Mir orbits Earth. Photo: NASA

Contact with the unmanned Russian Mir space station was lost early today after a computer in the lab's motion control system allowed the station to drift out of its normal orientation, depleting on-board batteries.

During subsequent passes over Russian ground stations, commands were uplinked to re-orient the station to maximize the amount of sunlight falling on Mir's solar arrays. Russian flight controllers told their NASA counterparts the station was well on the road to recovery.

It's not yet known what caused the motion control system computer, which was installed last summer, to allow the station's orientation to change in the first place. But engineers believe it was a "single event upset" type of event, i.e., some sort of one-time spurious command, and that Mir's systems appear generally healthy.

The brief scare, however, raised concern once again about the Russians' ability to control the abandoned station's upcoming re-entry and breakup.

The current plan calls for the launch of an unmanned Progress fuel tanker around Jan. 10 that would dock with Mir two days later. The Progress would be used first to raise Mir's orbit slightly to set it up for a controlled re-entry over the Pacific Ocean between Feb. 26 and 28.

But that assumes Russian flight controllers can communicate with the station and that its main central computer and motion control system are operational.

If contact was permanently lost, and if nothing else was done, Mir would re-enter the atmosphere and break up sometime in late March or early April, according to Russian projections.

Debris from Mir is expected to survive the heat of atmospheric entry and impact Earth somewhere along the station's ground track. Mir's orbit carries it 51.6 degrees to either side of the equator where 70 percent of the world's population lives.

Two Russian cosmonauts - Salizhan Sharipov and Pavel Vinogradov - are trained to make an emergency flight to Mir if conditions warrant to make repairs or take over manual control of the station.

But it's not yet clear how soon they could be launched in a real emergency or even whether intermittent communications would warrant such a mission.

Russian engineers believe the glitch today occurred in a motion control system computer that was installed last year to control the station's attitude until the lab's main computer is reactivated for re-entry.