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SDI, a Two-Edged Sword: Defense or Destruction?

December 28, 1986|PETER D. ZIMMERMAN | Peter D. Zimmerman, a physicist, is a senior associate at the Carnegie Endowment for International Peace.

In arguing his case for the Strategic Defense Initiative, President Reagan maintains that it would produce weapons that only destroy other weapons. Not so. Some of the technology works almost as well for offensive purposes. In fact, space-oriented strategic arms would be almost perfectly suited to strikes against population centers, or as instruments of coercion and destruction to be employed against smaller adversaries. And they would not add much to the cost of a defensive system.

The value of these weapons lies in power and position, classic ingredients for military success. From orbit above the Earth, almost any space-based missile defense system under consideration today could bring its weapons to bear on targets on the planet as well as in the atmosphere. Designs for the smallest laser defense systems call for at least 100 platforms, and the "smart rock" kinetic kill systems may have thousands of battle stations and tens of thousands of missiles.

Because the most vulnerable phase of ballistic missile flight is so short, only those satellites within range of the missiles when they are launched can participate in the battle. Consequently, very many lasers or missiles must be in orbit because each battle station would be in the right position for only a small portion of each day. Ten or 20 times as many lasers would be needed in the system as would be needed in battle, and perhaps 30 times as many rocket-launching anti-missile satellites because rockets are slow compared to the speed of light at which lasers deliver their punch.

How can the enormous energies of SDI weapons be transmitted through the atmosphere? For kinetic kill vehicles the answer is easy. It takes a rocket weighing about 450 pounds to power an 11-pound kill vehicle to the 16 miles-per-second speed needed to give it a chance to intercept and destroy a ballistic missile. But a slightly different 450-pound rocket could carry a payload of 200 pounds or more, if the objective were only to send a re-entry vehicle from orbit back down to Earth, where it would hit with the power of a large meteorite. Such a vehicle would be adequate for carrying a guidance system accurate enough to strike very small targets--aircraft, buildings, even early-warning radar antennas.

Many laser systems proposed for use in space cannot be used against targets on Earth. The wave lengths of their beams, their colors, are such that the light is absorbed high in the atmosphere. But there is promise in a "directed energy" anti-missile system; one such system is said to be the ground-based "free-electron laser," a combination of an accelerator, a sophisticated magnet assembly and a set of relay mirrors on the ground and in space. The beams are produced on the ground and are then sent up to the mirrors, which direct them at their targets. If the beams can go up through the atmosphere with minor losses and scattering, they can go back down again. Mathematicians call this a "self-proving theorem."

Space-based weapons and ground-based lasers with space-based relay mirrors offer both versatility and selectivity. A laser system that could handle 1,400 booster rockets--the number now in the Soviets' ICBM force--also could start more than 1 million fires over the course of 12 hours. (Only 6,400 fires break out each day in the United States.) An adversary could choose to concentrate his arson in densely populated areas, swamping the fire departments afflicted. Or lasers could be used to start brush fires in widely scattered areas. We know what one match can do during Southern California's dry season; imagine the effect of just 100 of the 1 million fires that a Soviet SDI system could set.

Such weapons could be used with pinpoint precision against oil tankers, storage depots, communications facilities--all manner of targets. In a guerrilla war they could be used to burn off jungle cover; in a future attack on Libya, Moammar Kadafi's tent could be targeted directly, with few or no other casualties, anywhere, and at no risk to the attacking forces. The drawback: These weapons also could destroy the bomber "leg" of our strategic triad while it's on the ground during the opening seconds of war; our airborne command posts might be vulnerable as well.

The strategic role of space-based multi-mission weapons is small in a world dominated by tens of thousands of nuclear warheads. But if most ballistic missiles and most nuclear weapons are eliminated at about the time SDI is deployed, the system's strategic offensive capabilities will appear far more important.

The offensive uses of space are not arguments for or against SDI; they are consequences of the technology, if the technology succeeds. In deciding whether to construct a system for strategic defense, the American people will have to take into account both edges of the SDI sword.

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