First in a sporadic series.
A traditional assumption in science fiction is that ships in orbit have an enormous advantage over enemies on the ground. Heinlein, evoking the Hornblower-verse, called it the gravity gauge. In general the advantages of high ground are obvious, and the image of fighting from the top of a gravity well versus fighting from the bottom is vivid. But it is also very likely wrong.
In the rocketpunk era of the 1950s there was little reason to think so. The development of bombers up to that time emphasized speed and altitude, from the B-17 and its British counterparts to the pressurized B-29, then the jet powered B-47, B-52, and V-bombers, then the supersonic B-58 and Mach 3 B-70. But the B-70 ended up getting canceled, and by the time 'Dr. Strangelove' came out in 1964 the B-52s were flying near treetop level. 'Under the radar' has now become a common metaphor.
To be sure, the B-70 fell victim not just to improvements in Soviet air defense but to competition from ICBMs that flew much higher and faster still. Even after four decades of effort and enormous expense, strategic missile defense remains uncertain at best. But this is largely a matter of short warning times, compact warheads, and most of all the conundrum that strategic nuclear defense is ineffectual unless it is perfect.
Large spacecraft arriving from deep space (such as an interplanetary armada) are quite another matter. They are much bigger targets, perhaps 100-1000 times the cross section of a warhead, and their approach track will probably be known long in advance. Once they arrive they have nowhere to hide and nothing to take cover behind. In a rock throwing contest at the gravity well, holding the high ground means no concealment, while the low ground means being able to take cover in the underbrush.
Not even the gravity well itself, as it turns out, offers much advantage. Sure, an orbiting ship can easily throw rocks at the surface, the rocks needing only a small kick to send them down from orbit. But by the standards of space tech, throwing rocks up to orbital altitude is also not very hard. Simple, single-stage boosters in the V-2 / SCUD class will do the job nicely. You'll still need a target seeker in order to hit anything, but sending it up there is not a big problem.
Nor is delivering a punch – the target's own kinetic energy will see to that. Suppose the target seeker is hanging like a popup fly, stationary relative to the Earth at the moment of impact. The target runs into it at, say, typical low orbit speed of 7.8 km/s. As Sancho Panza said in the musical play and film Man of La Mancha, 'whether the pitcher hits the rock or the rock hits the pitcher, it is going to be bad for the pitcher.' Each kilogram of impactor delivers a kinetic whack of about 30.5 megajoules, equivalent to rather more than 7 kg of TNT. (In some circles, I am honored to say, this has come to be known as 7 ricks.)
The target ship can engage the (relatively) oncoming missile, but it has only a short window, perhaps two or three minutes from launch to intercept. What really tilts the contest in favor of the side on the ground, however, is that as US Navy doctrine says, it is better to shoot the archers, not the arrows – and the archers can conceal themselves amid a planetful of ground clutter. Surface-to-space missiles can be carried by stealth aircraft or by submarines, or they can be launched from the backs of trucks. In this last case, even if the spacecraft engages and destroys the 'archer' once revealed by launch, all it has achieved is to blow up a truck. The planetary defenders' tracking system can use passive scan, hard to detect, and the command and control system is even harder to detect.
Some of these factors apply most strongly against ships in low orbit. Against ships in higher orbit, surface-to-space missiles – in contemporary usage ASATS, anti-satellite missiles – must be larger and more expensive, take longer to reach their targets, and have lower relative impact velocities. Hitting a target at an orbital altitude of one Earth radius, about 6400 km, requires an ICBM class booster, and the target will have about half an hour to engage it (or take evasive action). If the target ship is in high orbit, hitting it requires the equivalent of a deep space mission. But the ship is also farther away from any surface target it might be intending to attack.
If beam weapons are available, some of these calculations change. The ship can use lasers (or whatever) against missiles coming up from the surface. But the defender can also fire beams from the surface, and photons are untroubled by any gravity well short of a black hole. Laser cannons are less expendable than a truck launcher, but surface-based lasers are still likely to be much cheaper than similar lasers in space that need a spacecraft to carry them. Power supply and waste heat disposal are also easier to arrange on the surface than in space.
Yes, for story purposes you can handwave all this as needed. It is also true that if the attacker's objective is sheer devastation, nuclear bombardment from high orbit will do the job. (So will the ever popular asteroid toss, but this is just a Rube Goldberg way to get the same effect.) The technical possibility of overcoming enormous attrition rates by sheer overkill is a general fact of life in the nuclear age.
Nevertheless, the general upshot – so to speak – of the shortage of ground cover in space is that the presumed high-ground advantage in a gravity well fight pretty much evaporates. Instead, space forces engaging a planetary surface defense face disadvantages comparable to those historically faced by naval forces engaging coastal batteries. They are both exposed and vulnerable, far more so than the defenders. Their one strategic advantage is that they can change their mind and leave, an option not open to surface/shore defenses. Hostile space forces can also plausibly blockade a planet from high orbit, 'distant, storm-beaten ships' cutting it off from interplanetary commerce. But getting close to a hostile planet (or even one just partly hostile) places spacecraft at a severe disadvantage.
Unlike most of the issues we'll take up when we discuss deep space warfare, this has fairly immediate practical implications. Surface-to-orbit warfare disadvantages all spacefaring powers, because space access is easier denied than defended. Second and third-tier powers, with no space presence themselves, can hold space hostage by threatening orbital installations and departing or returning orbiters. The one piece of good news here is that while SCUD class boosters are already all too proliferated, this is not the case for orbital target seekers capable of zeroing in to hit a ship. Moreover, such target seekers cannot effectively be developed in secret, because they can only be reliably tested in space.
Which is why I did not start this piece with a pro forma objection to militarizing space. For anyone interested in real world(s) spacefaring, the objection is not pro forma; it is eminently (and imminently) practical. All spacefaring powers, and those that wish to become spacefaring, have a shared interest in a treaty regime that discourages development and testing of ASATS that could end up denying space to everyone. Whether this will actually happen is of course anyone's guess, but the fact that some such treaty is in the interest of all major players cannot hurt its prospects.
Related link: I shoot down space fighters here.
Sunday, June 7, 2009
First in a sporadic series.