The United States and the Soviet Union could be moving toward a more stable, less hair-trigger balance of nuclear forces, says a top American negotiator. Edward L. Rowny, the chief US strategic arms negotiator, says it is now reasonable to assume that the Soviets will begin making some of their land-based , long-range intercontinental ballistic missiles (ICBMs) smaller and mobile.
Such missiles would be less powerful, less accurate, and therefore less threatening than the heavy missiles that now make up most of the Soviets' land-based force.
''If you look at it from a Soviet planner's point of view, he's got 75 percent of his eggs in one basket,'' General Rowny says, referring to Soviet dependence on heavy missiles in fixed silos. ''So he's going to try to hedge his bets.''
''To the extent they do move toward small missiles, they move to a more stable situation,'' he says. ''If they are convinced that we can't hit their missiles, they should feel more secure and more stable. So it helps out the situation.''
Mr. Rowny's delegation to the strategic arms reduction talks (START) in Geneva has just completed a fourth round of negotiations with the Soviets.
In a breakfast meeting with reporters on Aug. 19, Rowny argued that the Soviets were being pushed toward the development of a mobile ICBM by Washington's decision to produce the MX missile and the highly accurate D-5 warhead for the Trident II submarine-launched ballistic missile.
The Soviets are currently testing a solid-fuel missile that is the capabile of being moved around on a mobile launcher, Rowny said.
But the retired Army lieutenant general warned that the Soviets would not go completely mobile with their missiles. They will hold onto many of their big multiwarhead missiles, in part because such weapons are ''cost effective,'' he said.
It costs almost as much to build and maintain a small, single-warhead missile as it does a multiwarhead missile. Rowny added that going to small, mobile missile creates problems for arms control verificatoin.
If the Soviets decide to reduce their dependence on ''blockbuster'' missiles and shift toward small, mobile missiles, it will add to a list of positive developments that have occurred in recent months in the US-Soviet relationship.
The list includes a new grain pact that increases the minimum level of Soviet purchases, a compromise agreement on human rights issues at the East-West talks in Madrid, and some movement on secondary issues in START in Geneva and the East-West negotiations on conventional forces in Vienna.
Last week, Soviet officials were reported to have shown an interest in American proposals, made during meetings in Moscow, for an improvement in the Washington-Moscow ''hot line'' communications link.
And in the latest development, President Reagan decided on Aug. 19 to lift licensing restrictions on the sale of American pipelaying tractors to the Soviets. Such sales had been opposed by Defense Secretary Caspar W. Weinberger. But Secretary of State George P. Shultz and Commerce Secretary Malcolm Baldrige argued that the tractors, produced by the Caterpillar Tractor Company of Peoria, Ill., can't be converted to military use and that the Soviets can easily obtain the tractors from Japan.
But some US officials are convinced that there will be no real movement on the central issues involved in US-Soviet arms control talks until the end of the year. At that point, the Soviets must be convinced that the NATO alliance is going to deploy new medium-range US missiles in Western Europe. That deployment is scheduled to begin in December.
Meanwhile, some observers see a positive sign in Soviet President Yuri Andropov's porposal last week to ban anti-satellite (ASAT) weapons from space.
Kurt Gottfried, a Cornell University professor who chaired a 10-member panel on anti-satellite weapons for the Union of Concerned Scientists, said Mr. Andropov's offer was ''a concrete step forward.''
''I think the Soviets came to realize that developing ASAT was a very foolish thing to do,'' he said in a telephone interview.
US State Department reaction to Andropov's offer was cool. State Department spokesman Alan Romberg pointed out that the Soviets have the world's only operational ASAT system, which they have tested for more than 12 years.
The US has yet to test its own ASAT weapon, which would be fired from an F-15 fighter flying at high altitude. It looks as though the American ASAT weapon would ultimately be more effective than the system the Soviets now possess.
But Gottfried, a professor of physics and nuclear studies, argues that the Soviets would eventually catch up in ASAT development, putting American national security at greater risk, because the Americans are more dependent on satellites than the Soviets.