.  The other pods will be vulnerable to such systems for approximately 1,000 km before entry interface, although the exact number will depend on the angle.  It might be possible to come in at a steeper angle and use rockets to reduce the angle at the last minute.  However, this would significantly increase the cost, mass, and complexity of the pod, and wouldn’t address the lower-altitude vulnerability issues.

A more careful analysis of the aerodynamic data for various spacecraft reveals that there are potential shapes which could outperform the chosen Apollo-based and X-38-based pods.  A bent-bicone shape has a potential hypersonic L/D (lift-to-drag ratio) of approximately 1.4, and potentially improved packing efficiency relative to a lifting body, reducing the amount of dead mass that must be carried, and there are several other simple shapes with L/D of 0.8-1.  There exist examples of winged shapes with hypersonic L/D of as much as 2.6, although these are likely to be even heavier than the lifting body described above.

Compared to lifting body/winged shapes, simpler conical shapes will suffer from poor subsonic aerodynamics.  The obvious solution is to use a Rogallo wing, similar to the systems originally proposed for Gemini, or a parafoil as used on the X-38.  While the Apollo capsules landed precisely enough that NASA began offsetting the point of aim from the location of the recovery ship for fear of hitting the ship, the typical miss distance was on the order of a kilometer, which requires an infeasibly large drop zone.  While this would be adequate if targeting a dry lakebed or something of the sort (water landings are impractical without support already on the surface), such features are often not conveniently placed, and Apollo did not have to worry about collisions with other descending capsules.  Modern military parafoils have L/D values of 4 to 6, which is competitive with most lifting bodies, and Rogallo wings have wide L/D values, depending on construction, ranging from 4 up to as much as 12-16, although higher L/D wings may not be particularly good for the uses under consideration here.

The original paper that described the MOOSE concept has several interesting facts relevant to landing troops on planets.  A figure on pg. 380 shows the variation in downrange distance with percentage variations in deorbit delta-V.  From a 200 nm circular orbit, a 1% variation would produce a dispersion of approximately 35 nautical miles and a 3% variation in delta-V from a 300 nm orbit will produce a 200 nm dispersion.  This is another reason to suspect that all pods will have some degree of lifting capability, to allow them to compensate not only for thrust variations during insertion, but also the other variations that may come up during the drop.  

The paper also includes a slightly less minimal design for a one-man non-lifting pod than MOOSE, as well as a 3-man lifting pod.  The ‘life raft’ (MOOSE was supposed to be a ‘life jacket’) massed about 230 kg to the 110 kg listed for MOOSE in the paper (note that these figures vary from those used in the original calculations on drop pods).  This design is not studied in great detail, as its only real advantage over MOOSE was that it didn’t have to be foamed in space, but it massed twice as much.  

A very interesting figure was included in a description of the 3-man lifting ‘lifeboat’, and is reproduced below

Vs is the fraction of satellite velocity (or circular orbit velocity in a LEO of altitude that is not explicitly called out in the paper) the spacecraft begins to maneuver at.  The craft described began at .8Vs to reduce heating load, and a crossrange of 500 nm.  It had an L/D of 1.5, a dry mass of 1,005 kg, and a payload of 450 kg.  This is broadly similar to the winged pods described above, with a slightly worse payload fraction (although the study was conducted in the early 1960s, and better technology could improve this) and somewhat better L/D.  

An ABM system could potentially cover a substantial area.  The exact range will depend upon a number of factors, but a missile in the class of the THAAD block 4 should have a coverage footprint of approximately 300 km.  The SM-3 Block IB is estimated to have a footprint of approximately 400 km, and the Block II should be able to reach about 500 km.   These are rough estimates, but ranges on the order of 500 km are entirely reasonable for such weapons.  Unfortunately, more precise values are generally classified or otherwise unavailable, even for more mundane SAMs.  It should be noted that these are the ranges for warhead (or drop pods) landing short of the launcher.  For objects that fly over the launcher, the coverage range is much greater.  All of the listed missiles are capable of reaching low orbit, which means that the capsule could be shot at any time after it is inserted into low orbit.  It is theoretically possible to come in fairly steeply from a high orbit, but this will mean either more heating and higher deceleration forces, or very significant expenditures of delta-V to insert the capsules into their entry trajectories if they instead come in much more slowly than usual.  ABM systems are not a case where there are grounds for reasonable expectation for massive improvements in the weapons performance.  There is no propulsion system that could replace chemical rockets for the purposes of short-range missiles, and the other systems involved show no signs of significant improvement.  This is closely related to the problems seen with deep-space missiles, but made worse by the absolute requirement for high thrust-to-weight ratios.  The range is limited by the fact that the target must be shot down before it gets too low in the atmosphere for the missile to function properly.  In such cases, the radar horizon is the biggest limit on rage, and if forward-based sensors are available, range could as much as double.  Of course, radar, being an active sensor, is vulnerable to bombardment itself, and range might instead be limited by passive optical detection of entering pods.  Laserstars overhead could shoot down some of the missiles, provided that they are not being shot at themselves, but the protection they provide is almost certain to be incomplete.  That is not to say that having as many spacecraft as possible overhead during the drop is not a good idea.  At the very least, they will attract missiles that otherwise would have taken out pods.  

Even if the pods, of whatever size, have high (>1) lift to drag ratios during entry (which probably means they are lifting bodies), they still very vulnerable to missile defenses.  The pod spends a significant amount of time at altitudes above the sensible atmosphere (That part of the atmosphere that offers resistance to a body passing through it), where it has essentially no maneuverability, and would be easy pickings for any of the missiles described in Section 4.  Even after it gets lower, its maneuverability is still limited by the fact that it is unpowered, and any turns will scrub valuable energy, leaving it vulnerable to SAMs.  The best strategy is to stay entirely out of the range of defenses, which can be accomplished because of the ability of the pods to either come in at a steep angle or maneuver after entering the atmosphere.

The use of lasers against drop pods is a somewhat dubious proposition.  With proper planning of the approach, the pods will have a large thickness of atmosphere between them and the laser site, even when they are above the horizon.  A laser site will suffer from the same problems that a radar site does, as well as the issues raised by propagation through the atmosphere and potential problems penetrating the plasma shell around the pod.  This plasma would tend to absorb the laser, causing slightly more heating to the pod, but nothing more.  Even if the pod was still above the atmosphere, the fact that it has to be designed for the heating environment of atmospheric entry will mean that it will prove a significantly harder target than a conventional spacecraft.

Other forms of hypervelocity projectile launcher are also potential candidates for use in defenses.  In theory, passive projectiles should be cheaper and much harder to detect.  The exact velocity achievable is a complicated question.  In theory, most systems should be capable of significant velocities, probably more than a typical ABM.  However, there are significant drawbacks to firing projectiles at such speeds.  It is likely that high velocity flight at such low levels will produce a plasma trail would give away the projectile, and might well be hard on the surroundings.  The other drawback is that unlike a typical missile launcher, the launcher is expensive, and potentially vulnerable.  This is likely to make them a less-attractive option for planetary defense, with the possible exception of ram accelerators, which do not require a sophisticated launcher.  The ram accelerator might also be able to repurpose the projectile into a conventional ramjet for sustainer work during atmospheric flight.

The efficacy of individual drop pods is highly doubtful, however.  Even if only minimal losses are suffered, there are still the problems encountered during the airborne landings in Normandy on D-Day.  Troops were scattered, and most of the airborne forces spent their time wandering about as small groups of men from different units instead of fighting as formed units.  This type of confusion drastically reduces combat effectiveness.  It could be argued that maneuvering drop pods could place troops closer together, but at the speeds involved in spaceflight timing errors of a second can scatter pods by 7 km or more.  

Another significant problem with individual pods is the lack of heavy equipment for the troops on the ground.  Anything that is in a pod larger or heavier than a man will be both an easier target and a more prominent one, and a mass-optimized equipment pod will follow a different trajectory from a mass-optimized individual pod.  The defense would likely shoot at such pods on general principal, denying the drop units support.  Even if some way were found to combat the dispersion problem, light casualties could still compromise combat effectiveness significantly.  Even losses as low as 10% can have a significant effect on the combat power of a unit, particularly an airborne unit that has had most of its vehicles destroyed.

Some people would raise powered armor as the solution to this problem.  After all, if an infantryman can be given the firepower of a vehicle, there is no need for vehicles.  The problem with that is that there is virtually no reason to expect that practical powered armor will be developed in the PMF (Plausible Mid-Future).

First, we must define powered armor.  Powered armor is a suit that provides the infantryman with greater strength and protection than an unarmored infantryman while not interfering with his function as an infantryman.  The last part is critical.  The armored infantryman must still be able to do the jobs required of infantry, such as clearing buildings and going up stairs.  This in turn sets size and weight limits on the armor.  Current OSHA guidelines state that the design load for stairs is 510 lb.  Even assuming that all of that limit is available (ignoring things like old or rotten stairs, or stairs not built to code), an average combat-loaded modern infantryman (sans armor) still weighs approximately 225 lb., leaving 285 lb. available for the armor.  This number includes not only the armor itself, but also all of the various servos and power supplies necessary to run it.  As an example, the Lockheed HULC currently weighs 53 lb. without batteries and can carry about 200 lb.  However, it is only a lower-body system and must include its own structure, so given various developments, a total of 50 lb. for the entire power/servo system does not seem entirely out of the realm of possibility.  This leaves 235 lb. for armor.  Taking as a baseline current ESAPI (Enhanced Small Arms Protective Insert) plates, this translates to about 35 square feet of armor or 3.2 m2.

A typical adult male has a surface area of 1.9 m2, so this is a vaguely practical number for armor area once all the other stuff under the armor is taken into account.  The ESAPI plates are rated to resist WWII-vintage M2 .30 caliber armor-piercing rounds, but only when backed by the various plate carrier vests.  This means that the total surface area available would have to drop again, which in turn reduces the practicality of the system.  Even then, more modern 7.62 mm AP ammo would likely be able to defeat it, although solid information on this is difficult to find.  At one point, rifles in this caliber were standard-issue, and could be again if a need (such as defeating targets in powered armor) was there.  Such an evolution of weapons to counter increased armor has happened before.  In the 1500s, the standard gunpowder weapon was called an arquebus, and it was incapable of penetrating the increasing thicknesses of armor being worn on the battlefield.  A heavier gunpowder weapon, called the musket, was developed to defeat such armor.  Muskets made armor more or less obsolete, and once they had done that, they shrank to the size of the arquebus, absorbing it in the process.  

Increasing the weight of armor protection to defeat such threats moves the armor out of the category of “powered armor” and into the realm of “small vehicle”, which has the side-effect of removing the operator from the infantry.  As a friend of the author’s said “if you plan on having your infantry armed like tanks, and armoured like tanks, you shouldn't be surprised that they weigh as much as tanks.”  The small vehicles that would result have no parallel in modern warfare, casting doubt on their utility, and even if they were to prove useful, it is likely that they would not look like powered armor, due to the complex actuators and control systems required of such armor.  A small tracked or wheeled vehicle with a turret would be much more efficient, although it has been pointed out that it might also look quite a lot like a Dalek.

All of the above analysis assumes modern armor and weapons, and the assumption for application to the PMF is that the balance between armor and weapons will remain more or less constant.  This could obviously be flawed, but even if armor increases in power relative to weapons, the weapons used will be tailored to deal with the threat.  Small (~25 mm), low-power weapons that fire shaped charges would probably be effective if all else fails, absent special authorial pleading.  

The above is a best-case analysis. There are likely to be other complications from powered armor, such as reduced mobility (a problem in urban combat), increased ground pressure (a problem anywhere there is mud), increased logistics burden (a problem anywhere) and the fact that not all steps are built to OSHA specs.  The combat load of a soldier will also likely increase, and the number used above was for a basic rifleman only.  Grenadiers in the study referenced carried an extra 8.5 lb, and SAW gunners an extra 16 lb, to say nothing of the heavy weapons personnel, or even personnel who are simply heavier than average.  Add to this the fact that powered armor, both in fiction and in real life, is often touted as not only protecting the soldier, but also increasing his carrying capacity.  All of these combine to render powered armor a dubious proposition.  This is not to say that exoskeletons will not be useful for increasing the carrying capacity of soldiers, or that powered armor might not have a role in peacekeeping/counterinsurgency operations, where the enemy does not have access to modern weapons.  The problems of reliability and maintenance will also be major issues for a force that relies so much on very high-tech equipment.  Without real-world experience, it is difficult to determine how much maintenance powered armor would require, but even the most basic powered armor will be very complex compared to virtually all systems the infantry use today.  This is not a good thing when the system will be exposed to dirt, mud, debris, insufficient maintenance, and near-continuous use.  This in turn indicates that additional maintenance facilities above and beyond what is standard today will have to be dropped with the unit, exposing them to the orbital defenses (see above).

An alternative is to drop a more conventional mechanized unit, complete with vehicles.  This unit will tend to land in bigger chunks, improving effectiveness, but the reduced number of targets for the defenders is likely to result in greater losses.  Unless all pods are of the same mass, the defenders will still be able to discriminate between them, and guess at their payloads.  This would likely allow them to focus their attacks on bigger, heavier pods, which could be assumed to carry things like tanks.  Careful design of a unit’s equipment could mitigate this problem, but only at a cost in mass-efficiency for both the pods and the equipment.  The exact tradeoff is heavily technology-dependent, and thus outside the scope of this paper.

In either case, once the attackers are on the ground, they still have to move to their target and capture it.  The movement in question will be over hundreds if not thousands of kilometers of terrain, and impeded by enemy resistance, terrain features, and a lack of roads.  If the drop zone was a relatively undefended area, it was probably also lightly inhabited, and thus lacking in transportation infrastructure.  Sabotage would also contribute to this lack, delaying the advance even more.  The US Army estimates that a typical rate of march (including rest and maintenance halts) of between 16 and 32 km/h depending on the quality of the roads and the time of day, with a practical maximum daily range of approximately 200 km.  Note that this is for a road march in peaceful conditions, not a combat advance.  Even the “high-speed” advances in the 2003 Iraq War averaged somewhere around 15 km/h, against light opposition.  However, taking the average daily range and a distance of 1000 km (through some combination of landing distance and not being able to take the shortest route), the unit will take 5 days to reach the target.  This does not take into account the possibility of resistance, and the various problems that could occur during what will be at least a semi-tactical march.  During this period, the enemy will know where they are, and where they are going, and be able to move forces to reinforce the objective.  It seems reasonable that the defender will be able to manage at least twice the attacker’s movement rate, giving a huge radius in which troops can be drawn from to reinforce the defenses.  This assumes that the landing zone is a complete surprise to the enemy, which is unlikely to be the case if any serious preliminary bombardment is done.  For that matter, extended preliminary bombardment might be counterproductive, giving an enemy the warning he needs to move local defense systems in to slaughter the drop pods.  These systems could be small and relatively-low performance, resembling modern SAMs, as they would only have to intercept the pods at low speed and altitude.

Once the assault force arrives at the target, they must overwhelm the troops defending it.  Assuming that the attacking force has about three times the per-man effectiveness of the defenders (which is not unreasonable, as training masses nothing and making equipment better is often cheaper than shipping more of it), they will need about even numbers to overcome them.  This ignores potential losses in effectiveness due to fatigue, losses in key personnel, and general confusion during the drop and march.  Training, equipping, transporting and supplying that many troops is going to get expensive very fast.

 Exactly how expensive is an interesting question.  Taking as our baseline a Stryker Brigade Combat Team (chosen because it seems a reasonable analog to a future space-transportable unit with integrated support units), the total mass of vehicles and heavy equipment is at least 12,025 tons (time and information constraints prevented a better number, although this estimate was intended to be a reasonably conservative best-case).  There are a total of 4,236 men, and assuming that light equipment and people amounts to 500 kg per man, the total “combat mass” comes to at least 14,145 tons.  There are roughly 1500 individual vehicles/pieces of heavy equipment, so at least that many drop pods are required (assuming crews drop with their vehicles).  If we assume 4.5 kg/man/day shipboard, the unit requires 572 tons/month in transit.

The combat supply requirements are somewhat more involved.  The first assumption made is that all water is being procured on the surface, instead of being dropped from orbit.  The second is that the vehicles do not require fuel, and use some form of lightweight power source, which is likely to be ultimately nuclear-derived.  A typical man-day’s supply will total 31.8 kg (of which 14.2 kg is ammo and 6.8 kg is equipment attrition replacements), with an additional 24.4 kg if the vehicles use shipped fuel.  This totals 134.7 tons/day of combat, although this number may be low (the source value for supplies/man/day probably includes higher-echelon troops than are present in the SBCT and who don’t use as much ammo as those on the front lines).  After major combat operations are completed, the daily requirements will drop to around 13 kg/man/day, or 55.1 tons/day, which can be reduced by another 1.8 kg/man/day if food is procured locally.

If combat is expected to take 30 days, then the total supply requirements will be 4,041 tons.  This will also have to be dropped, for a total drop mass of 18,186 tons.  However, this number ignores the mass of the fuel systems that would need to be deployed.  While the paper referenced above contains some details on the proposed scheme, details on the exact weights involved are fairly sparse.  Reference is made to the system breaking even for weight with gasoline after 30 days of combat.  If this is correct, then the mass required for the fuel production system would be approximately 3,100 tons, or about 17% more drop mass.  However, the report in question dates back to the early 1960s, and it is likely that the technology of the future (or even of today) would allow significant reductions in that mass, although how significant is impossible to estimate precisely.  An assumption of a fuel system mass of 1,000 tons is probably as reasonable as is possible without detailed study, bringing the drop mass to 19,186 tons (assuming that no additional personnel above and beyond the brigade’s normal complement are required to operate the machinery).

Assuming that the drop pods have a total mass equal to 50% of the payload (which is a somewhat generous, but generally in line with the numbers given above), that means a total drop pod mass of 9,593 tons.  It might be possible to reduce the drop pod mass slightly by finding more mass-efficient ways to drop supplies, such as reusable shuttles.  However, this does require improved security around the drop zone, and planners would probably assume that this would generally not be the case.  Also, we need to account for supplies consumed during transit.  If we assume a total transit time of 6 months, this mass (which does not have to be dropped) will amount to 3,432 tons.  The total mass that must be launched into space for this mission is a minimum of 32,211 tons.  The troops will probably require at least 3 tons/man in hab space (keeping in mind that they must be fit to fight at the other end), so the total hab mass is a further 12,708 tons, although a fair bit of this can probably be provided by requisitioned civilian ships, and would not be included in the launch budget.  Assigning a further 10% of payload mass for general spacecraft structure, the total payload mass that must be moved from one planet to another is approximately 49,411 tons.  This is a total of 11.7 tons/man, of which 7.6 tons must be launched specifically for this mission.  Even if launch costs approach current grid energy costs ($100/3.6 GJ, which is theoretically possible if using laser launch, a space elevator, or a launch loop), the cost of putting the necessary equipment and personnel in orbit will be $26.84 million.  If the transit velocity is about the same as orbital velocity, the transit energy cost will be $82.34 million; although a more realistic number for such a transit would be twice that (the above ignores the energy costs of the ships themselves).  Given the other costs of running a spacecraft, the total shipping bill for the brigade could easily pass $300 million in even the most optimistic case.  This totally ignores the costs of the drop pods and supplies themselves, although the cost of supplies for transit can be traded off against the energy cost of using a faster, higher-energy transit.

To move multiple brigades, which will be required for all but the smallest worlds, many times the amount of stuff described above will have to be moved, to say nothing of the various combat support elements.  Heavy artillery, combat support, and air units will all need drop pods, habs, and cargo spacecraft.  The shipping bill alone would rapidly rise into the billions or tens of billions of dollars, and the heavy equipment is more vulnerable during the drop.

Training the forces is also non-negligible.  It is likely that the troops would need to be trained in an environment that has the characteristics of the target world.  The best way to do this appears to be an orbital hab with a rotation rate set to give the appropriate gravity.  Terrain can probably be approximated at home, and the hab can have an appropriate atmosphere.  The spin rate of a power’s habs might be an important piece of intelligence data to back up signs of preparations for an invasion, giving an indication of who they expect to go to war with.

At this point, it would be logical to suggest the use of robots as an alternative to human troops, and there are significant factors to recommend this approach.  A robot not would require habs during shipping, would (presumably) take no training, and could be considered expendable.  However, there are problems with this approach, as it rests on the assumption that a suitable ground-combat robot could be created.  Many of the reasons cited in Section 2 in support of unmanned warships do not apply on the ground.  The largest issue is that while it is practical to propose that every spacecraft be run by remote control, doing the same for a robotic invasion force removes entirely the logistical advantages accrued therein.  This in turn requires the creation and deployment of autonomous robots in an environment that is tactically far less clean than space, overcoming formidable technical and moral/political obstacles.  Nor should the difference in physical environment be overlooked.  A robot would have to deal with dirt, mud, and other hazards of military life with little maintenance, as well as being capable of fulfilling all the roles of the person it is replacing, in an environment where versatility is far more important.  The cost of this is non-trivial, although it does offer a vaguely-plausible alternative for those willing to imagine that robotics will advance so far.

Another, often overlooked issue with robots is their effectiveness in replacing humans during counterinsurgency operations.  A robot advanced enough to be effective at winning hearts and minds is unlikely to be the cheap and disposable device described above.  Morally, it will have to be almost equivalent to a person to win the trust of those it works among, which more or less erases the line between robot and human, except from a logistical perspective.  And the overall logistical requirements of a robot-based force are unlikely to be that much better than an equivalent human-based one.

If the landing were to take place, it would be the attacker’s ultimate gamble.  All of his troops would be landed in one area, and there is no practical way to get them back.  Laser launch and robust SSTOs would offer the capability to evacuate some of the men, but all of the heavy equipment would probably have to be abandoned.  Even such an evacuation would be risky, as the defender would want to trap as many men as possible, if for no other reason than to make it more difficult to attack again later.  Any spacecraft taking off would do so through a barrage of missiles, and laser launch sites would be prime targets for any number of different methods of attack.

If the attacker made a successful landing, he would have to face the defender’s surviving forces.  One major advantage the defender has is that not only does he not have to pay shipping costs for his units, he can also use quantity to overcome quality.  Most of the defender’s army would be draftees, given a few months of training and some basic weapons.  One on one, the attacker’s units would have no problem destroying them thanks to better training and equipment.  However, they do not have to pack as much combat power into as little mass as possible, which allows them to be deployed in large numbers at the optimum ratio for cost to combat power.  Furthermore, they are on the defensive, which is less difficult for the inexperienced troops that make up the majority of their ranks.

One alternative to a serious invasion is to stage a change of government to one that is more favorable to you.  This can either be done by encouraging a coup, or by supporting an insurgency.  The coup would be encouraged by threats against the planet if the planet does not surrender, along with promises of good treatment if it does.  Insurgency holds great story potential.  Small teams would be inserted onto the planet, probably through normal space travel, and used to create or support local guerilla movements, with the aim of overthrowing the government.  This is not practical in all situations, as it either requires a weak government (which might not be able to resist a conventional attack), a large existing insurgent movement, or a great deal of both patience and luck.

It has been suggested that the costs of effective space forces and orbital defenses are large enough that a defender cannot field sufficient ground forces to effectively resist an invasion.  The problem with this is that ground forces are relatively cheap, and sufficient forces to make invasion very difficult can be funded out of the leftovers from the Navy.  As mentioned above, the defender’s equipment can be designed to be as cheap and reliable as possible, and stockpiled well before the battle.  Furthermore, unless the potential attacker is very close to the defender’s planet, the long lag between the invasion force departing home (when it is detected) and landing on the defender’s planet would allow the Army to normally exists as a cadre with stockpiled equipment, further reducing operating costs.

Local irregular units might supplement the defender’s conventional forces.  The efficacy of this type of force in harassing conventional units has been demonstrated in Iraq and Afghanistan in recent years, and their effectiveness is multiplied many times over by the fact that the attacker is racing against the clock imposed by his supplies and the defender’s response.

All of the above discussion assumes a homogenously-defended world, which no allies for the attacker.  If there are any allies, the situation changes significantly.  The best option in that case is simply to ship the men to the ally, and buy the equipment and supplies from him, or just pay him to make the attack in the first place.  Even if some special equipment has to be shipped in, a large proportion of the vehicles in any large military unit are simple trucks, or slightly more complex variations on generic vehicles.  All supplies should be procured on-planet, as tooling up to make munitions is generally much simpler than making vehicles.

The problem with this plan is that the potential target is unlikely to fail to notice the preparations and will strenuous, and probably violently, object.  Also, any power on a balkanized world will have a much stronger army than one that is in control of a homogenous world, all else equal.  The best way to invade is probably by using the ally as a proxy.  However, if, for whatever reason that is not practical, the invader would probably have to fight through any orbital defenses the defender would have, with the ally presumably joining in.

Orbital defenses of Balkanized worlds are a very complicated matter, with the potential for battles between various sets of orbital defenses.  There is the possibility that the world will have a more-or-less unified set of orbital defenses, similar in concept to NORAD.  The problem is that if the powers are close enough to set something like that up, they are unlikely to turn on each other to the extent of supporting an invasion.  More likely, each power will have its own orbital defense system, which, being in orbit, has global coverage, preventing the attacker from going around it, as well as ground-based defenses in their own territory.  It would also have the secondary purpose of destroying the orbital infrastructure of any of the other powers that attack it.  Supporting an invasion (even if not as an active participant) is a de facto act of war, so the off-planet attacker would somehow have to protect his ally in the opening stages of the war.

Even after the orbit-based defenses are destroyed, the fact that the defenses are limited to their own territory does not necessarily mean that the attacker will be safe on the other side of the world.  Today, many nations hold small islands scattered around the world, which would make ideal bases for such defenses.  The defensive submarines mentioned in Section 4 would also be ideal for a balkanized world, particularly as the attacker might well have to exercise more restraint in hunting them for fear of hitting neutrals.

The best way to use an ally might be merely as a staging point.  The attacker would come in on his own at first, and attempt to gain space superiority. The other power(s) on the planet would be induced to declare neutrality, and when the orbital battles were over, any allies would declare for the attacker and probably conduct the bulk of the invasion themselves, with the aid of limited orbital fire support and possibly occupation troops.  At the same time, the defender would surely figure out what’s coming, and probably would declare war preemptively.  The only way to make an alliance work is if the extraplanetary attacker was somehow able to pre-position his forces without the target being overly suspicious.  Deployments of ground troops might be concealed under the pretense of joint training missions, although the logistics of shipping them to the target planet makes this approach problematic.  A more likely scenario is a port visit by a naval squadron of some sort.  Provided that such visits happen regularly, it is possible that an attacker and his local ally could gain a very significant advantage in orbit.

The defender can still make life complicated and landings difficult even if the attacker has a planetary ally.  As noted in Section 4, surface defenses have very long ranges, and it is entirely possible that a defender could hit targets over an attacker’s ally.  This would make landing troops difficult even with an ally, or force the attacking force to move a significant distance overland.  While this is probably preferable to the dangers of an opposed landing, even administrative movements are difficult and slow.  There are also likely to be prepared border defenses that would have to be dealt with, a problem that would be avoided if they were landing directly in the enemy’s territory.

In many ways, the easiest scenario for a ground invasion is a planet that is not homogenously defended, but also not balkanized, so there are few or no surface defenses in place.  The attacker can land away from the defenses and move overland to attack the target.  The biggest problem is likely to be transportation.  As mentioned above, areas that are poorly defended are likely to be of little consequence and have poor transportation infrastructure.

Assuming that a method besides a straight-up invasion is chosen, the attacker will of course have to move some ground troops to be able to occupy the planet after it has surrendered.  The analysis of moving a ground force provided above applies, but the problems for occupation troops are less severe.  First, the supply load for occupation troops is approximately a third of that of troops in combat, and might be reduced farther by the transportation of small factories or the use of local industry.  Second, the drop pods can be replaced with conventional surface-to-orbit shuttles.  The first units would probably land in pods, but follow-on ones would be landed at no mass penalty.  Third, the units themselves can be equipped differently than they would be for facing military units.  This could result in substantial mass savings, as heavy vehicles like tanks can be left behind.  The exact employment of the occupation force is outside the scope of this paper, but there are numerous works on the subject.

All of this begs the question of why exactly the attacker wants the world.  An attempt to add to one’s territory is probably best accomplished through diplomatic means, possibly backed up by some display of military force.  Resources are plentiful enough that, barring McGuffinite (plot-dictated special resources), invading a planet for them is not a sensible plan.  The exception to that rule is humans, but in that case, the defender will probably fight to the bitter end rather than accept slavery.  However, if humans are the target, invading a low-tech world is by far the most sensible plan.  Another possibility is the world itself.  If habitable worlds are rare (moving somewhat outside the PMF) then conflict over them is a possibility, but the question then becomes why the attacker would not use a bioweapon instead, wiping out the population and leaving the infrastructure (and probably the biosphere) unharmed?  The most likely answer is that bioweapons are viewed as abhorrent and use of them gives one the status of an outlaw state, combined with the potential problems of the agent either surviving to render the planet uninhabitable or escaping to other worlds.  If the population is more important than the infrastructure, bombarding the infrastructure should send the planet back to a technological level equivalent to (probably) the late 1800s in short order, removing the ability to resist the invasion.

Strategic needs might also be the basis for an invasion, but will obviously depend heavily upon the exact circumstances in question, and fall outside the scope of this paper.  All that can be analyzed here is the effect of technological constraints on such strategic requirements.  

by Byron Coffey (2016)


The common trope in science fiction is a specialized spacecraft designed to insert troops into combat, called a Dropship. The topic is covered exhaustively in an article at the always impressive Future War Stories. Also well worth reading is the entry on Tactical Transports. Mr. Frisbee has some notes about insertion and extraction here. For a variety of reasons, dropships tend to be spherical in shape.



The final entry in this section, affectionately known to the Imperial Legions as the “Big Ugly Breakfast 1” — and less affectionately known to almost everyone else as “Good gods, what is that thing?” — is the Flapjack-class cavalry dropship (Eye-in-the-Flame Arms/Artifice Armaments). Uniquely among Imperial starship designs, the Flapjack has adopted the rare “disk” or “saucer” hull form. It does this because the Flapjack-class is equipped with not merely a single, but a pair of nuclear-pulse drives, using the relatively environmentally friendly laser-fusion or (in the Flapjack II) antimatter options, the descent and deceleration drives; the dorsal and ventral hulls of these ships are in effect simply the pusher plates for these drives. The main body of the vessel, suspended between these on hydraulic dampers, is a short, wide cylinder, heavily structurally reinforced and itself surrounded by “sidewall” armor as thick and refractory as the pusher plates.

The intended usage of the Flapjack is orbital insertion of armored vehicles, en masse, into hot zones. To enable this, after being decoupled from a carrier in the high orbitals of a planet under attack, the Flapjack uses its descent drive to accelerate downwards through the atmosphere, minimizing dwell time within range of orbital and anti-air defenses. In addition, while the descent of a Flapjack obviously has far too bright a sensor signature to be concealed, the combination of the radiation hash from the descent drive’s thrust bombs and the plasma sheath formed by its hypersonic atmospheric transit together render it extremely difficult for weapons systems to attain successful guidance lock, and terminal guidance (especially to the fine degree necessary to insert a weapon into the narrow window of vulnerability between the pusher plates and the sidewall armor, even if the weapon is capable of surviving and maneuvering in the immediate environment of an active nuclear-pulse drive) virtually impossible.

At the end of its descent trajectory, the Flapjack uses the more powerful thrust bombs of its deceleration drive to perform a “suicide burn”; i.e., maximal deceleration at minimum altitude, compatible with lithobraking in a manner which preserves the integrity of the ventral pusher plate. This deceleration burn serves the additional functions of preparing the drop zone for the arrival of the dropship by flattening any structures or prepared defenses, and eliminating any but the most heavily armored, secured, and radiation-proofed resistance in the immediate area. Once the ground is reached, multiple armored cargo access doors with integral ramps and excavation drones permit the Flapjack to be actively discharging combat vehicles within minutes of a successful landing.

A proposal for an infantry dropship along the lines of the Flapjack, tentatively designated the Pancake-class, has been advanced by Eye-in-the-Flame Arms, but at the present time the high-radiation aftermath of such a vessel’s landing is not considered viable for personnel wearing M-70 Havoc combat exoskeletons or N45 Garrex field combat armor, the current legionary standards. While this would not be a problem for troops equipped with the specialized N45r Callérás high-rad field combat armor, its associated disadvantages and the expense of refit ensure that, for the foreseeable future, infantry will continue to be landed via drop shuttle (q.v.)

— Naval Starships of the Associated Worlds, INI Press, Palaxias, 421st ed.

1. A statistically improbable number of combat drops take place at planet dawn.

Robbie-Yarber: So how do you land a nuclear pulse rocket? On all the diagrams I've seen there are no landing jets, so I guess they intended to leave them in orbit permanently? But that raises the question of how do the passengers get back to the ground?

Alistair Young:

Well, so far as the Flapjack is concerned, the answer is... hard.

Disclaimer: under ordinary circumstances, this would not be the recommended procedure for landing any sort of nuclear pulse-drive ship, and it's definitely not the one any other such designs in the 'verse use, such as the old Phoenix stack. But the Flapjack-class is an odd duck because of its peculiar requirements: namely, having to survive a descent into a hot zone, and thus implicitly wanting to decelerate as hard as possible as low as possible (a "suicide burn"); which in turn implies both that you want a drive with the high thrust qualities of a nuclear pulse drive to do the job, and also that you probably want to avoid vulnerabilities such as those that auxiliary landing thrusters might create.

So, if you take a look at a Flapjack, it looks like the ventral pusher plate is massively overbuilt; which it is, because once it goes into the suicide burn part of landing, it's got to fly through its own nuclear fireballs (not usually a good idea in atmosphere), and then once it gets too close to the ground to decelerate with the nuclear pulse drive any more, it just cuts off the drive, drops under gravity, and smacks straight into the ground, finishing its deceleration by lithobraking right into the crater its drive just cut and relying on that same plate and its shock-absorption system to soak up enough of the ensuing damage. (The theory being that the ground has been nicely softened up for it by the burn, and that in any case, you don't land a Flapjack near anything that you're worried about damaging...)

If you think that sounds like an extremely rough ride, well, you'd be right. ;)

It also solves their taking off from the ground problem, in a manner of speaking: by and large, this sort of landing is very hard on the ship, too, so Flapjacks aren't designed to take off again under their own power, or indeed be used again at all without some extensive maintenance and refurbishment. When possible, they get hauled off the ground by salvage tenders and refitted in orbit, but for all practical purposes, it's a stripped-down, single-use dropship.

[And while I haven't yet done a detailed design work-up on it, I suspect the pusher plate is flat or near-flat; allowing part of the blast to escape to the sides should, I suspect, make it possible to get rather lower before needing to cut the drive. But don't quote me on that, it's yet to have numbers put to it.]

(ed note: so this thing is a two-ended Orion nuclear pulse rocket. One Orion rapidly moves the carrier from orbit to the ground, while the second combines the function of rapid deceleration and nuclear daisy cutter. Totally insane but I totally love it.)


“Frisian Vessel Obadiah to FDF commander Cantilucca,” crackled an unfamiliar voice through Cokes commohelmet. “Come in FDF Cantilucca. Over.”

“There is an Obadiah,” said Johann Vierziger as he watched the rear and sides of the van for possible dangers, “on the FDF naval list. She’s a Class III combat transport.”

“— FDF Cantilucca. Over,” as Coke switched on the transmission from orbit again.

“Survey team commander to FDF vessel Obadiah” Coke said. “We’re glad to hear from you, boys, because we’ve got the Heliodorus Regiment looking for our scalps. Can you drop a boat to pick us up? The Heliodorans have secured the spaceport. Over.”

Obadiah to FDF Cantilucca,” the helmet responded. “You bet we’ll drop a boat. Hold what you’ve got, troopers. Help is coming in figures one-five minutes. Obadiah out.”

Niko Daun looked up, toward the sound of the incoming boat. Coke, suddenly fearful that Pilar would follow the direction of Daun's gaze, shot his hand over her unprotected eyes. “His visor will darken automatically,” Coke said.

Pilar pulled his hand down with a firm motion. “I’ve worked in spaceports for twelve years, Matthew,” she said. “I know that plasma exhausts can be dangerous to my eyesight.”

“Blood and martyrs, sir!” Niko said. “It’s not a boat, it’s the whole ship! They’re coming straight in and there’s no port here!”

“Class III?” Coke snapped to Vierziger as the penny dropped.

“That’s right, Matthew,” Vierziger agreed. “The Obadiah’s a battalion-capacity combat lander. She’s got pontoon outriggers, so she doesn’t require a stabilized surface to set down. And armor, in case the landing zone’s hot.”

The transport swept overhead at a steep angle. The roar and glare of her engines were mind-numbing. Foliage at the tips of trees beneath her track curled and yellowed.

The vessel's exhaust was a rainbow flag waved at Madame Yarnell and the Heliodorans, some ten klicks to the west. Either the Obadiah’s commander expected to lift again before anyone could react, or —

Or the commander didn’t care what a regiment of light infantry might attempt. The Obadiah was coming in with her landing doors open. The troops she carried were ready to un-ass the vessel as soon as the skids touched, or maybe a hair sooner.

“Bloody hell!” Mary Margulies shouted over the landing roar. “She’s coming in loaded! She’s coming in with troops!”

The Obadiah landed a hundred meters away, like a bomb going off in the forest. Her exhaust and armored belly plates cleared their own LZ. Dirt and shattered trees flew away from the shock. Coke caressed Pilar’s head closer to his chest to protect her from the falling debris.

From THE SHARP END by David Drake (1993)

Exotic Attacks

One must keep in mind the objective, and do not get caught up in the details of a standard solution. Sometimes one has to think outside of the box. If the objective is to eliminate the current inhabitants of a given planet, there might be more efficient methods than using zillions of nuclear weapons to turn the continents into glassy slag. I have a small selection below, but you can find much more at TV Tropes under "How To Invade An Alien Planet".

Divide and Conquer

If the planet is Balkanized (that is, composed of many mutually suspicious nations like the situtation that currently obtains on Terra) you have an opportunity. Make covert contact or send your agents into a couple of the most powerful nations and encourage them to attack each other. The loser of the war will be in no condition to resist your invasion, and the winner will be vastly weakened. The idea is "Let's you and him fight." In The Art of War, Sun Tzu called this "attacking alliances." A common colloquialism is "play both ends against the middle".

This is an example of Unconventional Warfare.

A balkanized planet is just full of flaws and vulnerabilities for an invader to take advantage of. The invaders can try to covertly inflame old hatreds and grievances, corrupt a nation into doing the invader's bidding by dangling riches or valuable alien technology in front of their nose, frame one nation with something it didn't actually do, the possibilities are endless.

Isaac Kuo points out that this also has implications for the invaders. If the invaders do not have enough troops to conquer an entire planet, but only enough for one nation, the dynamic shifts. As he puts it:

If alien forces are overwhelming but localized, then you don't need to be strong enough to defeat them. You just need to be stronger than your neighbor.

Isaac Kuo

This is a variant on the old joke "I do not have to run faster than the lion, I just have to run faster than you."

Isaac also mentions that if the various balkanized nations hate each other enough, when the invaders attack one nation, that nation's enemies might actually pay the invaders in gratitude.

When The Twerms Came

We now know (little consolation though this provides) that the Twerms were fleeing from their hereditary enemies the Mucoids when they first detected Earth on their far-ranging Omphalmoscopes. Thereafter, they reacted with astonishing speed and cunning.

In a few weeks of radio-monitoring, they accumulated billions of words of electroprint from the satellite Newspad services. Miraculous linguists, they swiftly mastered the main terrestrial languages; more than that, they analysed our culture, our technology, our political-economic systems — our defences. Their keen intellects, goaded by desperation, took only months to identify our weak points, and to devise a diabolically effective plan of campaign.

They knew that the US and the USSR possessed between them almost a teraton of warheads. The fifteen other nuclear powers might only muster a few score gigatons, and limited deliver systems, but even this modest contribution could be embarrassing to an invader. It was therefore essential that the assault should be swift, totally unexpected, and absolutely overwhelming. Perhaps they did consider a direct attack on the Pentagon, the Red Fort, the Kremlin, and the other centres of military power. If so, they soon dismissed such naive concepts.

With a subtlety which, after the event, we can now ruefully appreciate, they selected our most compact, and most vulnerable, area of sensitivity ...

Their insultingly minuscule fleet attacked at 4 a.m. European time on a wet Sunday morning. The weapons they employed were the irresistible Psychedelic Ray, the Itching Beam (which turned staid burghers into instant nudists), the dread Diarrhoea Bomb, and the debilitating Tumescent Aerosol Spray. The total human casualties were thirty-six, mostly through exhaustion or heart failure.

Their main force (three ships) attacked Zürich. One vessel each sufficed for Geneva, Basle, and Berne. They also sent what appears to have been a small tugboat to deal with Vaduz.

No armourplate could resist their laser-equipped robots. The scanning cameras they carried in their ventral palps could record a billion bits of information a second. Before breakfast time, they knew the owners of every numbered bank account in Switzerland.

Thereafter, apart from the dispatch of several thousand special delivery letters by first post Monday morning, the conquest of Earth was complete.

From "When The Twerms Came" by Sir Arthur C. Clarke (1972)

Internal schism


(ed note: Miranid of the Farla empire has just finished explaining to Henlo why conventional theory holds that a decisive interstellar war is impossible. Now he explains the sneaky trick they are going to do in order to avoid conventional theory and destroy the upstart barbarian Vilk empire.)

"Our barbarian friends have another weakness, which we have up to this point not been able to utilize without compromising its existence. I 've carefully saved it until now, and they have considerately not discovered it within themselves."

"The Vilks, of course, were able to make war quite successfully. Since they were operating as a horde of mobile independent principalities, and since they were after loot and glory only, they were never forced to gain what civilized nations would term 'victory', or 'conquest.'"

"They were reapers, harvesting the same field again and again, and gradually extending their boarders. They had no time for the re-education of subject peoples to their own ideals or patriotic causes -- a fact further implemented by their total lack of such civilized appurtenances. They merely informed their vassals that they had become the property of whatever Vilk it happened to be, and levied tribute accordingly. They left it to the natural fertility of the Vilk soldier to gradually erase all traces of independent nationality among such nations as could interbreed, and to the natural inertia of generations of slavery among such as could not."

"The result has been the gradual accumulation, in Vilk ranks, of a number of Vilks who are not Vilks."

Miranid seemed anxious to stress the point.

"And these Vilks may be good, barbarian Vilks like all the rest of them. But some of them inevitably feel that their particular kind of Vilk is better fitted to rule the communal roost."

"A situation, you will agree, which does not apply among such civilized communities as Farla, which may have its internal dissensions, but no special uniforms of hide-color, limb-distribution, or digital anomalies around which infra-nationalistic sentiments may be rallied."

Miranid stabbed the chart with his dividers. "We will slice here, here, and here, with most of our lighter units supported by some heavier groups. You and I, Henlo, will take the remainder of the main fleet and spit right through to Vilkai, where we will crown some highly un-Vilkish Vilk king of the Vilks, and then leave him to perish."

"The entire sorry mess will slash itself to suicide in the petty nationalistic squabbles which are sure to follow the precedent we set them. We will be enabled to do so quite easily by the allies which our housewifely intelligence corps have neatly suborned for us."

From SHADOW ON THE STARS by Algis Budrys (1954)

Biological Warfare

The unpleasant fellow of the Four Horsemen of the Apocalypse who rides the white horse is pedantically known as "Conquest" but popularly as "Pestilence." Genetically engineered plagues have the advantage of scalability, that is, the deadly disease will multiply to fit any sized population. Care must be taken by the attacker if they are of the same species at the defenders, since the plague will probably work equally well on them. And no matter how virulent the disease, there will probably be a few survivors who are immune or who manage to avoid infection.

Note that sometimes the biological weapon is in the form of an insect instead of a disease, and sometimes instead of the target being the defenders it is crops or food animals.


The rider of the white horse had a buddy riding a black horse. As a general rule, the defenders have to eat (unless they are intelligent robots or something like that). Destroy their ability to make food and they will eventually all starve. You can introduce biological warfare agents that kill crops, interfere with the influx of sunlight to the planet via huge mirrors or inducing nuclear winter, drop in the equivalent of genetically engineered super-locusts that will devour everything, there are several methods. Bobby Coggins mentions you can kill off a large percentage of the population just by destroying means of food transport (highway and railway lines, junctions and port facilities).

This will not wipe out 100% of the defenders because they will start a crash course of making food with hydroponics, yeast, or something like that. But the probability is that only a small fraction of the defenders will survive.

Von Neumann Machine

Specifically a Von Neumann universal constructor, aka Self-replicating machine. These are machines that can create duplicates of themselves given access to raw materials, much like biological organisms. Whatever sabotage they are programmed to do against the defenders is magnified by the fact that they breed like cockroaches.

In the TV series Stargate SG-1, the Replicator are self-replicating machines that are ravaging all the planets in the Asgard galaxy. In Greg Bear's novel The Forge of God and the sequel Anvil of Stars, an alien species systematically destroys planets detected as possessing intelligent life by attacking the planets with self-replicating machines.


Nanotechnology is machines the size of molecules. They are pretty nasty just like that, but the become a million times worse if they are also self replicating machines. This is the dreaded Gray Goo scenario.

Killer SETI

Children are taught "Sticks and stones may break my bones, but names will never hurt me." While this is true of school playground interactions, it may not hold in the world of communicating with aliens, i.e., SETI.

What can aliens do to harm us over a radio wave? Plenty.

They might attack individuals via transmitting a Medusa Weapon image. They might send blueprints for a device they claim will produce free energy but will actually turn half the planet into antimatter. It might even send instructions which will covertly create an alien agent here on the planet, such as in the science fiction story A for Andromeda.


Even if star travel is impossible; "mere" communications could do a lot of damage. After all, this is the basis on which all censors act. A really malevolent society could destroy another one quite effectively by a few items of well-chosen information. ("Now, kiddies, after you’ve prepared your uranium hexafluoride…")


      While it has been argued that sustainable ETI is un-likely to be harmful (Baum et al. 2011), we can not exclude this possibility. After all, it is cheaper for ETI to send a malicious message to eradicate humans compared to sending battleships.
     If ETI exist, there will be a plurality of good and bad civilizations. Perhaps there are few bad ETI, but we cannot know for sure the intentions of the senders of a message. Consequently, there have been calls that SETI signals need to be “decontaminated” (Carrigan 2004, 2006).
     In this paper, we show that it is impossible to decontaminate a message with certainty. Instead, complex messages would need to be destroyed after reception in the risk averse case.
     If such a message is received only in one place, and only once, it might be possible to contain it and its harmful consequences, or even destroy it. If it is received repeatedly, perhaps even by amateurs, containment is impossible. As a further complication, the International Academy of Astronautics has adopted a “Declaration of Principles Concerning Activities Following the Detection of Extraterrestrial Intelligence” which states1 that “These recordings should be made available to the international institutions listed above and to members of the scientific community for further objective analysis and interpretation.” This view is shared by the majority of SETI scientists (Gertz 2017).


     We continue with a hypothetical message which appears to be, at first sight, positive and interesting, and shall be analyzed in depth. Any message could, in principle, be examined on paper. For many plausible message types, however, it is much more convenient to use a computer. Even the simple LATEX notation is difficult to read as code. Consider the proof of the Riemann hypothesis, which begins with the equation


     which is much easier to read when interpreted and finely printed as

     Even a typesetting system such as TEX is a Turing-complete programming language (Greene 1990), so that the message is in fact code, and may contain a malicious virus. Messages may contain large technical diagrams, equations, algorithms etc. which can not reasonably be printed and examined manually. In addition, the message itself might be compressed to increase interstellar data rates, and the decompression algorithm would be code. Executing billions of decompression instructions cannot plausibly be performed manually and requires the use of a computer. But then, the computer would execute potentially harmful ETI code. For this case, it was suggested to use isolated, quarantined machines for analysis (Carrigan 2004, 2006).
     In the following section, we explain why these measures are insufficient, and no safety procedure exists to contain all threats.


     Consider a large ETI message with a header that contains a statement such as “We are friends. The galactic library is attached. It is in the form of an artificial intelligence (AI) which quickly learns your language and will answer your questions. You may execute the code following these instructions...”
     We assume that the message is available only to a small group of people, part of a government body, who decide to keep it private, but follow their curiosity and examine it with utmost care. A computer in a box on the moon is built to execute the code. Safety devices are in place, their design by choice of the reader, such as remote-controlled fusion bombs to terminate the experiment at any time.
     This scenario resembles the Oracle-AI, or AI box, of an isolated computer system where a possibly dangerous AI is “imprisoned” with only minimalist communication channels. Current research indicates that even well-designed boxes are useless, and a sufficiently intelligent AI will be able to persuade or trick its human keepers into releasing it (Armstrong et al. 2012; Dawson et al. 2016).
     For the escape, we have to assume that researchers engage in a conversion with the AI (without, there would be no benefit in running the experiment in the first place). In such a text conversion, the AI might offer things of value, such as a cure for cancer, and make a small request in exchange, such as a 10% increase in its computer capacity. It appears rational to take the offer. When we do, we have begun business and trade with it, which has no clear limit. If the cure for cancer would consist of blueprints for nanobots: should we build these, and release them into the world, in the case that we don’t understand how they work? We could decline such offers, but shall not forget that humans are involved in this experiment. Consider a nightly conversation between the AI and a guard: “Your daughter is dying from cancer. I give you the cure for the small price of...”. We can never exclude human error and emotions. After all, is it ethical to keep a sentience in a prison when it expresses incredible pain due to small manufacturing errors from building the box?
     Even in a military-style, adamant experiment, there will still be humans involved who go home after examination work with their own feelings. Even if everything is officially secret, whistle-blowers might get some news out to the public. Quickly, there could be a community on Earth in favor of letting it out for religious, philosophical etc. reasons. If the AI promises to cure cancer, or offers a message of salvation, a cult could form. Maybe (or maybe not) a majority of the population would be in favor of releasing the AI. Should, or even could, a democratic government work against the majority of its people? Dictatorships are unstable and eventually over-thrown; the AI will be eventually released.


     With a non-zero prison escape probability in any single time period, the AI will be free at some point of time. Then, the worst possible result would be human extinction or some other unrecoverable global catastrophe (Bostrom 2014). The main argument is that the human species currently dominates planet Earth because of our intelligence. If ETI-AI is superior, it might (or might not) become more powerful and consider us as irrelevant monkeys (or maybe not).


(ed note: In the story, SETI scientist make radio contact with the alien inhabitants of Mu Cassiopeiae, a spectral class G5Vb star about 25 light-years away. Sadly the scientist cannot get any scientific information from the aliens since the entire planet is under a religious dictatorship. They receive nothing except the alien equivalent of evangelical proselytizing pamphlets. Kind of like an interstellar version of The Watchtower.)

“ ‘—begat Manod, who reigned over the People for 99 years. And in his day lawlessness went abroad in the land, wherefore the Quaternary One smote the People with ordseem (Apparently a disease—Tr.) and they were sore afflicted. And the preacher Jilbmish called a great prayer meeting. And when the People were assembled he cried unto them: Woe betide you, for you have transgressed against the righteous command of the Secondary and Tertiary Ones, namely, you have begrudged the Sacrifice and you have failed to beat drums (? —Tr.) at the rising of Nomo, even as your fathers were commanded; wherefore this evil is come upon you.’ Sheemish xiv, 6.

“Brethren beyond the stars, let us ponder this text together. For well you know from our previous messages that ignorance of the Way, even in its least detail, is not an excuse in the sight of the Ones. 'Carry Our Way unto the ends of creation, that ye may save from the Eternal Hunger all created beings doomed by their own unwittingness.’ Chubu iv, 2. Now the most elementary exegesis of the words of Jilbmish clearly demonstrated—

Father James Moriarty, S.J., sighed and laid down the typescript. Undoubtedly the project team of linguists, cryptographers, anthropologists, theologians and radio engineers was producing translations as accurate as anyone would ever be able to.

Father Moriarty had been assured that the different English styles corresponded to a demonstrable variation in the original language. So he accepted the edited translation of the messages from Mu Cassiopeiae. “Only why,” he asked himself as he stuffed his pipe, “must they use that horrible dialect?” He touched a lighter to the charred bowl and added, “Pseudo-King James,” with a bare touch of friendly malice.

“And the biology, biochemistry, zoology, botany, anthropology, history, sociology … and who knows how far ahead of us they may be in some technologies? Sure. Those hopes were expressed before I was born,” snapped Strand. “But what have we actually learned so far? One language. A few details of dress and appearance. An occasional datum of physical science, like that geological information you spoke of. In more than a hundred years, that’s all!”

“Religious ranting, you mean,” said Strand sourly.

Moriarty grimaced. “Correct word, that. I was reading the latest translation you’ve released, on my way here. No sign of any improvement, is there?”

"Nope,” Okamura said. "As of twenty-five years ago, at least, Akron’s still governed by a fanatical theocracy out to convert the universe.” He sighed. "I imagine you know the history of Ozma’s contact with them? For the first seventy-five years or so, everything went smoothly. Slow and unspectacular, so that the public got bored with the whole idea, but progress was being made in understanding their language. And then—when they figured we’d learned it well enough—they started sending doctrine. Nothing but doctrine, ever since. Every message of theirs a sermon, or a text from one of their holy books followed by an analysis that my Jewish friends tell me makes the medieval rabbis look like romantic poets. Oh, once in a great while somebody slips in a few scientific data, like that geological stuff which got you so interested. I imagine their scientists are just as sick at the wasted opportunity as ours are. But with a bunch of Cotton Mathers in control, what can they do?”

‘Tes, I know all that,” said Moriarty. "It’s a grim sort of religion. I daresay anyone who opposes its ministers is in danger of burning at the stake, or whatever the Akronite equivalent may be.”

Okamura seemed so used to acting as dragoman for visitors who cared little and knew less about Ozma, that he reeled off another string of facts the priest already had by heart. “Communication has always been tough. After the project founders first detected the signals, fifty years must pass between our acknowledgment and their reply to that. Of course, they’d arranged it well. Their initial message ran three continuous months before repeating itself. In three months one can transmit a lot of information; one can go all the way from ‘two plus two equals four’ to basic symbology and telling what band a sonic ‘cast will be sent on if there’s an answer. Earth’s own transmission could be equally long and carefully thought out. Still, it was slow. You can’t exactly have a conversation across twenty-five light-years. All you can do is become aware of each other’s existence and then start transmitting more or less continuously, meanwhile interpreting the other fellow’s own steady flow of graded data. But if it weren’t for those damned fanatics, we’d know a lot more by now than we do.

“As it is, we can only infer a few things. The theocracy must be planet-wide. Otherwise we’d be getting different messages from some other country on Akron. If they have interstellar radio equipment, they must also have weapons by which an ideological dictatorship could establish itself over a whole world, as Communism nearly did here in the last century. The structure of the language, as well as various other hints, proves the Akronites are mentally quite humanlike, however odd they look physically. We just had the bad luck to contact them at the exact point in their history when they were governed by this crusading religion.”

The next word in the sentence from Aejae xliii, 3 which we are considering is ‘mchiruchin,’ an archaic word concerning whose meaning there was formerly some dispute. Fortunately, the advocates of the erroneous theory that it means ‘very similar’ have now been exterminated and the glorious truth that it means ‘quite similar’ is firmly established.”

     Strand leaned back in his swivel chair. His glum hostility was dissolving into bewilderment. “What’re you getting at? Look here, uh, Father, it’s physically impossible for us to change the situation on Akron—"
     “What d’ you mean?”
     “We can send a reply to those sermons.”
     “What?” Strand almost went over backward.
     “Other than scientific data, I mean. I assure you. Dr. Strand, all I want is a free scientific and cultural exchange with Mu Cassiopeiae.”
     The director reseated himself, leaned elbows on desk and stared at the priest. He wet his lips before saying: “What do you think we should do, then?”

“Why, break up their theocracy. What else? There’s no sin in that! My ecclesiastical superiors have approved my undertaking. They agree with me that the Akronist faith is so unreasonable it must be false, even for Akron.

“Maybe I got you wrong,” he said grudgingly. “But, uh, how do you propose to do this? Wouldn’t you have to try converting them to some other belief?”

“Impossible,” said Moriarty. “Let’s suppose we did transmit our Bible, the Summa, and a few similar books. The theocracy would suppress them at once, and probably cut off all contact with us.”

He grinned. “However,” he said, "in both the good and the bad senses of the word, casuistry is considered a Jesuit specialty.” He pulled the typescript he had been reading from his coat pocket. “I haven’t had a chance to study this latest document as carefully as I have the earlier ones, but it follows the typical pattern. For example, one is required ‘to beat drums at the rising of Nomo,’ which I gather is the third planet of the Ohio System. Since we don’t have any Nomo, being in fact the third planet of our system it might offhand seem as if , we’re damned. But the theocracy doesn’t believe that, or it wouldn’t bother with us. Instead, their theologians, studying the astronomical data we sent, have used pages and pages of hairsplitting logic to decide that for us Nomo is equivalent to Mars.”

“What of it?” asked Strand; but his eyes were kindling.

“Certain questions occur to me,” said Moriarty. “If I went up in a gravicar, I would see Mars rise sooner than would a person on the ground. None of the preachings we’ve received has explained which rising is to be considered official at a given longitude. A particularly devout worshipper nowadays could put an artificial satellite in such an orbit that Mars was always on its horizon. Then he could beat drums continuously, his whole life long. Would this gain him extra merit or would it not?”

“I don’t see where that matters,” said Strand.

“In itself, hardly. But it raises the whole question of the relative importance of ritual and faith. Which in turn leads to the question of faith versus works, one of the basic issues of the Reformation. As far as that goes, the schism between Catholic and Orthodox Churches in the early Middle Ages turned, in the last analysis, on one word in the Credo, filioque. Does the Holy Ghost proceed from the Father and the Son, or from the Father alone? You may think this is a trivial question, but to a person who really believes his religion it is not. Oceans of blood have been spilled because of that one word.

“Ah…returning to this sermon, though. I also wonder about the name ‘Nomo.’ The Akronite theologians conclude that in our case, Nomo means Mars. But this is based on the assumption that, by analogy with their own system, the next planet outward is meant. An assumption for which I can recall no justification in any of the scriptures they’ve sent us. Could it not be the next planet inward—Venus for us? But then their own ‘Nomo’ might originally have been Mu Cassiopeiae I, instead of III. In which case they’ve been damning themselves for centuries by celebrating the rising of the wrong planet!”

Strand pulled his jaw back up. “I take it, then,” he said huskily, “you want to—”

“To send them some arguments much more elaborately reasoned than these examples, which I’ve simply made up on the spot,” Moriarty answered. “I’ve studied the Akronist faith in detail… with two millennia of Christian disputation and haggling to guide me. I’ve prepared a little reply. It starts out fulsomely, thanking them for showing us the light and begging for further information on certain points which seem a trifle obscure. The rest of the message consists of quibbles, puzzles, and basic issues.”

“And you really think— How long would this take to transmit?”

“Oh, I should imagine about one continuous month. Then from time to time, as they occur to us, we can send further inquiries.”


(ed note: about 75 years later)

“Time sure passes. But I had to call you right away, Jim. Transmission from Akron resumed three hours ago.”

“What?” Moriarty glanced at the sky "What’s their news?”

“Plenty. They explained that the reason we haven’t received anything from them for a decade was that their equipment got wrecked in some of the fighting. But now things have quieted down. All those conflicting sects have been forced to reach a modus vivendi.

Apparently the suggestions we sent, incidental to our first disruptive questions seventy-five years ago—and based on our own experience— were helpful: separation of church and state, and so on. Now the scientists are free to communicate with us, uncontrolled by anyone else. They’re sure happy about that! The transition was painful, but three hundred years of stagnation on Akron have ended. They’ve got a huge backlog of data to give us. So if you want your geology straight off the tapes, you better hurry here. All the journals are going to be snowed under with our reports.”

From THE WORD TO SPACE by Poul Anderson (1960)

Memeweave: Threats and Other Dangers/Perversion Watch/Open Access
Classification: WHITE (General Access)
Encryption: None
Distribution: Everywhere (Bulk)
As received at: SystemArchiveHub-00 at Víëlle (Imperial Core)
Language: Eldraeic->Universal Syntax
From: 197th Perversion Response Board


Given the high levels of uninformed critical response to our advisory concerning handling potential refugees arriving sublight from regions within the existential threat zone of the Siofra Perversion, or Leviathan Consciousness as it is becoming popularly known, the Board now provides the following explication.

The present situation is an example of what eschatologists refer to as the basilisk-in-a-box problem. The nature of the mythological basilisk is that witnessing its gaze causes one to turn to stone, and the challenge therefore to determine if there is a basilisk within the box and what it is doing without suffering its gaze. The parallel to the Siofra Perversion’s communication-based merkwelt should be obvious: it won’t subsume you unless you alert it to your existence as “optimizable networked processing hardware” by communicating with it.

Your analogous challenge, therefore, is to determine whether the hypothetical lugger or slowship filled with refugees is in fact that, or is contaminated/a perversion expansion probe, without communicating with it – since if it is the latter and you communicate with it sufficiently to establish identity, you have just arranged your own subsumption – and unless people are subsequently rather more careful in re communicating with you, that of all locally networked systems and sophonts.

Currently, the best available method for doing this is based on the minimum-size thesis: i.e., that basilisk hacks, thought-viruses, and other forms of malware have a certain inherent complexity and as such there is a lower limit on the number of bits necessary to represent them. However, it should be emphasized that this limit is not computable (as this task requires a general constructive solution to the Halting Problem), although we have sound reason to believe that a single bit is safe.

This method, therefore, calls for the insertion of a diagnostician equipped with the best available fail-deadly protections and a single-bit isolated communications channel (i.e., tanglebit) into the hypothetical target, there to determine whether or not perversion is present therein, and to report a true/false result via the single-bit channel.

If we leave aside for the moment that:

(a) there is a practical difficulty of performing such an insertion far enough outside inhabited space as to avoid all possibility of overlooked automatic communications integration in the richly meshed network environment of an inhabited star system, without the use of clipper-class hardware on station that does not generally exist; and

(b) this method still gambles with the perversion having no means, whether ontotechnological or based in new physics, to accelerate its clock speed to a point which would allow it to bypass the fail-deadly protections and seize control of the single-bit channel before deadly failure completes.

The primary difficulty here is that each investigation requires not only a fully-trained forensic eschatologist, but one who is both:

(a) a Cilmínár professional, or worthy of equivalent fiduciary trust, and therefore unable to betray their clients’ interests even in the face of existential terror; and

(b) willing to deliberately hazard submitting a copy of themselves into a perversion, which is to say, for a subjective eternity of runtime at the mercy of an insane god.

(Regarding the latter, it may be useful at this time to review the ethical calculus of infinities and asymptotic infinities; we recommend On the Nonjustifiability of Hells: Infinite Punishments for Finite Crimes, Samiv Leiraval-ith-Liuvial, Imperial University of Calmiríë Press. Specifically, one should consider the mirror argument that there is no finite good, including the preservation of an arbitrarily large set of mind-states, which justifies its purchase at infinite price to the purchaser.)

Observe that a failure at any point in this process results in first you, and then your entire local civilization, having its brains eaten.

We are not monsters; we welcome any genuine innovation in this field which would permit the rescue of any unfortunate sophonts caught up in scenarios such as this. However, it is necessary that the safety of civilization and the preservation of those minds known to be intact and at hazard be our first priority.

As such, we trust these facts adequately explain our advisory recommendation that any sublight vessels emerging from the existential threat zone be destroyed at range by relativistic missile systems.

For the Board,

Gém Quandry, Eschatologist Excellence

From EVIDENCE by Alistair Young (2016)


This is a variant on biological warfare. Obviously if you took Terra and terraformed it to have the climate of Mars then the bulk of the population would die. However that would take thousands of years and be subject to constant sabotage by the inhabitants.

But an ecosystem is more fragile. In David Gerrold's series The War Against the Chtorr Terra has been invaded by an alien ecosystem, one far more evolutionarily advanced than the native one. In Philip E. High's No Truce With Terra, a small group of aliens teleport into Terra, throw some seeds and eggs from their ecology out onto the ground, and wait. The metal based ecosystem spreads like wildfire, threatening the entire planet.

The Web of Hercules

In James Blish's The Triumph of Time (fourth novel in the Cities in Flight series) the alien empire The Web of Hercules has spread from the Great Globular Cluster in Hercules to conquer the galaxy. They have a unique weapon used to kill planets.

The Triumph of Time

     He heard Miramon draw in his breath slightly to answer, but he was never to know what that answer would have been; for at the same moment, Miramon’s whole board came alive at once.
     “Hey!” Amalfi squalled. “Wait for orders down there, dammit!”
     “What do they mean?” Miramon said, trying to read every instrument on his board at once. “I thought I understood your language, Mayor Amalfi, but—”
     “The City Fathers don’t speak Okie, they speak Machine,” Amalfi said grimly. “What they mean is that the Web of Hercules—if that’s who it is—is coming in on us. And coming in on us fast.”
     With a single, circumscribed flip of his closed fingers, Miramon turned off the lights.
     Blackness. Then, seeping faintly over the windows around the tower, the air-glow of the zodiacal light; then, still later, the dim pinwheels of island universes. On Miramon’s board, there was a single spearpoint of yellow-orange which was only the heater of a vacuum tube smaller than an acorn; in this central gloom at the heart and birthplace of the universe, it was almost blinding. Amalfi had to turn his back on it to maintain the profound dark-adaptation that his vision needed to operate at all in the tower on his mountain.
     While he waited for his sight to come back, he wondered at the speed of Miramon’s reaction, and the motives behind it. Surely the Hevian could not believe that a set of pilot lights in a tower on top of a remote mountain could be bright enough to be seen from space; for that matter, blacking out even as large an object as a whole planet could serve no military purpose—it had been two millennia since any reasonably sophisticated enemy depended upon light alone to§see by. And where in Miramon’s whole lifetime could he have acquired the blackout reflex? It made no sense; yet Miramon had restored the blackout with all the trained positiveness of a boxer riding with a punch.
     When the light began to grow, he had his answer—and no time left to wonder how Miramon had anticipated it.
     It began as though the destruction of the inter-universal messenger were about to repeat itself in reverse, encompassing the whole of creation in the process. Crawls of greenish-yellow light were beginning to move high up in the Hevian sky, at first as ghostly as auroral traces, then with a purposeful writhing and brightening which seemed as horrifyingly like life as the copulation of a mass of green-gold nematode worms seen under phase-contrast lighting. Particle counters began to chatter on the board, and Hazleton jumped to monitor the cumulative readings.
     “Where is that stuff coming from—can you tell?” Amalfi said.
     “It seems to come from nearly a hundred discrete point-sources, surrounding us in a sphere with a diameter of about a light year,” Miramon said. He sounded preoccupied; he was doing something with controls whose purpose was unknown to Amalfi.
     “Hmm. Ships, without a doubt. Well, now we know where they get their name, anyhow. But what is it they’re using?”
     “That’s easy,” Hazleton said grimly. “It’s antimatter.”
     “How can that be?”
     “Look at the frequency analysis on this secondary radiation we’re getting, and you’ll see. Every one of those ships must be primarily a particle accelerator of prodigious size. They’re sending streams of stripped heavy antimatter atoms right down the gravitational ingeodesics toward us—that’s what makes the paths the stuff is following look so twisted. They’ve found a way to generate and project primary cosmics made of antimatter atoms, and in quantity. When they strike our atmosphere, both disintegrate—
     “And the planet gets a dose of high-energy gamma radiation,” Amalfi said. “And they must have known how to do it for a long time, since they’re named after the technique. Helleshin! What a way to conquer a planet! They can either sterilize the populace, or kill it off, at will, without ever even coming close to the place.”
     “We’ve had the sterility dose already,” Hazleton said quietly.
     “That can hardly matter now,” Estelle said, in an even softer voice.
     “The killing dose won’t matter either,” Hazleton said, “Radiation sickness takes months to develop, even when it’s going to be fatal.”
     “They could disable us quickly enough,” Amalfi said harshly. “We’ve got to stop this somehow. We need these last days!”

From The Triumph of Time by James Blish (1958)

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