Crew members on the Mir space station worked to restart their computer Monday - hoping they can avoid postponing the Russian space station's last rendezvous with a U.S. shuttle.

After running smoothly for months, Mir's main computer shut down Saturday. But Monday's first tests ran successfully and the crew hoped to get the computer back on line later in the day, said Valery Lyndin, spokesman for Russian Mission Control.The space shuttle Discovery is scheduled to lift off Tuesday and dock with the station Friday to pick up astronaut Andrew Thomas, the seventh and last American to live on the Russian outpost.

But that trip to the Mir is in doubt, since the shuttle can only dock with the station when its computer-controlled steering system keeps it properly aligned.

Lyndin said space officials have not yet determined why Mir's computer broke down Saturday. Russian cosmonauts Talgat Musabayev and Nikolai Budarin replaced it with another unit, but the new unit failed Sunday when they tried to switch it on.

During a chat with ground controllers, Musabayev said the situation on board "isn't bad," but added he was concerned that it occurred on the eve of the shuttle's launch. "I'm worried not for myself, but for the flight program," he said.

Frank Culbertson, director of NASA's shuttle-Mir program, said Thomas was a little antsy about the chance that his ride home might be delayed.

"He's just waiting for the next shoe to fall," Culbertson said. "But we're going to go get him."

Thomas has been living on the space station since January.

Mir's computer repeatedly failed last year, prompting Russian space officials to send up a replacement on a shuttle flight in January. That unit failed Saturday after five months of trouble-free operation.

Mir's main computer operates the station's motion control system, and when it fails, the station's energy supply is threatened because it can't keep its solar panels in optimum alignment with the sun.

When the latest trouble occurred, the station was an orbit that puts it in direct sunlight round the clock. The crew also shut down some systems to save energy, and as a result Mir's batteries were full Monday.