A sophisticated American experiment to analyze the dust of Halley's Comet is riding aboard the Soviet Union's Vega-1 spacecraft launched from Kazakhstan in Central Asia last Saturday, it was disclosed here today.

Pleased scientists at the University of Chicago broke more than a year of silence to describe what had developed into extraordinary behind-the-scenes cooperation and a unique episode in East-West relations.

It is the first known Soviet-American space venture since the docking of the manned Apollo and Soyuz spacecraft in 1975 and comes amid heightened public discussion of militarized competition in space by the two superpowers. The Soviets on Wednesday successfully tested a prototype space shuttle expected to be put to military use, and next month a U.S. shuttle is to place a massive new military satellite in stationary orbit over the Soviet Union.

The cosmic dust analyzer -- and a twin that will be launched aboard Vega-2 on Saturday -- were built by university scientists under the leadership of John Simpson, one of this country's preeminent astrophysicists who has designed experiments for more than 30 space missions over the past 25 years.

The joint project was born at an international symposium in Holland in September 1983 where Simpson first outlined vastly improved comet dust measuring methods. About a month later, he said, the Soviets "surprised me" by inviting him to put his analyzer aboard their Vega.

Simpson received Reagan administration approval, and on March 5 "we got the formal go ahead" and about $300,000 in funds from the National Aeronautics and Space Administration. That touched off a frantic search for components.

"We scoured the U.S. and within two weeks, got all the pieces," Simpson said.

Plugging American electronics into a Soviet spacecraft required unusual cooperation. A direct telex "hotline" was set up between Simpson's Laboratory for Astrophysics and Space Research here and the Soviet Academy of Sciences' Space Research Institute in Moscow.

Moscow shared telemetry coding data, and Simpson was able to conduct high-speed computer analysis of Soviet rocket launch characteristics to aid in designing the dust analyzer so it could swrvive blastoff and flight.

The instrument was invented and fabricated here and "on May 7, we walked into the Soviet space lab with working instruments," which were then bolted aboard the Soviet spacecraft by U.S. scientists, he said.

The Americans made seven trips to Moscow, but Simpson was not invited to the launch. "Who'd want to go there in the middle of the winter, anyway," he joked.

Simpson said he has already received computer tapes from the instrument during Soviet tests of the device while it awaited launch from the Baikonur cosmodrome. He said the Russians have invited him to be on hand in Moscow when the comet encounter begins in March 1986.

Halley's Comet follows an elliptical orbit around Earth's sun and enters our skies every 75 years. With its immense tail of incandescent gases and matter, it is one of the heavens' most dramatic sights.

But very little is known about comets. Simpson said astrophysicists assume they are composed of material traceable to the earliest moments of the solar system.

"This will be the first time man can get close enough to the comet to get a good look at what we believe is primordial matter, matter that belongs to the formation of our solar system," Simpson said. "That should give us some clues on how the Earth and the other planets were formed."

Five missions to the comet are planned during its time in the sun's vicinity -- two Soviet, two Japanese and one European. American hopes for a Halley's mission were scrubbed when the administration sharply reduced unmanned space missions in favor of the shuttle and a planned orbiting space station.

The two Vegas should rendezvous with Halley's Comet in March 1986. When they do, the Simpson instruments -- each about the size of a kitchen food processor -- will report the presence of comet dust particles as small as one micron, one millionth of a meter.

The data will be shared by scientists around the world and help determine how close to Halley's the last three missions can safely be sent.

Today's disclosure seemed as unusual for its informality and unofficial nature as the project itself. Asked why the news was not released earlier, Simpson said, "Both sides just didn't know what they were getting into.