A buddy of mine from the air biz used to work at Lawrence Livermore Labs, and he was once at a luncheon where Edward Teller was holding forth. Since there were several atmospheric scientists at that particular lunch, at one point Teller speculated on whether it would be possible to set up a series of nuclear explosions that would cause atmospheric particulates to precipitate out of the air.
My friend was a little nonplussed, because this was a truly loony idea. But after thinking about it for a while, he chalked it up to Teller having a little fun with his own reputation. He had, after all, basically invented the thermonuclear bomb, and had then spent much of his remaining career overseeing its refinement, and looking for some place to use it. From proposed massive canal building projects to attempts to get more natural gas out of geological formations, Teller always had that single tool that he was trying to use: the H-bomb.
Later, when we all heard about the Teller’s backing of the Strategic Defense Initiative (called “Star Wars” in the popular press), some of us immediately wondered, “Where’s the bomb?”
We learned soon enough about the proposed X-ray (or gamma ray) laser, which was supposed to be pumped by a thermonuclear explosion, so there you are and bob’s your uncle. As I've noted in another essay, I didn’t expect that to work, for technical reasons, and it didn’t.
SDI did not die with the gamma laser failure, however. We’ve had various debates about the feasibility of “hitting a bullet with a bullet” vs “smart rocks” or “brilliant pebbles,” (or “sentient sand” for all I know). In any case, there’s really no idea so lame that a DOD bureaucracy won’t champion it, but there are some things that generally don’t get said, so I’m going to say them here.
The fact is that there are certain paths of least resistance in engineering. Some ideas, no matter their soundness or unsoundness, will never happen, because something else that is technically easier will happen first. It’s important to know what it is that will happen first.
A ballistic missile’s brief career is divided into several important phases: launch, boost, ballistic, re-entry, target, then boom. There were actually some studies in the early 1970s, during the ABM (anti-ballistic missile) debate, that suggested that it might barely be possible to stop a single bomb in the near-target phase, using what is essentially massive anti-aircraft fire, putting a more or less continuous shroud of shrapnel as an umbrella near the target. No one really thought of this as a good solution, for several thousand reasons, including the fact that it would only work on one bomb, and an early trigger would then blow all your anti-aircraft weapons to hell and gone.
Similarly, despite the PR graphics of SDI as a “shield,” there was never much intention to try to get at ballistic weapons in the re-entry phase, not least being that a single thermonuclear explosion at the edge of the atmosphere creates a good sized EMP pulse that will then blind subsequent defense radar.
That argument also applies to defensive measures during the entire ballistic phase, when the warheads are outside of the atmosphere in free-fall. But there’s actually a worse problem in the ballistic phase, camouflage.
In the absence of an atmosphere, anything, no matter how lightweight, follows a ballistic trajectory. It is very easy, therefore, to create decoys, simple balloons with the same radar signature as the warhead. In fact, you can put a balloon around the warhead and make it look exactly like the balloon. Since the balloon/decoys weigh only a few ounces, you can put hundreds of them in the same ballistic trajectory as your warhead, turning the problem from “hitting a bullet with a bullet” to “shooting a needle in a haystack.”
So no one really expects to take out a warhead during the ballistic phase. That leaves us with launch and boost. Launch is over in a few seconds, so the real development work is on stopping missiles in the boost phase, when they are conveniently located far away from the target (us) and near the launch site (them).
But how? First you have to sense the launch, then find the missile, then target it, then put something near to it, then kill it. That implies a really good sensor network, plus the ability to put your kill vehicle near the target very quickly.
The sensor network is easy, or, more accurately, it’s so difficult that there’s really only one way to do it, and that must be space-based. You need orbiting infrared sensors to see the launch, then something akin to radar to track it. The radar will need to be close to the boost, and that too is almost necessarily space-based. There have been arguments about “pop up” systems, but those are mostly red herrings; it’s a lot easier to do it from space.
Likewise, it’s a lot easier to target a high velocity vehicle with something that starts off at high velocity. If your initial sensor is in space, and your radar net is in space, the same arguments tell you that your kill vehicle needs to be from space.
Along this development pathway, as your identification and tracking systems get better and better, there will come a time when only the most effective type of kill vehicle will work. You can talk all you want about “brilliant pebbles,” and “kinetic kill” vehicles, but nothing beats a nuke for destruction at a distance. At high altitudes, the energy from a nuclear weapon is primarily in the form of hard X-rays, with an attendant electromagnetic pulse. The hard X-rays can melt or crack a warhead by uneven heating, and a nuke doesn’t really care how many decoys you put up, it’s going to blow them all away. The electromagnetic pulse will probably even wreck any putative missile guidance system from a much greater range.
So let me be very blunt here. There’s nothing secret about any of this. It is the inevitable result of any feasibility analysis. SDI is about putting nuclear weapons in orbit. It always has been.