The failure of Russian computers which control the international space station's positioning have NASA managers considering another extension of space shuttle Atlantis' visit to the orbiting outpost, officials said.
Since the computers failed earlier this week, thrusters on the docked space shuttle have been fired periodically to help maintain the space station's positioning.
NASA managers hoped to have the computers back up before Atlantis and its seven crew members undock from the space station next Tuesday.
But if the computers are not functioning, NASA may look into extending the space shuttle's stay a day or two.
Atlantis' mission, originally scheduled for 11 days, was extended by two days already so that astronauts can go on a spacewalk to repair a thermal blanket covering an engine pod that peeled up during launch.
Space station programme manager Mike Suffredini said on Wednesday he expected the 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.
"We always have an option to depart," Suffredini said.
On Wednesday, two astronauts went on a spacewalk to complete two tasks. They helped fold up a solar wing and tried to bring to life a rotating joint that will allow a new pair of solar arrays to track the sun.
The spacewalk began at 2358 IST as the astronauts were 332 kilometres above eastern Europe and ended more than seven hours later. Space shuttle Atlantis astronauts Patrick Forrester and Steve Swanson spent the first two hours helping to put the 35-metre solar wing away in its storage box.