The record 139 days two Soviet cosmonauts spent in space has convinced U.S. experts that the Russian will try to keep cosmonauts in orbit for six months sometime next year.

"They've said all along their goal is to keep stepping up their stay time in space," said the Library of Congress' Dr. Charles Sheldon, a leading Capitol Hill authority on the Soviet space program. "There's no reason to think they won't aim for six months."

Of more interest to U.S. space officials than endurance time is the purpose behind it.

"There are some people who think the Soviets are preparing for something big, like a space station or sending men to Mars," a highly placed source in the Carter administration said. "It won't happen next week or even next year but why would you keep-men up there that long if you didn't have something in the works. It puzzles us."

Most U.S. watchers think the Soviets are developing plans to orbit a large space station, one much bigger than the 20-ton Salyut the cosmonauts have visited and larger even than the 85-ton Skylab the U.S. put into orbit five years ago.

The Soviets do not have the rocket power to launch a large space station into orbit infact but could place four or five big structures in similar orbits to be assembled into a space station.

The Soviets have said they plan to orbit space stations that will be manned for long periods by the same crew, rather than visited every few months by different crews. They have said that it is not economic to revisit the same space station four or five times a year.

The Soviets also have said they would like to send men to Mars, a trip that could take a year. The Soviets have no timetable for a manned Mars excursion but earlier this year announced they tested a closed life-support system to keep men alive in deep space for as long as a year.