American space officials left for Europe over the weekend in the hopes of securing the most significant space agreement with the Soviet Union since three astronauts and three cosmonauts flew together in earth orbit three years ago.

The U.S. delegates will attempt to reach an agreement with their Soviet counterparts in which American scentific instruments would be flown in an unmanned Soviet spacecraft put into orbit around the moon. The Soviet spacecraft is expected to fly to the moon no earlier than 1981.

Space delegations from both countries will be in Innsbruck,Austria, all this week and part of next, attending a regular meeting of the Committee for Space Research, called COSPAR. One of the top matters on the agendas of both the U. S. and Soviet delegations is the use by both of the Soviet Lunar Polar Orbiter.

The instruments the American delegation is talking about for the Soviet lunar mission are understood to be refinements of instruments carried into lunar orbit by the last three crews of Apollo astronauts. Those instruments measured radioactivity in the lunar crust and identified mineral distribution around the portion of the moon that covered 10 to 15 degrees on each side of the moon's equator.

The Soviets are planning to put a spacecraft into polar orbit around the moon, meaning it would follow a path around the north and south poles of the moon. That would take it over the entire lunar surface during the course of its flight.

The American instruments, which have already been proven in the flights of Apollo 15, 16 and 17, would do the same thing for the unchanted regions of the moon that they did during their Apollo flights.

Scientists estimate that the last three Apollo missions mapped the radioactivity and mineral distribution of about 20 percent of the moon. A joint Soviet-U.S. mission in polar orbit would map the remaining 80percent.

The National Aeronautics and Space Administration tried to start its own lunar polar orbiter mission this year, but was unable to get it approved by the White House because of its cost. The fact that the Soviets seem intent on flying the same mission opened up the possibility of a joint flight.

Most of the expense involved in the lunar mission would be borne by the Soviet Union. Only the scientific instruments carried in the spacecraft would be American.

Space scientists are eager to get an agreement with the Soviet Union, even in light of the conviction of Soviet physicist Yuri Orlov on charges of slandering the Soviet state. Orlov's conviction has already resulted in 28 American physicists'cacenling their visits this summer to the Soviet Union.

"We want to keep as many doors open between the two countries as we can," said one space scientist who asked not to be identified. "This agreement to fly to the moon together is one of our best chances to do that."