CAPE CANAVERAL, Florida (CNN) -- A failed computer aboard the Russian space station Mir was replaced with a new one Sunday, but cosmonauts couldn't get it to work properly -- possibly delaying the homecoming of NASA astronaut Andy Thomas.

Mission managers were to meet Monday morning to decide whether to proceed with Tuesday's scheduled launch of the space shuttle Discovery, which is slated to pick up Thomas from Mir on Thursday.

Mir's computer-controlled automatic steering system must be working for Discovery to dock with it.

Earlier Sunday, Thomas told NASA officials he is nervous that Mir's computer problems could delay his return to Earth.

Thomas on Mir

Thomas told Scott Gahring, head of operations for NASA at Russian Mission Control, that he wants the shuttle to come and get him as soon as possible. Thomas, 46, has spent 4 1/2 months aboard Mir and is the seventh and final U.S. astronaut to visit the space station.

"He's a little nervous, but no more so than I think anybody would be when your ride looks like it could be (affected)," Gahring said. "He's going pretty good."

Thomas' first question on Sunday was whether a computer failure aboard Mir would affect the launch of Discovery, said Frank Culbertson, program manager for the shuttle-Mir program.

"You expect somebody to come pick you up and then they call and say, well, they're having second thoughts," Culbertson said. "That's what a person in that situation would probably think. He's just waiting for the next shoe to fall. But we're going to go get him."

But NASA flight rules say a shuttle can't launch for a docking mission with Mir unless it is expected that the station is going to be under control within 60 hours of launch. Discovery is scheduled to launch at 6:10 p.m. on Tuesday.

The problems began when Mir's motion-control computer failed on Saturday, causing Mir to lose the ability to point its solar panels toward the sun automatically. The station is now in "free drift," where the station is allowed to slowly rotate as it orbits the Earth.

Cosmonauts aboard Mir have replaced the failed computer with a new one that was brought up on the last shuttle ferry flight in January. But as of Sunday night, comonauts were unable to get the new computer to work properly.

NASA officials said engineers believe the problem lies with the computer or a related component, considering the sudden shutdown of the original computer the day before, NASA spokesman Bruce Buckingham said.

"Right now, the flight controllers don't know what the problem is," Buckingham said Sunday night. "It's left them kind of scratching their heads."

As a precaution to conserve power, the crew shut down non-necessary electrical equipment, making the atmosphere aboard Mir hot and muggy.

Discovery's flight will be the ninth and final time a NASA shuttle will dock with the aging space station. The schedule calls for Discovery to dock with Mir on Thursday and remain docked for four days.

Besides picking Thomas up, the shuttle is bringing several new projects to Mir, including a Russian biotechnology experiment.

· MIR MAIN PAGE · RELATED SITES · HISTORY · TIMELINE

· GALLERY · SOYUZ · CREW · REPAIR MISSION