Ever since the bombing of Hiroshima 40 years ago, nuclear warheads have been the heart of superpower arsenals. Their awesome destructiveness has made them seem the final word in weapons for global war. That may be changing. According to some analysts, advanced technology now offers the prospect of conventional weapons that can do the job of some nuclear missiles and bombs.
Such ``nonnuclear strategic weapons'' would be able to travel across continents and hit hard targets with the accuracy of a baseball pitcher throwing a strike. For the most part, they are not related to President Reagan's proposed space-based defense against ballistic missiles.
These nonnuclear weapons could have vast implications for US security policy -- and the Pentagon is already beginning to study what those implications might be.
``There is little doubt that nonnuclear strategic weapons are emerging, and that they will greatly complicate our ideas about how to prevent or wage wars,'' concludes a little-reported Air Force study.
Nonnuclear strategic weapons, in this sense, do not involve lasers shot from space, plasmoid guns, or other Luke Skywalker technology. Instead they would be based on today's ``smart'' munitions, which use advanced microelectronics to guide themselves to such targets as tanks and bridges.
For destructive power, these weapons might use shaped charges, which concentrate an explosion on a particular spot, or ``self-forging projectiles,'' which spit a piercing glob of metal as they approach a target.
These technologies are now relatively mature -- the US has already developed such weapons as SKEET, an antiarmor warhead that homes in on a target using infrared sensors. But to do some of the military jobs now assigned nuclear weapons, they must be further refined.
``The key technology is delivery precision,'' says Cecil Hudson, a co-author of the Air Force study, which was done under contract by the Rand Corporation and was conducted last winter.
A workable nonnuclear strategic weapon would have to travel thousands of miles and strike within feet of the weak point of a factory or early-warning radar, says Mr. Hudson. Nuclear weapons, whose explosive power is many times greater, require less accuracy to accomplish the same mission.
Some analysts play down the prospect of such weapons. ``We might be able to begin talking about them in 10 years,'' says a government scientist with access to classified data about weapon accuracy.
Others say the technology in this area is advancing more rapidly than is widely known -- and that in any case 10 years is not very long, in military planning terms.
``The technology is coming along. The important question is do you want them?'' says Carl Builder, an analyst who has written on the subject for the International Institute for Strategic Studies.
A summary of the Rand report, declassified only recently, concludes that nonnuclear strategic weapons will never completely replace nuclear warheads. It reaches no firm conclusions on how such weapons might be used.
But the report says certain concentrated strategic targets might be vulnerable to modern conventional warheads. Among them might be submarines, bombers, communication systems, key factories, and masses of enemy troops.
These new weapons, if deployed, would likely have ``profound'' strategic implications, according to the Rand study. Among other things:
They might make US commitments to Europe's defense appear more credible. If the Soviet Union invaded West Germany, for instance, nonnuclear strategic weapons would enable the United States to strike against targets in the Soviet heartland without making the wrenching decision of going nuclear, and risking nuclear retaliation.
They might make ``limited'' war more dangerous. If a war in Europe started, nonnuclear strategic weapons would throw yet another variable into the already-complicated war plans of both sides. Their use by the US might seem easy, but could in fact confuse the Soviets, resulting in escalation to nuclear war.
They might make the arms control process even more difficult than it is today. By adding yet another category of arms, and by blurring the lines between strategic and tactical weapons, they could make it harder for the US and the Soviets to strike a mutually acceptable weapons balance.
The authors of the Rand report could not decide whether in the long term nonnuclear strategic weapons would be a good or bad thing for the US to have. They did, however, agree that trends -- such as political feeling against nuclear warheads -- favor the future deployment of nonnuclear alternatives.
Some analysts feel the weapons would partly free mankind from the shadow still cast by Hiroshima's mushroom cloud.
``I would prefer the world go in this direction,'' says Mr. Builder.
Others say the weapons would be more dangerous than they seem.
``You might begin to think you could fight a war, with conventional forces. Then you may just trigger nuclear war,'' says John Mearsheimer, a University of Chicago political scientist.