A `star wars' primer

YOU will be reading and hearing more about ``star wars'' during the coming days. It is the headline news subject for the talks in Geneva next week between US Secretary of State George Shultz and Soviet Foreign Minister Andrei Gromyko. The Soviets want the United States to give up the ``star wars'' project. President Reagan is reluctant to do so. The White House has been saying that the project is negotiable one day, and nonnegotiable the next. No one can be sure whether the subject is negotiable, or at what price. Here, for the benefit of the layman, is some information about the matter.

Technically, the name for the project is Strategic Defense Initiative, or SDI for short. It means spending about $25 billion over the next five years for research and development, or devices which might someday make it possible to set up a system for intercepting enemy missiles and destroying them in the air or in space during their flight from launcher to target.

The main elements of the system would be sensors capable of detecting and locating an incoming missile from moment of launch and throughout its entire flight. These would target and guide defense interceptor missiles. With enough sensors, guidance systems, and interceptors the incoming missiles could be destroyed harmlessly.

To be effective the system would require powerful information processing equipment, precision guidance, high velocity interceptors, and both land- and space-based platforms for the equipment and the interceptors.

A complete SDI system would consist of several layers of defense. The first layer would aim at the incoming missile in its initial boost phase before it launches its MIRVs (multiple independently targeted reentry vehicles -- i.e., warheads). A second layer would aim at the enemy missile at the peak of its trajectory as it turns over and begins the MIRV launch. A third layer would aim at the warheads during their descent.

Supporters of the project think that with adequate research and development it might be possible to begin deployment of the system within 20 or 25 years. Others say 30 or more. Many technical experts doubt that it would ever work well enough to justify the cost.

There is no firm estimate of ultimate cost. Most guessing is that it would run into the hundreds of billions of dollars.

But even the most enthusiastic do not claim that the system could protect all US cities. The enthusiasts like to think that a multiple-layered system could achieve 95 percent, or conceivably even 99 percent accuracy. But the Soviets have about 8,000 warheads. If 1 percent got through the defense system, there would still be 80 warheads landing on US cities. That would be more than enough to wipe out all major cities. The more plausible estimate is about 95 percent accuracy, meaning 400 warheads getting through.

The R&D could have more limited uses. Most experts agree that a successful defense could probably be set up for the presidential command center in the Maryland mountains west of Washington, for various military command centers, and for some of the land-based ICBMs.

Any SDI would of course mean repudiating the ABM (anti-ballistic missile) agreement with the Soviets and thus clearing the way for them to do the same.

Among experts there is a tendency to assume that SDI will end up with limited defense systems for command centers and ICBMs.

Besides, what would be the use of spending vast sums of money on a system which could knock out long-range intercontinental missiles when the Soviets could get underneath the defense umbrella by moving their strategic submarines near the coasts and fitting them with shorter-range and low-altitude weapons, such as cruise missiles?

The SDI is a politician's dream project. It sounds wonderful and is bound to be popular. And if it doesn't work, no one will know for at least 20 years.