NASA struggles with shuttle navigation unit

CAPE CANAVERAL, Fla. -- NASA struggled Thursday to understand a fleeting but perplexing problem with one of space shuttle Endeavour's key navigation devices.

One of three inertial measurement units aboard Endeavour malfunctioned early Thursday, setting off an alarm in the cockpit after the astronauts had gone to bed.

It was immediately taken off-line, another unit took over, and shuttle commander Dominic Gorie was told to go back to sleep.

The unit started working again, even though it was not being used to guide Endeavour, said flight director Wayne Hale.

He stressed that nothing is jeopardized by having just two reliable inertial measurement units -- only one is needed to fly the shuttle.

But if another one fails -- an unlikely event, according to Hale -- the shuttle would be forced to return to Earth because there no longer would be any redundancy.

At this point, engineers are merely trying to figure out what went wrong.

One of the gyroscopes in the device apparently started to rotate slowly, which disturbed the stability of the unit, Hale said.

An intermittent electrical failure in circuitry may be to blame.

"I've got to stress that the engineering community is still out there thinking about this," he said.

A Cape Girardeau County native, astronaught Linda Godwin, is aboard the Endeavour.