NASA struggling to understand problem with space shuttle Endeavour's navigation unit

<br>CAPE CANAVERAL, Fla. (AP) _ NASA struggled Thursday to understand a fleeting but perplexing problem with one of space shuttle Endeavour's key navigation devices. <br><br>One of three inertial measurement

Thursday, December 13th 2001, 12:00 am

By: News On 6

CAPE CANAVERAL, Fla. (AP) _ 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. ``While I think it's a long shot, they might come back and say this is an explained condition and we really don't have anything to worry about, although right now we're being very conservative with how we treat this particular black box.''

The departing international space station residents, meanwhile, ceremoniously handed over control to their successors Thursday.

``The ship is now your responsibility,'' said outgoing commander Frank Culbertson. He presented his replacement, Russian cosmonaut Yuri Onufrienko, with the ship's log and firmly grasped his right hand.

Departing cosmonaut Mikhail Tyurin noted that the next space station crew is bound to have difficulties during the next six months. ``So good luck, guys. The best wishes to you,'' he said.

Culbertson, Tyurin and Russian cosmonaut Vladimir Dezhurov actually moved out of the space station and into the docked Endeavour last weekend. At that point, Onufrienko and American astronauts Daniel Bursch and Carl Walz became the official space station residents.

But it wasn't until Thursday that the two crews held the customary change-in-command ceremony based on the traditions of the high seas. Unfortunately, the communication link was lost in the middle of the ceremony, and nothing could be seen or heard from the orbiting complex for 90 seconds.

The link was restored just in time to see the 10 space travelers embrace and shake hands.

Endeavour will undock from the space station Saturday and land two days later. The touchdown will end a 129-day mission for Culbertson and his crew, which began in August.