CAPE CANAVERAL, Fla. -- A suspect navigation unit aboard shuttle Endeavour was declared dead Thursday, leaving seven U.S. astronauts and Russian cosmonauts one failure away from an emergency return to Earth.
With the shuttle and the International Space Station flying in tandem high above the planet, one of Endeavour's three Inertial Measurement Units began acting erratically late Wednesday as the crews aboard the complex headed off to bed.
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Perhaps due to an intermittent electrical spike, the problem triggered an alarm in the shuttle's cockpit, prompting ground controllers and Endeavour mission commander Dom Gorie to attempt to recover the unit.
That effort took about 45 minutes, and the unit since has been operating normally. Mission managers, however, now consider the device a failed unit because engineers could not conclusively pinpoint the cause of the problem Thursday.
NASA flight rules, meanwhile, call for a mission to be cut short if two of the shuttle's IMUs fail."We would be looking at an early mission termination, and in fact we would be looking to land probably within 24 to 48 hours," said NASA flight director Wayne Hale. "At that point you would be on your last good Inertial Measurement Unit."
About the size of small file cabinet drawers, the three 43.5 pound (19.5-kilogram) devices provide crucial guidance and navigation information to the shuttle's primary flight computers.
Housed between the shuttle's nose and cockpit, the units generate precise information about the ship's speed, orientation and direction of travel, enabling an orbiting crew and ground controllers to keep the craft on course.
No spare units are carried onboard shuttles, and Hale said mission managers are concerned that the suspect unit could begin acting erratically again.
"It's working now, but we're kind of worried that it will have another episode," he said.
Hale stressed that the shuttle's other two IMUs are operating without problems and that engineers have no reason to believe they might fail over the course of the next several days. Engineers are continuing a bid to determine the cause of the fleeting problem with the failed unit.
"Frankly, it's a little bit of a puzzle here that will give the engineers something to look at for a few days," Hale said.
Launched last Wednesday from Kennedy Space Center, Endeavour and four shuttle astronauts ferried a new crew to the station and are scheduled to return to Earth next week with three men who have been on board the outpost since August.
Landing remains scheduled for 12:55 p.m. EST (1755 GMT) Monday here at NASA's coastal Florida spaceport.
An early return to Earth, meanwhile, would be only the fourth in shuttle program history.
Fuel cell failures forced NASA to cut short shuttle missions in 1981 and 1997, and a single IMU failure prompted a mission to be abbreviated in 1991.
Conservative NASA flight rules have been relaxed since 1991 mission, however. Two of the navigation units, consequently, have to fail now before a shuttle crew is ordered to make an early return to Earth.
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