Space defense problems. Talk of difficulties with antimissile system hints at possible moderation of US arms position at Geneva

As President Reagan avidly promotes his ``star wars'' concept, senior officials within the administration are candidly beginning to point out the obstacles to achieving it. By speaking about potential difficulties, say diplomatic and arms control experts, the administration could be positioning itself to back off from its present position in the Geneva arms talks.

American officials mention these factors as they discuss the program, formally called the Strategic Defense Initiative (SDI), designed to provide a nonnuclear space-based defense against nuclear missiles:

It will be extremely difficult to manage the transition period, during which the Soviets and the Americans begin to move away from deterrence, based on offensive nuclear weapons, to more reliance on defensive systems.

If SDI is to be put in place, it will be necessary not only to negotiate reductions in nuclear arms but to agree on a balance in levels of conventional weapons, which would add still another dimension to bilateral talks.

SDI will be viable only if it is cheaper to add more defensive arms than for the other side to add the offensive capability needed to penetrate the defense.

Any future defensive systems must be reasonably survivable.

``As you discuss SDI and as you discuss the reduction of theater and strategic weapons, I think that has to be coupled with . . . conventional arms control,'' says a ranking State Department official involved in arms control policymaking.

Without limits on conventional weapons, he says, a defensive system that eliminated nuclear weapons would simply increase the risk of conventional war.

Taking account of Soviet fears about SDI, the official also says that the transition period would have to be ``very, very carefully managed on both sides'' and would require step-by-step cooperation between the superpowers -- something experienced diplomats agree would be difficult to achieve.

Hinting at the possibility of a US compromise with Moscow on defensive systems, the official says that, falling short of understandings on a long-term SDI, he would be willing to have the US field a ``terminal'' (ground-based) defense along with reductions in offensive systems on both sides. This seems to suggest the US might consider an agreement trading off defensive and offensive systems, as the Soviets would like, instead of simply reducing offensive weapons.

One idea being discussed by arms control experts is that the US focus SDI research on the terminal phase -- the stage when warheads reenter the atmosphere. Current research has emphasized the ``boost phase,'' using space-based weapons aimed at stopping Soviet missiles at liftoff stage. This is what most concerns the Russians.

Some experts believe it would be possible to reach an agreement with the Soviets on ground-based defense systems, enabling the US to modernize its old, unused missile defense systems to defend hardened targets (such as missile silos) and at the same time not undercut the talks on offensive nuclear weapons.

``Let's not bad-mouth our successes,'' says the US official, referring to the fact that agreements based on deterrence (and encompassing offensive and defensive systems) have kept the peace a long time.

There clearly is a debate within the administration regarding SDI and how far the US can effectively press Moscow for the radical change in strategic thinking that SDI requries. The State Department official suggests that at Geneva the two sides would have to negotiate a road map of ways in which scientists in both countries could cooperate -- even in the research phase -- to achieve the long-term objective.

American scientists could concentrate on certain aspects of research, he says, while Soviet scientists deal with other elements.

``Why can't the negotiators talk about that?'' says the official. ``If we agree on a potential end game, maybe we can negotiate shared research . . . sharing techniques, `we do the endo-atmospheric [research] and you do the exo-atmospheric.' ''

Not everyone in the Reagan administration accepts this view. The official stresses that these are his personal ideas, extrapolated from Mr. Reagan's statement in 1983 that the US should be willing to share technology with the Soviet Union. But little is heard about such sharing since the President made his remark.

Paul Nitze, the President's senior arms control adviser, emphatically rejects the idea of sharing research tasks. ``That's impossible,'' he says, citing the problem of verifying Soviet research and the risk to the US of providing the Soviets with advanced technological information.

But Mr. Nitze also is stressing the conditions under which SDI would be feasible, and these point to the difficulties of selling the Russians on the plan. In a comprehensive lecture on US arms policy at London's International Institute for Strategic Studies recently, Nitze stated that if the superpowers moved to a situation in which nuclear weapons had been eliminated, ``the need for a stable conventional balance would become even more important than today.''

``Clearly, were we able to move cooperatively with the Soviet Union toward a nuclear-free world, that would presuppose a more cooperative overall relationship than exists at present -- one in which efforts to establish a conventional balance at lower levels should also be fruitful,'' Nitze said.

He further stressed the critical nature of the transition period and cited the criteria under which the US would decide to deploy an SDI system: cost-effectiveness and reasonable survivability. If the new technologies are not survivable, he told his audience, ``the defenses could themselves be tempting targets for a first strike. This would decrease, rather than enhance, stability.''

Independent arms control experts suggest that Nitze is trying to distance himself from SDI by pointing up the difficulties and criteria involved.

``Nitze is taking the lead in seeing the obstacles to SDI,'' says Thomas K. Longstreth of the Arms Control Association. ``Until now there was no acknowledgment that trying to get from A to Z is a difficult task and could increase instability. So this is a significant step back from many assertions before.''

Other experts, too, suggest that Nitze is trying to rework SDI in his own image. He knows he cannot totally cut off the program, which the President wants and which true believers in the administration are pushing. But, say analysts, having more experience in arms control, Nitze realizes SDI makes nuclear arms control impossible.

Administration officials themselves admit that the SDI program has complicated the arms talks in Geneva. Reagan's position is that SDI is only in the research stage and therefore there is nothing to negotiate yet -- only to talk abut.

But some administration experts are known to have doubts about the practicality of the star-wars concept. Their concern is that the program, aimed at eventually building a space-based shield against nuclear missiles, is unacceptable to the Soviet Union and therefore undermines the chances for achieving a treaty on strategic and medium-range nuclear weapons.

If the new nonnuclear defensive systems that the US develops are not cost-effective, said Nitze in London, they could spur a proliferation of Soviet countermeasures and additional offensive weapons to overcome the space-based defensive shield, ``instead of a redirection of effort from offense to defense.''

Debate over SDI is expected to step up sharply when Congress takes up funding for the program. The President is requesting $3.7 billion in fiscal year 1986, or more than double the 1985 budget appropriation of $1.4 billion. ----30{et