Russian computers controlling the International Space Station's (ISS) orientation have failed, Nasa officials say.
The station's three crew and seven visiting shuttle astronauts are not at any immediate risk; the ISS has not lost all vital systems.
But it could force Nasa to extend the current shuttle mission by one day.
Engineers are unsure why the computers stopped working as a failure of this type has not occurred before.
The US space shuttle Atlantis docked with the orbiting outpost at the weekend to begin its latest mission.
The computer malfunctions have affected a number of systems including the Elektron oxygen generation system.
Nasa's space station chief said he expected the problem to be fixed within the next few days.
"We have plenty of resources, so we have plenty of time to sort this out," said Mike Suffredini, ISS programme manager for the US space agency.
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," Mr Suffredini said.
Different scenarios
The station's Russian segment has a network of six computers, but on Wednesday, only two were functioning.
The computers have experienced problems before, but a system-wide re-boot usually solved the problem.
This time, the system has been unable to re-boot.
Under a worst-case scenario, where the computer problem persists beyond the shuttle's stay, and attempts to resolve it come to no avail, the station's three crew members could be forced to return to Earth early, the website Space.com reported.
The ISS is currently relying on its four gyroscopes to maintain its orientation in space, then shifting to using thrusters aboard the shuttle Atlantis when the gyroscopes are overwhelmed.
This dependency on the shuttle has caused Nasa managers to consider an extension of one day to Atlantis' 13-day mission.
If this goes ahead, the crew will have to conserve supplies - the shuttle mission has already been extended by two days in order to carry out repairs on a torn thermal blanket.
The crew of space shuttle Atlantis was originally due to spend 11 days at the ISS.
Heat shield
The mission was extended to 13 days in order to carry out repairs on a 10cm (4in) section of thermal blanket which peeled back as the shuttle blasted off from Cape Canaveral on Friday.
The blanket protects the shuttle from the intense heat of re-entering the atmosphere.
The repairs could be made during a planned third spacewalk or a fourth, extra one, Nasa managers have said.
Engineers think the blanket was loosened by aerodynamic forces during lift-off, and was not hit by debris.
Damage to the shuttle Columbia in 2003 during its launch led to the vehicle's disintegration as it returned to Earth, killing all seven crew.
This was supposed to be the second shuttle mission of 2007, but a freak storm over the Florida launch site in late February caused hail damage to the shuttle and delayed the mid-March flight.
Despite the delays, managers are confident they will be able to complete the ISS before the shuttles' 2010 retirement date.
Nasa plans to fly 15 more missions to the station to deliver large components, spare parts and other supplies. In addition, one final servicing call to the Hubble Space Telescope is planned for September 2008.
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