Space scientists in Western Europe are delighted with a Soviet-European agreement that should enable them to achieve a close encounter with Halley's Comet.
A spacecraft designed by the European Space Agency is due to fly past Halley's comet in March 1986 at a distance just 500 kilometers (310 miles) from the comet's nucleus, a ball of dirty ice that is 6 km in diameter.
The comet will at this point be some 150 million km from Earth. It gets this close only once every 76 years. The rest of the time is in a highly elliptical orbit that takes it into the cold wastes of the solar system far away from the sun.
A few days before the European probe - called Giotto - is due to glide past the comet, two Soviet spacecraft, Vega 1 and Vega 2, will also encounter this wanderer of the solar system.
But the Soviet vehicles, which are also to fly past Venus in a journey that begins this December, will get no nearer to the comet than 3,000 km. The Soviet-European agreement provides that information from the Soviet probes will be passed to the control center in West Germany that is attempting to guide Giotto.
In this way, Vega 1 and 2 will act as pathfinders for the West European craft and increase its chances of achieving an accurate fly past.
Because it is traveling so near Halley, Giotto is in danger of being thrown off course by the thick jets of dust the comet is believed to discard from its nucleus at the rate of 10 tons a second.
With navigational guidance from the Soviet vehicles, West European scientists think they will be able to direct Giotto with an accuracy of 150 km.
Using navigational techniques derived only from earth observations, the accuracy would be about 600 km.
Dr. Edgar Page Esas, head of Space Science, believes top-level approval from the Kremlin was required before the deal could go through:
''The Soviets were very reluctant for a long time to agree to give the data. If the cooperation comes off, it will be a very solid scientific achievement,'' he said.
Meetings to finalize the accord will be held in Moscow in June and in Tallinn (also in the Soviet Union) in October.