In a few days, the hands of the ''doomsday clock'' will be moved a bit closer to midnight. Since 1947, this traditional warning device of the Bulletin of the Atomic Scientists at the University of Chicago has measured the prospects of nuclear war. Today, experts represented by this organization (including 18 Nobel laureates) see troubling signs that the posture of the superpowers has become more dangerous. And this has prompted them to move the hands of the symbolic clock for the first time in three years.
Among the threatening indicators:
* The Soviet Union has walked out of the medium-range nuclear missile talks at Geneva in response to the deployment of new US Pershing II and cruise missiles in Europe. Jane's ''All the World's Aircraft'' has just reported a new large Soviet bomber capable of ferrying the SS-20 missile, thus making it even more mobile. The SS-20's portability was a factor in prompting NATO to begin new missile deployments and seek negotiations.
* The parallel US-Soviet strategic arms talks have adjourned with the two sides far apart and no date set for resumption.
* The conventional arms reduction talks in Vienna between the NATO alliance and the Warsaw Pact, which have been going on for a decade, also have been halted without favorable result. In a speech here this week, NATO's commander, Gen. Bernard Rogers, repeated his warning that the alliance may have to ''escalate fairly quickly to the first use of nuclear weapons'' if Warsaw Pact forces invade Western Europe.
* Defense Department officials this week told senators in a private session they intend to spend at least $18 billion (and perhaps half again this much) over the next five years developing the technologies called for in President Reagan's ''star wars'' speech of last March.
Many scientists and arms control advocates are especially concerned about the arms race breaking out in space even before the far-off protective ballistic missile ''umbrella'' sought by Mr. Reagan could be in place. The Soviet Union already has a somewhat unsophisticated rocket designed to attack satellites. And the United States is poised to test a satellite-killer missile to be fired from an F-15 jet interceptor.
''The frightening thing is that the United States and the Soviet Union are going about their testing and, I assume, deployment as though this was a reasonable area of confrontation,'' said Bernard Feld, professor of physics at the Massachusetts Institute of Technology and editor-in-chief of the Bulletin of the Atomic Scientists. ''This is a great mistake and is going to make the world an even more dangerous place than it already is.''
''We face a basic choice about the military use of space,'' said Walter Slocombe, a former US defense official who took part in the first US-Soviet negotiations on controlling anti-satellite (ASAT) weapons.
Such weapons would be ''particularly destabilizing,'' he said. ''There could, in a crisis, be a considerable temptation to make such an attack to gain immediate military advantage.''
It is generally acknowledged that the US relies much more than the Soviet Union on satellites for such militarily critical things as weather forecasting, navigation, and the command and control of weapons and forces. ''The United States has bought top-of-the-line satellites for a lot of good reasons,'' Mr. Slocombe said. ''But we've gotten ourselves in a situation where we are highly dependent on a small number of valuable targets.''
Slocombe and others see a direct connection between the more advanced ASAT weapons (which soon will be able to reach much higher altitudes) and ballistic-missile defense systems that could not be operational for years. The latter strategic defenses would rely on satellites to detect and destroy enemy warheads.
Yet there are indications that both the US and the USSR realize the seriousness of the impending arms race in space, and both seem hesitant to proceed without probing for ways to reduce the threat.
Last summer, Moscow offered a new proposal on controlling weapons in space. Critics say the Soviets merely want to head off the newer and more capable US ASAT system. But arms control advocates are urging the administration to be more forthcoming in response. And the Pentagon has delayed flight testing the new F- 15 ASAT missile.
The Soviet offer is a ''very considerable advance over earlier suggestions,'' says former defense official Slocombe. ''It's clearly a reasonable framework to begin to discuss these issues.''