The Soviet response to America's ``Star Wars'' could be called ``The Empire Strikes Back''. It would be a sequal every bit as important as the first installment, for Moscow's actions will greatly affect the value of any United States missile shield. The USSR's options range from implacable hostility to guarded cooperation. If the US decides to build a strategic shield, Soviet planners could go all out to defeat it by building more offensive weapons, antisatellite ``space mines,'' and other countermeasures. Or they could decide to move with the US towards a world where defenses play a large role in the superpower relationship.
Protecting their nation against nuclear weapons is something the Soviets have worked on for a long time. The USSR is blanketed with defenses against enemy bombers. A crude antiballistic missile system now stands guard around Moscow. A Soviet version of the US Strategic Defense Initiative (SDI) has long probed the utility of such exotic defensive weapons as lasers.
Reagan administration officials in fact charge that the Soviet Union is preparing to forge ahead on its own, and erect some sort of nationwide defense against nuclear missiles. But it seems clear that the USSR research effort into space-based weapons, while extensive, is in important ways inferior to its US countrpart.
``Our technology base upon which SDI rests is sufficiently far ahead of the Soviets that I would say we are certainly exploiting our edge here,'' says George A. Keyworth II, science adviser to President Reagan.
Why should Americans care about the Soviet response to SDI? Unlike today, when superpowers hold each other hostage with vast nuclear arsenals, wouldn't a working space shield allow the US to control its own destiny?
First, Soviet actions could well determine whether a US missile defense is feasible at all. The quality of Soviet countermeasures would have a large effect on whether strategic defenses can be made tough enough to survive at a reasonable price -- and SDI officials insist that a defense would have to be both survivable and cost-effective to be deployed.
Second, a missile shield could not be thrown up in a day like wallpaper; Soviet actions could make the world a more dangerous place during the time between a US decision to build defenses and actual deployment. The USSR could quickly build hundreds of new nuclear missiles and warheads before a US space shield was in place, perhaps unnerving Western publics and providing an opportunity for the Soviets to force political concessions from the West.
And even if the US and USSR decide that strategic defenses are worth pursuing, their moves toward a defense-dominated world would have to be carefully coordinated, like those of two men stepping into a canoe at once. Otherwise, either country might feel itself falling dangerously behind the other, greatly heightening world tensions.
Joint deployment would be much less tricky if it were scripted ``in advance by explicit agreement between the United States and the Soviet Union,'' points out a recent Congressional Office of Technology Assessment report. Overwhelming a defensive system
The Soviet Union's initial response to a US missile shield could well be to look for ways to overwhelm it. They might stack up new offensive weapons. They could do this relatively easily by churning out extra warheads for existing missiles.
The USSR's large SS-18 booster is today limited by arms agreements to a cargo of 10 warheads, but it is capable of carrying at least 18. Larger forces of cruise missiles and bombers might also be built, in an effort to skirt under a US space-based shield.
Attempting to fool the defense is another Soviet option. Missiles could carry cheap Mylar balloon decoys, as well as warheads. Warheads could be concealed inside balloons, to make the defense's spotting problems even more difficult. Armor might be tried, too: Coating missile boosters with some sort of extra protective layer could make them resistant to attack by lasers or other directed-energy weapons.
Perhaps one of the most effective countermeasures the Soviets could use would be fast-burn missiles, which would finish their flaming boost stage inside Earth's atmosphere in a short 150 seconds. Fast-burn missiles would be safe from neutral particle-beam weapons and X-ray lasers, which cannot penetrate air well; and there would not be much time for the defense to attack them at all before they released their warheads.
Perfecting a fleet of fast-burn missiles would, however, take many years and millions of rubles.
Possible Soviet countermeasures are being studied very seriously, says SDI chief scientist Gerald Yonas, and a US defensive system might well anticipate and handle them.
For example, one SDI concept calls for using pellets and artifical clouds of gas in space. Like Earth's atmosphere, the gas would strip away the lighter decoys, such as balloons, from a group of warheads. The pellets would actually shred the decoys.
Moscow might also simply try to destroy a US missile shield at the start of hostilities. Space mines, which blow up near satellites, could be developed. According to the latest edition of the Pentagon's ``Soviet Military Power,'' the USSR could have ground-based laser antisatellite (ASAT) weapons by the end of the 1980s; a Soviet ASAT using less exotic technology has already been deployed.
SDI officials admit that ensuring survivability of a missile defense is one of their hardest challenges.
A shield's toughness will depend on the physical protection of armor and self-defense weapons; tactics, such as evasion by maneuverable satellites; and national policies, ``agreements we might have with the Soviets in terms of how we operate in space,'' says Dr. Dino Lorenzini, head of SDI's in-house architecture study.
Critics often contend that one result of Mr. Reagan's push for missile defense will be a militarization of the heavens, which have heretofore been a relative sanctuary from humanity's clashes on Earth.
But with military reconnaissance satellites already coasting through space and ASAT weapons in both the US and Soviet arsenals, this militarization has in fact already happened, many Pentagon officers say.
``Clearly, most people would say in a better and decent world it would be nice if we could keep space pure and pristine. I don't think it's responsible for the Department of Defense to take such an altruistic point of view,'' says Air Force Col. George Hess, director of the SDI key technologies section. Soviet emphasis on strategic defense
Ironically, since the dawn of the nuclear age it has usually been the Soviet Union, not the United States, which stressed defense against nuclear weapons. Today Moscow complains bitterly about the US SDI program; in 1967, Soviet Premier Alexei Kosygin said, ``I think that a defensive system which prevents attack is not a cause of the arms race.''
Thus it is possible that the Soviets could work on countermeasures against a US shield while improving their own strategic defenses.
The USSR today maintains a large force of traditional air defense weapons intended to protect against US bombers and cruise missiles. According to the Pentagon, the Soviets have 1,200 interceptor aircraft and 1,200 surface-to-air missile sites dedicated to air defense missions.
This screen does not cause that much worry in the US Air Force. The Strategic Air Command predicts that for the foreseeable future a high percentage of American bombers would be able to reach their targets. Years ago, Pentagon planners decided that in today's ballistic missile age defending the US against bombers isn't worth that much effort. There are approximately 300 US-based fighter aircraft dedicated to strategic defense.
As Reagan administration officials are fond of pointing out, the USSR also has the world's only working antiballistic missile (ABM) system, deployed around Moscow.
Under the terms of the 1972 ABM Treaty, both the US and the Soviet Union have the right to erect one such small defense. The US built one around a ballistic-missile field in North Dakota, but soon scrapped it as costly and ineffective.
The Kremlin is currently upgrading Moscow's ballistic-missile defense. Fast nuclear-tipped SH-04 and SH-08 rockets are replacing sluggish Galosh interceptor missiles, the system's old standbys.
Still, ``the upgraded Moscow system would be ineffective against a determined American strategic strike,'' judges Stanford University arms control expert David Holloway. ``But it could provide some defense against theater [nuclear missile] systems such as the Pershing II.''
The Soviets are also developing new radar that could enhance their early-warning and missile-tracking capabilities. This includes a large phased-array radar near Krasnoyarsk, in Siberia, that US officials argue violates the ABM Treaty. The Soviets are also adding new mobile air-defense radars, which some analysts say may be able to perform missile defense duties as well.
Then there is Soviet research into exotic defense technologies, their version of the US Strategic Defense Initiative. A recent Pentagon study says the USSR is devoting far more plant space, capital, and manpower to such projects than is the US.
But while broader than its American counterpart, Soviet work in defense technology may still not be more productive.
``They are spending five to 10 times as much on agriculture as we are, but I don't think anybody is maintaining there is a grain gap,'' says John E. Pike, space policy analyst at the Federation of American Scientists and an SDI critic.
The Soviets have long been interested in directed-energy weapons, destructive rays that might form part of an advanced screen against incoming ballistic missiles.
Some 10,000 Soviet scientists and half a dozen research facilities are thought to be working on high-energy lasers, for instance.
Soviet laser work may have moved beyond basic research to the development of prototype weapons. At Sary Shagan, a missile range in Soviet Central Asia were some of the most advanced research is under way, there are now two lasers that could ``blind'' low-orbiting US satellites, charge Pentagon officials.
If the Soviets skip some testing, the US Defense Department estimates that they could deploy an Earth-based laser shield against missiles in 10 years, ahead of the SDI timetable. The Soviets might well choose to do this: Several times in the past they have prematurely deployed new systems of marginal use in order to beat the US.
The Central Intelligence Agency is a bit more skeptical of Moscow's prospects. In a report to Congress in June, the CIA predicted that the USSR could not deploy a missile-defense system until after the turn of the century.
Like the US, the Soviets have also long been interested in two other exotic technologies: particle beams and radio-frequency beams. Particle beams -- streams of atoms or subatomic particles -- are considered mainly useful for zapping targets in space. Radio-frequency beams, which use microwaves, hold potential for destroying the electronics of a missile or satellite. The Soviets hold an edge over the US in both technologies, according to US intelligence officials, but still have far to go before t hey can make actual particle-beam and radio-frequency weapons.
A broad shield against missiles, particularly one with some components in space, needs more than weapons. It also requires sensors to spot targets, secure communications links, and computers to run the battle.
SDI officials say progress in these miscellaneous technologies may make or break any missile shield, and in these areas the Soviet Union is probably far behind the US.
Writing reliable computer software to run a missile shield, for instance, is today far beyond the capability of US engineers. And Soviet computer technology is at least a generation cruder than its US counterpart.
``I don't care how big a laser they can build,'' says Stephen M. Meyer, a Soviet defense specialist at the Massachusetts Institute of Technology. ``If they aren't capable of pointing it at anything, who cares?''
Still, the Pentagon claims that the scope of Soviet strategic defensive programs suggests they may be preparing to burst the limits of the 1972 ABM Treaty and erect a nationwide missile defense.
At the very least, Pentagon officials say, Moscow's work on upgrading radars and surface-to-air missile sites means they are better positioned than the US to build a relatively crude shield, using off-the-shelf technology, in the next decade or so.
``It would really give us fits if they did,'' says one Pentagon official.
If the Soviet Union raced ahead with its own defense, while bolstering its offensive arsenal, Kremlin planners might come to believe they had strategic superiority and could attack the US, or threaten to attack it, without fear of retaliation.
Thus Pentagon officials from Defense Secretary Caspar W. Weinberger on down argue that the US SDI is really not an initiative at all, but a response to Soviet actions.
Critics of the US program argue that this view of the Soviets exaggerates their capabilities.
Kremlin officials are not about to order a ``breakout'' from the ABM Treaty, because they know that doing so would be like ``throwing gasoline'' on the US SDI effort, says one defense analyst. A very tricky transition period
Critics also are concerned about SDI's effect on superpower stability. Here they turn the Pentagon's argument around: If it would destabilize the nuclear balance for the USSR to deploy defenses ahead of the US, wouldn't it also be destabilizing for the US to erect defenses before the USSR?
The transition from today's strategic situation, where superpowers rely solely on offensive arsenals, to the world of strategic defense would be a very delicate dance -- even according to SDI officials. Few people say the US could just build defenses on its own.
``I think if we've convinced ourselves this is the way to go, the Soviets will have convinced themselves this is the way to go, too,'' says Dr. Yonas.
Each step toward defenses by both sides would have to be made gingerly to keep the other fellow from feeling he was becoming dangerously vulnerable by being left behind.
Many experts inside and outside government feel this transition period would have be to planned in advance by a superpower agreement.
A necessary part of this agreement, these experts say, will be strict limits on offensive arms. Otherwise, defenses might not be effective enough to make sense, or cheap enough to afford.
``In my view offensive restraints are necessary,'' says Cornelius (Cory) Coll III, head of an SDI study group at the Lawrence-Livermore National Laboratory.
Other officials argue that offensive arms cuts will in fact follow, not precede, deployment of defenses: As defenses are gradually strengthened, both sides will see their nuclear weapons becoming less and less useful, and will become amenable to greater and greater reductions in their arsenals. The arms spiral will go down, instead of up.
``Defensive technology is the enabling mechanism that will make this chemistry of arms reduction work,'' Yonas argues.
If strategic defenses turn out to be feasible, will the Soviets really agree to go along? That is difficult for Westerners to predict -- it is not for nothing that Winston Churchill called the Soviet Union ``a riddle wrapped in a mystery inside an enigma.''
It is clear that the Soviets are working on defenses. But in the near term, most observors don't expect the Soviet Union to radically accelerate its military programs, offensive or defensive, in response to SDI.
For one thing, too many political and technical uncertainties surround the nascent program. In addition, the Kremlin wouldn't find it easy to divert resources from other sectors of the economy -- many of which lag their Western counterparts.
``They can't afford to plow forward with an SDI on the American scale,'' says Jonathan Haslam, a Soviet specialist as Johns Hopkins School of Advanced International Studies in Washington.
But the very fact that SDI exists has already changed the superpower relationship, helping bring Moscow back to the arms bargaining table, while at the same time making those talks more complicated. The issue of defenses, which has now gone very public in the West, is likely to affect superpower relations for some time to come.
Writes MIT's Stephen Meyer in Survival, the journal of the International Institute for Strategic Studies in London:
``For the Soviet Union, the US SDI program is quickly becoming symbolic of a more fundamental challenge between states . . . calling into contention the political, economic and industrial, scientific and technological, and military potentials of the superpowers.'' -- 30 -- Drawing: Foundation for an ABM radar network?
Some analysts say the Soviet early-warning network could ultimately be coupled with dual-role antiaircraft missiles and antiaircraft radar to form a rudimentary nationwide ABM system. US Defense Department officials say one radar installation, near Krasnoyarsk, violates the 1972 ABM Treaty because of its siting, orientation, and capabilities.