This section is for defending a planet from orbit. The next section is for attacking a planet by ground assault.
After all the interplanetary battles are over, and the defender's space fleets have been reduced to ionized plasma or fled in panic, the pendultimate stage is entered. The defenders orbital and planetary fortresses have to be neutralized, or at least neutralized enough so that ground troops can be inserted to set up a beachhead.
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")
When it comes to defending a space colony instead of a planet, you can attach weapons and defenses so the thing becomes sort of an orbital fortress that people live inside. On Babylon 5 this was called the "defense grid".
In Long Shot for Rosinante, Alexis Gilliland points out there are military implications. If you have several unarmed space colonies you will need a fleet of warships roving around to defend the colonies. If the colonies are then retrofitted with weapons so they can defend themselves, oh my! You suddenly have a spare fleet of warships that can be repurposed to military adventurism! Enemy colonies will become alarmed.
However the politicians of the nation which established the colonies may be unenthusiastic about the idea. For one thing it makes it harder to collect taxes from the colonies, since they can shoot at the tax collectors. It also makes it harder for the politicans to control the military, since the latter will be eager for some delicious military adventurism. Regardless of what those cowardly civilian politicans back home have to say.
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.
A variant on the orbital fortress is the Space Superiority Platform. Instead of defending the planet from invading spacefleets, this is an armed military station keeping an eye on the planet it is orbiting.
If a planet is balkanized, the platform will watch military ground units belonging to hostile nations, and bombard them if required. Militarily they have the high ground.
If the planet is a conquered one, or the government is oppressing the inhabitants, the platform will try to maintain government control and deal with revolts. By bombarding them if required.
Many early SF stories fret about the military advantage an armed space station confer upon the owning nation. Heinlein says trying to fight a space station (or orbiting spacecraft) from the ground is akin to a man at the bottom of a well conducting a rock-throwing fight with somebody at the top. One power-crazed dictator with a nuclear bomb armed station could rule the world! Space faring nations would need space scouts for defense.
But most experts nowadays say that turns out not to be the case. A nation can threaten another with nuclear annihilation far more cheaply with a few ICBMs, no station is required. And while ground launching sites can hide in rugged terrain, a space station can hide nowhere. Pretty much the entire facing hemisphere can attack the station with missiles, laser weapons, and propaganda.
Phil Shanton points out that you don't need a huge missile to destroy an orbiting space station, either. In 1979, the U.S. Air Force awarded a contract to the Vought company to develop an anti-satellite missile. It was not a huge missile from a large launch site. It was a relatively small missile launched by an F-15 Eagle interceptor in a zoom-climb. Vought developed the ASM-135 Anti-Satellite Missile (ASAT), and on 13 September 1985 it successfully destroyed the solar observatory satellite "P78-1". This means that an evil-dictator world-dominator nuke-station not only has to worry about every ground launch site, but also every single fighter aircraft.
It has also been modeled that the U.S. Navy could take out a satellite with a Standard Missile 3.
Things are different, of course if the situation is an extraplanetary fleet that remotely bombs the planet to destroy all the infrastructure. The fleet can construct a space superiority platform while the planet is struggling to rebuild its industrial base. Then the platform can bomb any planetary site that is getting too advanced in rebuilding. This is known as "not letting the weeds grow too tall.
- A planet might be invested, meaning that the planet is under siege from whoever owns the space station. The station does not want planetary inhabitants escaping, nor does it want blockade runners entering.
- A planet might be interdicted because they contain something very dangerous (Xenomorphs, thionite, the City on the Edge of Forever, replicators, or 100% lethal plagues).
- A planet might be interdicted because it has something very valuable and the station owner does not want poachers sneaking in and stealing any.
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 many space operas, invaders will deal with planetary fortresses under a force field by constantly using weapons on the ground around the fort. The land will eventually become a sea of lava, which will put the fort at a disadvantage.
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 the inhabitants of the planet lack handwavium force fields, and the besiegers have no shortage of bombardment weapons, sooner or later the planet people are going to have to resort to living underground. The more frightful the bombardment, the deeper they will have to dig.
This is why in the real world the US NORAD Cheyenne Mountain Complex is built under six hundred meters of solid granite. Proof against EMP, and the blast doors are rated to withstand a 30 megaton thermonuclear explosion as close as two kilometers.
In the 1955 movie This Island Earth, the planets Metaluna and Zagon are at war. Metaluna's surface has been laid to waste by Zagonian asteroid bombardment. The Metaluna's cities are now all underground, but even then they are planning to seize and relocate to Planet Terra before Metaluna becomes totally uninhabitable.
In the 1974 anime series Space Battleship Yamato the warlike Gamilas have devastated Terra by a continual asteroid bombardment. But these asteroids are radioactive. Even though the people of Terra have retreated underground, the radiation is slowly seeping downward. The people of Terra have about one year left to live before the radiation kills them. Lucky for them they receive help from a certain Queen Starsha of the planet Iscandar in the Greater Magellanic Cloud.
The queen offers a wonderful machine that will cleanse Terra of all the radiation. The catch is that the Terrans have to travel to the GMC go fetch it. Starsha hopefully gives them blueprints for a faster-than-light-drive/unreasonably-powerful-Kzinti-Lesson, but the Terrans have to build the drive and the ship to house it.
Lucky for them, the asteroid bombardment has conveniently evaporated the seven seas, exposing all the historical shipwrecks in general, and the wreck of the 1940 battleship Yamato in particular. There is not enough underground space to build a huge spacecraft, but the Yamato is reasonably large and strong enough to be converted. The Terran secretly tunnel underneath it and covertly rebuilt it, so as to not tip off the Gamilas.
In addition to large planetary forts, there may be scattered anti-spacecraft weapons sited all over the planet. The main difference is these have no real protection except being very good at hiding. Instead of armor or magic force fields, they are either one-shot sacrificial weapons or capable of frantically scuttling away after they give away their position. Or they are weaponized spacecraft launching facilities that the enemy wants or needs to capture intact so they are loath to damage it.
Because as soon as a ground (or sea) based gun opens fire on an enemy ship in orbit, the enemy is going to plaster the entire area with ortillery.
If you have sufficient stealth technology, it might be a good idea to put some planetary defensive weapons inside submarines. This made good sense back in the days of Mutual Assured Destruction, but nowadays orbital observation satellites have made it much harder for submarines to hide. Be aware though that their stealth is destroyed the instant they fire their weapons, and the attackers in orbit will lob a nuclear depth-charge that will crush the submarine like an eggshell. US Navy Ohio-class submarines carry 24 missiles, a planetary defense submarine would probably be carrying a similar amount. The PD sub would be well-advised to launch all of its missiles at once, and preferably the sub should be a remotely controlled drone.
In the 1955 Operation Wigwam test, the US military discovered that a 30 kiloton nuclear depth charge could kill a modern submarine with a radius of a bit more than a mile.
Like planetary fortresses, surface defences are at a disadvantage with respect to hostile spacecraft in orbit due to the gravity gauge.
Rick Robinson is of the opinion that the gravity gauge is not quite as one-sided as it appears. In an essay entitled Space Warfare I - The Gravity Well he makes his case. The main point is that the orbiting invading spacecraft have nowhere to hide, while the defending ground units can hide in the underbrush.
Of course it is a bit easier to inflict damage on orbital person now that lasers have been invented. Keep in mind that if the planet in question has an atmosphere similar to Terra some laser wavelengths should be avoided.
And keep in mind that the defender's anti-orbit rocket also does not need a warhead, a bursting charge surrounded by nails and other shrapnel will do. The relative velocity between the more or less stationary cloud of shrapnel and the orbital speed of orbital person will do the rest. Orbit person will be riddled by shrapnel traveling at about 27,500 kilometers per hour (7,640 m/s) relative.
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.