Russian computers that control the International Space Station's orientation and supply of oxygen and water have failed, potentially extending the space shuttle's mission — or cutting it short.

Russian engineers are not sure why the computers stopped working. A failure of this type has never occurred before on the space station.

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

"We have plenty of resources, so we have plenty of time to sort this out," said Mike Suffredini, NASA manager of the space station program.

But the computer failure could extend space shuttle Atlantis's mission by at least a day and, in a worst-case scenario, force the space station's three crew members to return to Earth early if the computers are not fixed.

Atlantis's mission had already been extended from 11 to 13 days so that astronauts can go on a spacewalk to repair a thermal blanket covering an engine pod.

Suffredini said he expected the computer problem to be fixed in the next couple of days. In a worst-case scenario, if at least one of the computers was not operating after the shuttle left, the space station's three crew members could return to Earth, he said.

Thrusters on the docked space shuttle, along with the space station's gyroscopes, have been fired periodically to help maintain the space station's positioning since the computers failed earlier this week.

The space station needs the manoeuvring thrusters controlled by the Russian computers for docking and avoiding space debris.

Without the Russian oxygen-machine running, the space station has a 56-day supply of oxygen left. "If we are in that position, we have an option to depart," Suffredini said.

Russian engineers think the computers' failure could have been triggered by a power source. The space station earlier this week got a new pair of solar arrays that were delivered by Atlantis and unfolded Tuesday to help provide power.

During a spacewalk on Wednesday, astronauts Patrick Forrester and Steven Swanson began to prepare a rotating joint that will allow the new pair of solar arrays to track the sun. Astronauts will finish prepping the joint on another spacewalk.

Forrester and Swanson also helped retract a 35-metre wing of an old solar array that will be folded into a storage box and moved to another location later this year.

Only 13 of the array's 31 sections were folded up, so flight controllers and astronauts will try to fold up the rest of the solar wing by remote commands on Thursday.