CAPE CANAVERAL, Florida (CNN) -- U.S. astronaut Andy Thomas told NASA officials Sunday he is nervous that a last-minute computer problem could delay his return from Russia's Mir space station.

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 the last 4 1/2 months on Mir and is the seventh and final U.S. astronaut to visit the space station.

Thomas' first question on Sunday was whether a computer failure aboard Mir would affect Tuesday's planned launch of the space shuttle 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."

Thomas on Mir

Discovery is scheduled to pick Thomas up from Mir and return him to Earth. But NASA said Sunday they can't launch Discovery until Mir's current computer problems are solved.

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.

Mir must be under control for Discovery to dock with it, according to NASA officials. 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.

However, NASA officials said they expect Mir's computer problems will be corrected in time for Tuesday's launch.

Cosmonauts aboard Mir have replaced the failed computer with a new one that was brought up on the last shuttle ferry flight in January. It will be late Sunday night before officials know whether Mir will be able to operate on its automated guidance system for the docking with Discovery.

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

Discovery's flight is 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.

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