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Excimer lasers, which use a different kind of chemical reaction, produce beams of short wavelengths that could destroy a missile by focusing on it for only a second or so. But the generating apparatus is so bulky that it could not be lifted into orbit; the laser stations would have to be placed on mountaintops to put them above the densest layers of the atmosphere. Even the thin upper layers would cause the beams to shimmer, however, owing to the same phenomenon that makes the light from stars appear to twinkle. The excimer laser beams would have to be bounced...
...ideal weapon? Not quite. In theory, X-ray lasers could be based in space, but that might mean keeping something like 1,400 atomic bombs in low orbits constantly crisscrossing the Soviet Union. Says Coll, rather delicately: "I don't think it's going to be politically acceptable to put bombs in orbit." In practice, the X-ray lasers would have to be launched from earth at the first warning of attack in a "pop-up" defense (they are in fact the only laser devices compact enough for such a defense). To get high enough fast enough, they would probably...
...four miles in circumference. A device that could accelerate the particles to perhaps half the speed of light, which would be poky by laser standards, but adequate for missile defense, might still weigh 500 tons, and hundreds if not thousands of the contraptions would have to be lifted into orbit. Particle beams have even more trouble penetrating the atmosphere than X rays, so they would be more useful for post-boost and mid-course interception than for boost-phase kills...
Boost-phase interception would be carried out by low-orbit satellites firing rockets out of pods. A rocket would accelerate to the vicinity of a rising missile, then release a homing vehicle that would be guided by sensors and thrusters to a head-on collision with the missile. But as many as 20,000 rockets orbiting aboard many hundreds of satellites might be required to keep enough within range of Soviet launch sites at all times to fend off a full- scale missile onslaught...
...fired into the upper atmosphere. Each rocket would release a swarm of so- called smart rocks--vehicles powered by little thrusters and guided by tiny sensors--to hit warheads and decoys in space. An alternative is to fire the smart rocks out of devices called rail guns placed in orbit. The rail guns use a burst of electric current to accelerate the smart rocks along a rail. One problem is sheer numbers: immense swarms of smart rocks would be needed to hit warheads and decoys indiscriminately. The other option, picking out the warheads from the decoys, would require rocks that...