Exploring the High-Tech Frontier

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The basic idea of Star Wars is a "layered" defense that would attack the warheads throughout their flight, destroying more at each stage. No more than, say, 70% might be wiped out during any one phase, but in the end perhaps only 10% or less of the warheads launched would explode on or over their targets. That would cause mass loss of life, but could leave most U.S. retaliatory capacity intact. Boost-phase interception, however, is generally considered to be the key to an even partially effective defense. That is when the enemy projectiles are easiest to find: the intense heat of a missile's rocket thrusters, concedes the anti-S.D.I. Union of Concerned Scientists, makes it stand out "like a firefly in a darkened room." That is also when a missile defense is most efficient: a single hit, by a laser beam, for instance, can destroy ten warheads at once. In post-boost and mid-course phases, the separated warheads are vastly more difficult to find and distinguish from decoys. On re-entry, the decoys burn up, and only the warheads continue to plunge through the atmosphere. But if there are, say, 5,000 left out of an original launch of 10,000, they could easily overwhelm any conceivable "terminal" defense. Besides, by then it might be too late to prevent terrible damage. Warheads can be set to detonate if struck by interceptor rockets or projectiles, and though hardened missile silos are almost impervious to anything except a direct hit, nuclear explosions even in the upper atmosphere can rain ghastly destruction on cities.

Weapons that could attack the warheads differ somewhat by stage of flight, but--and this is a faintly cheering thought to Star Wars researchers--most are $ adaptable to more than one phase. The systems that could zap missiles in boost generally could also hit warheads in post-boost, mid-course and perhaps even re-entry phases. A rundown of the potential missile and warhead killers that are getting the most attention from scientists:

LASERS. They are devices that generate high-powered, concentrated beams of light, almost perfectly parallel and of a single wavelength. The light from a lamp, in contrast, is a fuzzy discharge wiggling at different wavelengths and scattering in every direction. Laser beams travel at the speed of light (not surprising: they are light, though not always visible) and can be focused over thousands of miles of space to burn a hole in the skin of a Soviet missile, destroying its guidance mechanism and deactivating its warheads.