Next June, when a Soviet spacecraft launches a balloon into the corrosive sky of Venus, 62 million miles from Earth, the Soviet Union plans to rely on supersensitive U.S. space agency antennas to track the balloon's course through the planet's stormy atmosphere.

When the same Vega spacecraft encounters Halley's Comet in 1986, NASA's Deep Space Net precision antenna is to gather information crucial to Soviet and European scientists as they plan the orbital path of a spacecraft they hope to fly later as close as possible to the comet without hitting it.

These two new examples of unprecedented U.S.-Soviet interplanetary cooperation came to light today in the aftermath of Thursday's revelation here that a sophisticated U.S.-made instrument to measure comet dust is aboard the Vega, which blasted off from a Central Asian launch site Sunday. A second Vega mission was successfully launched today, the official news agency Tass said.

In three weeks, the precision antennas are to be trained on the outbound Vega1 craft in the first acknowledged use of the Deep Space Net to track a foreign-made interplanetary probe.

News that the Vegas are carrying measuring devices designed and built by astrophysicists at the University of Chicago, whose Dr. John Simpson designed the dust instrument, comes amid heightened public concern that the two nations are embarked on competitive militarization of space.

The Reagan administration and Kremlin have exchanged such accusations, and a bilateral agreement on space cooperation has not been renewed since it lapsed in 1982.

However, informal U.S.-Soviet cooperation on the Vega mission is far greater than is commonly known, according to interviews with U.S. space officials and scientists at universities and laboratories nationwide.

This month, Soviet scientists and technicians brought a Vega spacecraft to the National Aeronautics and Space Administration's radio-antenna facility near Madrid so the Deep Space Net's receivers could be adjusted to receive Soviet telemetry signals. It was the first time the Soviets had allowed such thorough analysis of one of their spacecraft.

In addition, U.S., Hungarian and Soviet scientists are cooperating on precision image-making instruments aboard Vega. Data from these instruments are to assist the Deep Space Net's electronic ears in determining the comet's position and velocity as it speeds away from the sun.

Such information would allow the European Space Agency to fine-tune the intercept trajectory of its ambitious comet probe, which is to follow the Vega spacecraft by several days. The Europeans want to fly their spacecraft, called Giotto, within 300 miles of the comet's center.

The complexity of the encounter with the comet, which passes near Earth only once about every 75 years, has drawn together scientists from many countries in unexpected ways.

"Walking into a Deep Space Net control center and seeing Soviet manuals in Russian spread all over was something," recalled Dr. Robert Preston, leader of the U.S. project to track the Vega probe's Venus-bound balloon. NASA antennas in Spain, Australia and New Mexico are to aid Soviet and French space antenna networks monitoring the experiment.

The NASA antennas are sensitive enough to detect a 2-watt radio signal 62 million miles away and can measure angular movement with astonishing accuracy.

Officials said this is important when tracking the balloon, designed to float for about two days high in Venus' turbulent atmosphere of carbon dioxide and sulfuric acid clouds. The speed and direction of drift is expected to tell much about the composition and nature of the planet's atmosphere.

Preston said the Deep Space Net's sensitivity to movement is equivalent to that of a person in Chicago noticing the movement of the minute hand on a wristwatch in Los Angeles.

U.S. participation in the balloon experiment is being coordinated by France's national space agency. "Technically, there is no explicit relationship between the U.S. and the U.S.S.R.," said an official at the Jet Propulsion Laboratory in Pasadena, Calif.

Neither government publicly had acknowledged the informal bilateral arrangements between NASA and Soviet space authorities that makes even limited scientific space exchanges possible.

However, NASA official Peter Smith said of the lapsed space accords, "Not all cooperation was obliged to cease. We have been able to reach agency-to-agency agreements between us ," after clearing it with NASA and the State and Defense departments.

NASA's Venus-balloon tracking effort will cost about $1 million, Preston estimated.

Other joint space efforts have included the 1975 orbital linkup by the manned Apollo and Soyuz spacecraft and a cooperative project using satellites to detect ships and aircraft in trouble. The system is credited with saving about 300 lives