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.
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 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 poka-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".
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.
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)
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".
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.
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.
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|
|Blast||40% to 50%|
|Thermal Radiation||30% to 50%|
(unless this is a neutron bomb)
|5% to 10%|
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
- 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)
|Effects||Explosive yield / detonation height|
|1 kt / 200 m||20 kt / 540 m||1 Mt / 2.0 km||20 Mt / 5.4 km|
|Thermal radiation—ground range (km)|
|Third degree burns||0.6||2.5||12||38|
|Second degree burns||0.8||3.2||15||44|
|First degree burns||1.1||4.2||19||53|
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).
|20 psi||Heavily built concrete buildings are severely damaged or demolished.|
|10 psi||Reinforced concrete buildings are severely damaged or demolished.|
Small wood and brick residences destroyed.
Most people are killed.
|5 psi||Unreinforced brick and wood houses destroyed.|
Heavier construction severely damaged.
Injuries are universal, fatalities are widespread.
|3 psi||Residential structures collapse.|
Serious injuries are common, fatalities may occur.
|1 psi||Light 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 overpressure||Maximum Wind Velocity|
|50 psi||934 mph|
|20 psi||502 mph|
|10 psi||294 mph|
|5 psi||163 mph|
|2 psi||70 mph|
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)
|Effects||Explosive yield / detonation height|
|1 kt / 200 m||20 kt / 540 m||1 Mt / 2.0 km||20 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|
(values for other than 20 kt are extrapolated
using the cube-root scaling)
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.
|Effects||Explosive yield / detonation height|
|1 kt / 200 m||20 kt / 540 m||1 Mt / 2.0 km||20 Mt / 5.4 km|
|Effects of instant nuclear radiation—slant range (km)|
|Lethal total dose (neutrons and gamma rays)||0.8||1.4||2.3||4.7|
|Total dose for acute radiation syndrome||1.2||1.8||2.9||5.4|
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.
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.
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".
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 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.
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.
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.
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.
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.
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.
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.
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 relative.
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 12and 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) 100 150 200 250 300 500 750 1000 Rise Time (sec) 142.8 174.9 201.9 225.8 247.3 319.3 391.0 451.5 View Range (km) 1,122.1 1,369.9 1,576.8 1,757.3 1,919.0 2,447.0 2,952.1 3,359.4 View Time (sec) 145.3 179.4 208.9 235.5 260.1 346.6 441.2 528.7 Clear Rise Time (sec) 71.4 123.7 159.6 188.9 214.2 294.4 371.0 434.3 Clear View Range (km) 567.1 979.1 1,260.0 1,486.2 1,679.9 2,280.4 2,831.0 3,266.1 Clear View Time (sec) 72.6 126.8 165.0 196.8 225.0 319.3 418.2 508.1
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) 100 100 100 150 150 150 200 200 Missile Apogee (km) 150 200 250 200 300 500 400 800 Rise Time (sec) 73.9 59.1 50.9 101.0 72.4 52.2 83.6 54.1 View Range (km) 1,122.1 1,122.1 1,122.1 1,369.9 1,369.9 1,369.9 1,576.8 1,576.8 View Time (sec) 145.3 145.3 145.3 179.4 179.4 179.4 208.9 208.9 Clear Rise Time (sec) 22.7 16.9 14.0 58.7 39.3 27.2 55.5 34.7 Clear View Range (km) 567.1 567.1 567.1 979.1 979.1 979.1 1,260.0 1,260.0 Clear View Time (sec) 72.3 72.3 72.3 125.3 125.3 125.3 161.9 161.9 Vertical Velocity (km/s) 0.990 1.401 1.716 0.990 1.716 2.620 1.981 3.431 Impact Energy Factor 1.02 1.03 1.05 1.02 1.05 1.11 1.06 1.19 Altitude (km) 300 300 500 500 750 750 1000 1000 Missile Apogee (km) 400 600 750 1000 1000 1500 1500 2500 Rise Time (sec) 142.8 102.4 165.3 132.2 225.8 162.0 233.7 160.9 View Range (km) 1,919.0 1,919.0 2,447.0 2,447.0 2,952.1 2,952.1 3,359.4 3,359.4 View Time (sec) 260.1 260.1 346.6 346.6 441.2 441.2 528.7 528.7 Clear Rise Time (sec) 114.6 79.9 145.2 115.0 208.5 148.0 219.7 150.1 Clear View Range (km) 1,679.9 1,679.9 2,280.4 2,280.4 2,831.0 2,831.0 3,266.1 3,266.1 Clear View Time (sec) 217.4 217.4 299.6 299.6 378.6 378.6 444.4 444.4 Vertical Velocity (km/s) 1.401 2.426 2.215 3.132 2.215 3.836 3.132 5.425 Impact Energy Factor 1.03 1.10 1.08 1.17 1.09 1.26 1.18 1.54
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.
The final stage is usually landing your invading army on the ground to capture key targets and force the planet to surrender.
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)
(bulkheads and floors)
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).
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.
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.
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 Type Payload
CD CL BC 1-man ballistic 200 310 3 1.4 0 73.81 1-man lifting 200 350 3 1.3 0.5 89.74 1-man winged 200 1,000 5.5 0.25 0.3 727.27 12-man ballistic 2,500 3,850 18 1.4 0 152.78 12-man lifting 2,500 4,350 20 1.3 0.5 167.31 12-man winged 2,500 11,000 30 0.25 0.3 1466.67 HMMWV ballistic 4,500 6,800 30 1.4 0 161.90 HMMWV lifting 4,500 7,800 32 1.3 0.5 187.50 HMMWV winged 4,500 19,000 56 0.25 0.3 1357.14 Stryker ballistic 18,500 27,750 60 1.4 0 330.36 Stryker lifting 18,500 32,000 64 1.3 0.5 384.62 Stryker winged 18,500 77,500 125 0.25 0.3 2480.00
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.
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.
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.
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 2in 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.
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.
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".
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:
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.
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.
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.
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.