While engineers try to determine what caused the failure of Russian computers that control the international space station's orientation, oxygen and water supplies, a NASA official said Thursday he doesn't consider the situation critical.

The computers were up and running briefly early Thursday, said Bill Gerstenmaier, NASA's associate administrator for space operations.

But since engineers are still troubleshooting, they were expected to go down and come back up throughout the day.

"I think we're stable. In my world, this is space station operations," Gerstenmaier said.

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It will probably be several days before engineers figure out what caused the computers to fail Wednesday, he said.

"We've got a challenge. We'll figure out a way to get this behind us," he said. "We've got time. There's no criticality that we have to figure this out in a certain amount of time."

This type of computer failure had never been seen before on the space station. The addition of a new solar array to the space station might be a cause, he said. Other possibilities include electromagnetic interference and a software problem.

A new solar array had been unfolded outside the station Tuesday to help provide power for the orbiting outpost, and astronauts spent Wednesday hooking up a joint that will let the arrays track the sun.

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The crew got a scare early Thursday while the computers were being reconnected: A fire alarm went off on the Russian Zarya module, but Gerstenmaier said there was no indication of fire or smoke.

Since Atlantis is still docked at the space station, its thrusters can help, if needed, to maintain the station's position while computers are down, Gerstenmaier said.

NASA has not decided whether to extend the mission because of the problem, he said. But Atlantis astronauts were asked Thursday to begin conserving power in case the shuttle has to remain an extra day.

The mission had already been extended from 11 to 13 days to repair a thermal blanket that peeled up during launch.

"Ideally we'd like to have the computers up and operating in some function before the shuttle departs," Gerstenmaier said.

Systems can be reconfigured on the space station to help it continue operating if the shuttle leaves and the problem still hasn't been fixed, he said.

In a worst-case scenario, NASA said, the station's three crew members might have to return to Earth early if the computers aren't fixed.

Without the Russian oxygen-machine running, the space station has a 56-day supply of oxygen left.

"We're a long way from that scenario," Gerstenmaier said.

[The space station has been continuously occupied since the first permanent crew arrived aboard in November 2000.]

The space station is operated primarily by the Russian and U.S. space agencies, with contributions from the Canadian, European and Japanese space agencies.

During a spacewalk on Wednesday, astronauts Patrick Forrester and Steven Swanson started to bring to life a rotating joint that will allow the new solar arrays to follow the sun.

They also helped retract a 115-foot wing of an old array that will be folded up into a storage box and moved later this year.

Only 13 of the array's 31 sections were folded up. Flight controllers and astronauts on Thursday resumed efforts to fold up the rest of the solar wing by remote commands. The procedure ran into some problems Thursday, with bunching of panels and tangling of guidewires.

NASA managers decided Wednesday to use a spacewalk Friday to repair a torn thermal blanket, located over an engine pod near the shuttle's tail.

Engineers don't think the damaged section of the thermal blanket, which protects part of the shuttle from the blazing heat of re-entry, would endanger the spacecraft during landing. But it could cause enough damage to require schedule-busting repairs.

NASA has focused intensely on any problems that could jeopardize a shuttle's re-entry into Earth's atmosphere since shuttle damage resulted in the 2003 Columbia disaster that killed seven astronauts.