WASHINGTON — With last-minute progress on strategic missiles and "Star Wars," the third summit meeting between President Reagan and Soviet leader Mikhail S. Gorbachev ended Thursday by greatly advancing prospects for a treaty slashing the biggest and most dangerous weapons in the superpowers' nuclear arsenals.
Some negotiators even held out hope that the strategic arms reduction, or START, treaty might be completed in time for signing next May or June, when Reagan is scheduled to visit Moscow for the fourth superpower summit of his presidency.
The achievement--whether a "breakthrough," as one senior U.S. official said Gorbachev called it, or "real progress," as Reagan called it in a televised address Thursday night--did not consist of any single dramatic concession or negotiating triumph by either side.
Rather, it was the cumulative result of several arcane and highly technical diplomatic compromises: a series of seemingly small agreements that eliminated or substantially reduced important stumbling blocks on START, and a murky formulation that finesses--at least for now--the long-divisive issue of anti-missile defenses.
In several cases, what the working groups that toiled through Wednesday night accomplished was not an overt, directly stated agreement but instead a tacit understanding not to press points previously insisted on.
The major new points of progress were these, according to U.S. officials:
--Within the overall limit of 6,000 nuclear-armed bombers and land-based and sea-based ballistic missiles that the two sides had already accepted, the sub-limit of the latter two is not to exceed 4,900 on each side. This sub-limit, lower than Moscow had wanted and very close to the total sought by U.S. negotiators, is important because the Soviet Union has far more land-based missiles than this country does, and far fewer nuclear-armed bombers and submarines.
The lower the sub-limit, the more Moscow will have to reduce its huge arsenal of land-based missiles and the greater freedom the United States will have to spread its weapons among bombers and submarines, armaments in which the United States enjoys technological superiority.
--In the area of verifying compliance with the START treaty, the superpowers agreed to build upon the unprecedented system of on-site inspections and satellite monitoring specified in the newly signed treaty eliminating medium-range nuclear missiles.
Allows Permanent Inspectors
Those procedures, the most intrusive ever accepted by the Soviets, allow inspectors from each side to be stationed permanently on the other's soil.
--Partial accords were reached on several of the complex but important rules to be used for counting each side's strategic missiles under the proposed treaty.
--The two countries agreed to negotiate limits on submarine-launched cruise missiles, which are becoming increasingly threatening as their range and accuracy increases, if ways can be agreed on to determine which of these missiles carry nuclear warheads and which carry conventional explosive.
The United States, which has both kinds, enjoys a substantial lead in this field and previously has declined to enter negotiations aimed at putting limits on SLCMs, as they are called.
--The two leaders agreed to put aside their quarrel over so-called "narrow" versus "broad" interpretations of the 1972 Anti-Ballistic Missile Treaty and--without explicitly adopting one interpretation or the other--move ahead with the START accord.
Formula to Be Developed
Negotiators at Geneva are to work out a formula binding the two superpowers not to withdraw from the ABM Treaty for an extended but still-to-be-agreed-on period of time, at the end of which the treaty would lapse unless the two agreed to extend it.
Until now, Moscow has insisted that the ABM Treaty must be extended indefinitely unless formally abrogated by one side--considered a politically difficult step, especially for the United States.
The Soviets also dropped their vow not to carry out major cuts in their strategic arsenals until restraints had been negotiated on Reagan's Strategic Defense Initiative, the space-based anti-missile defense system popularly known as "Star Wars."
Neither Side Gave Up
"I don't feel that we've given up, or that they've given up," said a key senior Administration official, but he said the understanding reached will improve prospects for the START negotiations.
--The Soviets, in the joint statement issued at the end of the summit, did not insist as they have in the past that SDI testing in space be explicitly prohibited during the 7-to-10-year period envisioned for non-withdrawal.
--After the seven to 10 years, each side would be free to deploy space-based defenses unless both had already agreed that deployment would be prohibited. Previously the Soviets had sought to keep the non-deployment period in force indefinitely.