The MX debate (continued)

In trying to thread one's way through all the complexities of the arguments about the MX there is one particular fact to be borne in mind.

There has of late been an enormous improvement in the technology for aiming a modern, lethal missile hurtling down through the air.

Back in the early 1970s any such missile, either American or Soviet, was considered to be fairly accurate if it had an even 50-50 chance of falling within 3,000 feet of its target.

During the balance of the decade first the United States, then the Soviets, managed to improve their guidance systems until they could expect to put half of their shots within 600 feet of a target.

Soviet SS-18 and SS-19 and the US Minuteman III missiles are all said to be able to get within the 600-foot range of target, half the time.

There are various versions of the latest guidance systems. Some work from satellites. Some work on star fixes. The assumption is that improvement in guidance technology will mean that the individual warheads mounted on an MX missile will be able to do even better. In theory, they will be so accurate as to be able to hit so close to a missile silo that no amount of ''hardening'' of the silo could save it.

Note at this point that in the early 1970s both the US and the Soviet Union were mounting nuclear warheads with blast yield measured in magatons. One megaton means the equivalent of 1 million tons of TNT. The US Titan II missile is rated at a yield of nine megatons. The farther your shot is expected to land from the target the more blast needed to affect the target. But if your shot can get within a few feet of the target a mere few hundred kilotons (a kiloton blast is the equivalent of a thousand tons of TNT blast) is sufficient to take out a missile in its silo.

The MX is designed to lift 10 warheads. Each one of these will have an estimated yield of 550 kilotons.

The MX should be ready for deployment and become operational sometime around 1988 or 1989.

At present the US Navy's best submarine-launched missile is the Trident II with an accuracy rating of 1,200 feet. But the Navy believes that well before 1989 it will have a Trident III missile with the same accuracy rating as the MX. With that kind of accuracy the Trident's 100-kiloton warhead should be enough to take out an enemy silo.

The Trident missile has a range of some 10,000 miles. The Trident submarine has 24 firing tubes. Each tube has a missile with eight independently targetable warheads. Thus one Trident submarine can launch 192 ballistic warheads on 192 different enemy targets from a distance of 10,000 miles.

The US now has 33 missile-firing submarines in its fleet. The present fleet can launch 4,768 warheads. The US submarines are the least vulnerable conveyor in existence today for deterrent weapons. The Soviets are not able to locate and track them at sea. The Soviets have 1,040 warheads aboard their smaller fleet of strategic submarines. The US Navy fixes on them as they come out of port and tracks them at all times.

The cruise missile can home directly on target. Being unmanned and able to fly at treetop height it should have high surviva-bility even when running through anti-aircraft defenses. A cruise missile can be designed to carry almost any desired yield of warhead. But it is slow. If the ability to strike almost instantly, within a few minutes, is important, then the ballistic missile has the advantage over the slow, subsonic cruise missile.

The most vulnerable deterrent is now the fixed, land-based missile launcher. Targeting accuracy is making it obsolete.

The US has 1,052 fixed, land-based missile launchers.

The Soviet Union has 2,122 such launchers.

All of the Soviet launchers are vulnerable. The US is better off in having less of its deterrent in a vulnerable conformation than does the Soviet Union.

The MX was originally intended to be mobile. It is still theoretically mobile in that it is encased in and launched from a tube which can be pulled from the silo and moved to another. In practice it is virtually a fixed land-based weapon. The dense pack idea is supposed to give it some protection. But will it?

The same accuracy and blast yield can be achieved either from cruise missiles or from Trident II. The main difference is that you cannot have as many warheads of high blast yield on one missile with Trident or cruise as you can with MX. To get the same number of warheads would require more Tridents or cruise missiles than MX missiles.

But is the difference worth spending some $30 billion on the more vulnerable MX?