START: many nights at the round table

The United States hopes to push Soviet nuclear missiles out to sea and thus enhance stability (and America's considerable superiority in submarines).

The Soviet Union hopes to forestall a further surge of American technology.

And each side hopes to convince the Western antinuclear movements that it is the sole purveyor of peace in the superpower world.

These are some of the strategic aims of the US and Soviet negotiators as they sit down to Strategic Arms Reduction Talks (START) that opened here in plenary session June 30.

The real seriousness with which the two sides are negotiating cannot be judged until they have fully laid out their initial positions and revealed their flexibility or rigidity. This won't happen for at least half a year.

For now, however, the moderates on both sides are in charge.

On the American side this is measured by the State Department victory over Defense Department hard-liners a cliffhanging 11/2 days before President Reagan's May 9 Eureka College speech setting forth the American arms control position.

Thus, in the in-house Reagan administration settlement reached May 7, the Defense Department failed to win not only its proposed warhead figure but also its insistence on using nuclear weapons' ''throw weight'' - in which the Soviets hold a better than 2-to-1 lead in megatonnage - as a main ''unit of account'' in defining the START goal of Soviet-American equality.

The goal of equal throw weight was relegated to a ''second stage'' - and the relation of that stage to the first one that is now being negotiated was left ambiguous.

The first stage of the United States proposal therefore utilizes as units of account missiles (as in previous strategic arms agreements) and warheads (a new unit).

On the Soviet side, moderation is measured by the abandonment of the Kremlin's earlier refusal to negotiate with the US at all until Washington ratifies the second Strategic Arms Limitation Treaty (SALT II) signed by presidents Carter and Brezhnev in 1979.

Soviet moderation is also signaled in Moscow's merely criticizing the new Reagan proposal for radical missile and warhead reductions - and not rejecting the offer outright. The Kremlin did angrily reject the first surprise US proposal for deep cuts made by President Carter in 1977. Various Soviet diplomats have suggested to Western contacts that in retrospect the Russians think they made a mistake then.

The Reagan proposal currently being proposed would not immediately equalize the 9,480 American and 8,040 Soviet nuclear warheads, since it leaves aside those nuclear weapons carried on heavy bombers. What the US proposal seeks to do instead is to reduce by a third each side's 7,500 warheads on land-based intercontinental ballistic missiles (ICBMs) and submarine-launched ballistic missiles (SLBMs).

Within the overall ballistic missile ceilings of 850 and ballistic missile warhead ceilings of 5,000, it further seeks especially sharp reductions in the single most accurate and lethal (but at the same time most vulnerable) weapons, the land-based ICBMs. This category would be limited to no more than half of the total permitted warheads, or 2,500.

US spokesmen say that this scaling down of both sides' ICBMs would promote stability and overall deterrence of nuclear war, since it is the fear of losing one's ICBMs in a surprise attack that makes superpower trigger fingers especially itchy. Ever since land-launched ballistic missile accuracies reached unexpected precision a few years ago, the US has been worrying that a Soviet attack could wipe out 90 percent of US ICBMs and leave only America's less accurate SLBMs and slower, less penetrating bombers for retaliation (and thus for deterrence of an attack in the first place).

The Soviets will face this same threat - that a surprise US attack could wipe out 90 percent of its ICBMs - in the late '80s, as the US deploys the MX land missile and Trident II SLBM. The problem will be much more acute for the Soviet Union, since three-quarters of its intercontinental nuclear warheads are currently located on vulnerable ICBMs as against only a quarter of US intercontinental warheads.

(SLBMs remain invulnerable under current technology because they are mobile and cannot be located in time to be targeted in any surprise attack. Mobile land-based intercontinental missiles would achieve the same invulnerability, but neither the US nor the Soviets have yet solved the technical and political difficulties facing such deployment.)

The US proposal would not by any means eliminate mutual ICBM vulnerability. In a typical regime under the American proposal, the US might deploy some 1,875 ICBM warheads against some 415 Soviet ICBM missiles, while the Soviet Union might deploy some 2,495 ICBM warheads against some 435 US ICBM missiles. With the standard cross-targeting of two warheads per missile target, this would mean that both sides would have plenty of ICBM warheads to destroy the others' ICBM missiles in any surprise attack.

Nonetheless, the US proposal would rein in the current proliferation of ICBM warheads, both in restricting total numbers and, in restricting the ''fractionation'' of ICBMs with more and more warheads on a single missile.

Will the Russians then welcome the US proposal as a chance to reduce the dangers of nuclear war?

No - for one strong reason. The US proposal would require radical restructuring of the Soviet intercontinental nuclear forces. Moscow would have to reduce its present 75 percent reliance on ICBM warheads to 50 percent - and to make up the difference at sea, where American technology, instead of being five years ahead of the Soviet Union as on land, has perhaps double that lead.

Technology and increasing ICBM vulnerability might eventually force the Soviet Union into this shift anyway. But the conservative Soviet military and political leadership is reluctant in the extreme to get pushed into such a move prematurely. And it seems more inclined to look for a solution to ICBM vulnerability in mobile ICBMs.

The Soviet response is likely to be to turn down the US ICBM/SLBM proposal and to demand instead curbs on those weapons that have been left out of the US proposal: bombers and air-launched and sea-launched cruise (as distinct from ballistic) missiles now under development in the US.

Conclusion: It's going to be a long negotiation in Geneva.