How to Zap an ICBM

After years of sophisticated underground nuclear tests in Nevada, U.S. weapons scientists are confident that they have finally conceived an anti-ballistic missile (ABM) system that can be effective. Sidestepping the complex problem of directly intercepting an attacking 17,000-m.p.h. intercontinental ballistic missile with a defensive missile --a feat equivalent to hitting a flying bullet with another bullet--they have designed a system that will use great bursts of X rays from exploding nuclear warheads to destroy enemy missiles at a distance.

To increase the intensity of X rays produced by a nuclear explosion, physicists can reduce the amount of uranium 238 in the outer layer of ABM warheads and add more tritium, which raises the temperature of the blast, to the fissionable material. As a result, nearly 80% of the energy released by the explosion of the new warheads, believed to be in the one-megaton range, is in the form of high-energy X rays. To extend the lethal range of these rays, which are quickly absorbed or attenuated when traveling through air, the ABM warhead will be carried high above the atmosphere by the new Spartan missile and exploded in space in the vicinity of incoming ICBMs.

Plasma Sheath. During the brief instant of the nuclear explosion (which lasts only five ten-millionths of a second), X rays traveling at the speed of light emanate from the center of the blast. Although their effect diminishes sharply at increasing distances even in the vacuum of space,* the X rays from a one-megaton blast are intense enough to destroy an ICBM caught within a sphere extending two miles from the exploding ABM warhead.

Striking the ICBM, X rays instantaneously ionize a thin layer of its outer casing, causing the formation of a sheath of hot gas, or plasma. But only a small portion of X-ray energy is used to form the plasma sheath. Most of the remainder is converted into a shock wave that races through the missile. At a distance of two miles, the impact of the shock wave on a 6 1/2-ft. dia. 30-megaton warhead would be equivalent to the explosion of 2 or 3 Ibs. of TNT within the missile, which may be enough to set off some of the lens-shaped charges of conventional explosives inside (see diagram). These, in turn, would cause the remaining lens-shaped explosives to detonate. Because all of the conventional charges would not explode simultaneously, as they are designed to do, the resulting implosion would not be uniform enough to start a critical reaction in the core of the nuclear device; it would simply damage the warhead and turn it into a dud.

Slow Fission. Even if the shock wave fails to set off the warhead's conventional explosive, it can damage electronic components or cause sufficient changes in the critical shape of internal cavities within the warhead to prevent a nuclear explosion. In addition, the heating of the ICBM's exterior may so damage its heat shield that the missile would burn up upon entering the atmosphere.

Neutrons produced by the ABM blast could also cause crippling damage at a range of about two miles. Penetrating into the ICBM's outer shell of uranium 238, they can produce slow fission, causing heat that may deform the warhead or set off its lens charges. The neutrons may also whiz into the warhead's core of uranium 235, causing it to explode in a premature nuclear blast while still hundreds of miles from its target.

Because ABM-produced X rays and neutrons could sweep such large segments of the skies clear of threatening ICBMs, defense planners believe a relatively small number of Spartan missile batteries--costing a total of $4 billion--could defend the entire continental U.S. against the kind of primitive missile attack that China may well be able to launch by the mid-1970s. They could also provide protection against a few Soviet ICBMs that might be launched accidentally.

But even the installation of many Spartan batteries--backed up by smaller and faster Sprint missiles for short-range interception of ICBMs that penetrate the X-ray curtain--would not provide sufficient protection against a determined and massive attack by the Soviet Union. Using shielding, decoys, multiple and maneuverable warheads and radar-jamming chaff or nuclear explosions, the Russians could confuse and overwhelm U.S. defenses--just as the U.S. could overcome theirs.

* An ICBM two miles from the blast will receive only one-quarter of the X-ray energy that hits a missile one mile away. At a distance of three miles, the impacting X-ray energy will be only one-ninth as large.

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