This section is for attacking a planet by ground assault. The prior section is for attacking a planet from orbit.
After all the defender's orbital fortresses have been neutralized, the final stage is entered. If the defender still resists, their planet is now the new target. The attacker will attempt to insert troops onto the planetary surface to set up a beachhead. Then the ground assault begins. The attacker will attempt to advance past the final defenses in order to loot, conquer, or destroy the defender's planet.
Because no matter how many bombs you dump onto a planet, you have not captured it until you've got an eighteen-year old holding a rifle with their boots firmly on the ground.
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.
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).
Ever since the 1950s when Project Paperclip scooped up all those German rocket scientists and brought them to the US, the military had been looking at innovative ways to use rockets. Not just in sending bombs and warheads to rain death and destruction on the enemy, but also in the equally important fields of logistics and transport. Being able to rocket resupplies to troops on the front was a powerful concept.
But what really made a gleem in the eyes of the army generals was the idea of using rockets to rapidly deliver troops to suprise the enemy. Which is relevant to the topic of deploying invading forces to a planet's surface.
Alas, while there were several projects looking into this, none of them worked very well, and all where cancelled. Mostly because they were the functional equivalent of turning a soldier into a human cannonball. These are ballistic missiles after all.
Apparently during World War 2, Werner von Braun made a sketch showing three passengers in the nose of a V-2 rocket. However, given the short range and the price of a V-2 rocket, this was not a a cost-effective solution.
In 1956 the US Army briefly looked into transporting troops using a Jupiter intermediate range ballistic missile. The payload was a capsule carrying 18 soldiers and their weapons. The range of 500 miles with an apogee of 130 mies. The capsule would land using a parachute and landing rockets.
The concept went nowhere.
1958 brought the advent of the US Army's Redstone rocket. This was an updated version of the German V-2. The Redstone was steadily improved until it became one of the most reliable rockets of the era. It was, however, a bone of contention between the US Army and US Air Force. On the subject of nuclear warfare, the Army favored tactical nuclear weapons delivered at short battlefield ranges. The Air Force, on the other hand, wanted strategic nuclear weapons delivered between continents in order to rapidly obliterate the Soviet Union's ability to wage war. The Army liked the Redstone because its maximum range of 323 km was usable near a battle. The Air Force wanted missiles with a minimum range of 5,500 km.
The short-lived Army Ballistic Missile Agency (ABMA) looked into using the Redstone to deliver cargo and personnel. This was the even more short lived Army Missile Transport Program (AMTP).
The ABMA came to the startling conclusion "The cost versus effectiveness of rocket transportation compared to fixed-wing aircraft transportation appears to demand that rocket transportation be substituted for the conventional aircraft transport system in the immediate future."
This received further support from the Transporation Corps Combat Development Group, which recognized the need for resupplying troops via missiles and recommended missile development.
Alas, the AMTP failed to gain any additional support and was quietly closed.
|ITHACUS Payload breakdown|
(x1200 @ 180 lb each)
(bulkheads and floors)
"Ithacus" 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. Not if you have to ride that contraption.
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, saving the four crew but abandoning the 1,200 troopers to their doom. For that reason crew module ejection 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. Bad for morale, that.
Ithacus did have emergency flotation balloons so it could abort to a water landing, with the balloons preventing the thing from capsizing or plunging down into Davy Jone's Locker. This would save the 4 crew and the 1,200 troopers. But if the only option is aborting 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 would be useful if Ithacus could be given an escort of space fighters capable of following its sub-orbital flight.
It also would be possible for Ithacus to launch into a low polar orbit and loiter there. This would make the range global. It would also wage psychological warfare on the enemy as they nervously watched the orbital Sword of Damocles overhead, jam packed with marines. Such an orbital launch would reduce the allowable payload but in certain circumstances may be worth it.
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, from a random patch of ground it would just blast a huge crater and topple over. In theory, if the landing site could be fully secured and if the landing site was reasonably 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" or "Capsule Troopers." 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.
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.
These can be machines that were specfically designed as planetary attack weapons, or they could be some sort of benign von Neumann that mutated into something dangerous.
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.
Children are taught "Sticks and stones may break my bones, but names will never hurt me." While this is true of school playground interactions, it may not hold in the world of communicating with aliens, i.e., SETI.
What can aliens do to harm us over a radio wave? Plenty.
They might attack individuals via transmitting a Medusa Weapon image. They might send blueprints for a device they claim will produce free energy but will actually turn half the planet into antimatter. It might even send instructions which will covertly create an alien agent here on the planet, such as in the science fiction story A for Andromeda.
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.
Before you scoff at this, you might want to look at the history of inadvertent rabbit plagues in Australia.