Plans are under way for a Soviet-American space mission next year to demonstrate one country's ability to rescue marooned astronauts from the other country.

Unlike the 1975 Apollo-Soyuz mission in which Soviet and American spacecraft locked together for two days, the new project envisions a close approach by an American space shuttle and a Soviet space station and then the simulated transfer of crew members wearing spacesuits and jet backpacks.

No agreement has been signed, but discussions have taken place at the behest of the Reagan administration. President Reagan has signed legislation renewing the U.S.-Soviet space agreement that was allowed to expire in May 1982 in response to the Soviet-inspired crackdown on Poland's Solidarity labor movement.

"The idea of a demonstration rescue mission is appropriate at this time," a high-ranking source at the Johnson Space Center said. "We already demonstrated that we can rendezvous with the Soviets and dock with them, but in this case a rescue could be demonstrated without even going through the elaborate procedures that call for the two countries to dock their spacecraft together."

The United States and the Soviet Union conducted their only joint space mission in July 1975 when an Apollo spacecraft and a Soyuz spacecraft docked and hatches were opened between them to form a common tunnel.

The two circled the Earth together for almost two days while three astronauts and two cosmonauts visited each other's spacecraft. The American crewmen were Thomas P. Stafford, Donald K. (Deke) Slayton and Vance D. Brand. The Soviets were Alexei Leonov and Vasily Kubasov.

In the proposed space-rescue demonstration, the American shuttle would rendezvous with the Soviet Salyut space station. One or more American astronauts would fly to the Salyut wearing the jet-powered backpacks used on three shuttle missions. A Soviet cosmonaut might even leave Earth in the American craft and use the backpack to fly to Salyut.

"Both countries want it and both want to demonstrate it," the Johnson Space Center source said. "A space rescue demonstration is long overdue."

Several recent episodes on Soviet space flights -- including a brief scare that cosmonauts might be stranded in a crippled craft -- have raised concern about the potential need for space rescue techniques.

"We don't need a common docking system to do this anymore, the way we did in 1975," the source said. "This thing really can be done quite simply as soon as the two countries decide to do it."

Sources said negotiations might take no more than six months. "We started working with the Russians in 1970 to plan the 1975 mission," said one. "This time around there would be no need to discuss and arrange things that far ahead."

The first opportunity for a rescue demonstration would be in October when the space shuttle carries the European-built Spacelab on its third flight, sources said. Spacelab will carry a record crew of eight, including two Germans and a Dutchman.

"The thought has occurred to us that the rescue demonstration ought to be international," one source at the Johnson Space Center said. "The international idea is a good one."

If it doesn't occur in October, the demonstration rescue mission could come anytime the United States flies the space shuttle on an orbital path that is inclined to the equator by 51.6 degrees. Manned Soviet spacecraft always use that path, which is dictated by launch constraints inside the Soviet Union. U.S. shuttle missions have followed that inclination on two of the 14 shuttle flights.