Equipment failure to force shuttle home early

CAPE CANAVERAL, Fla. The space shuttle Atlantis will return toEarth three days early because of malfunctioning navigatingequipment, NASA said Saturday.

Landing was set for 2:58 p.m. Chicago time today at Edwards AirForce Base in California, the National Aeronautics and SpaceAdministration said.

Atlantis was launched from Florida Nov. 24 on what was to havebeen a 10-day mission for the Defense Department.

NASA officials declared a "minimum duration flight" within anhour after the navigating equipment, called an inertial measurementunit or IMU, started to malfunction.

Flight commentator Kari Fluegel said software engineers noticeda "bit error" in the unit, which gives speed and accelerationinformation to the shuttle's computers.

Minutes later, mission managers called the shuttle home.

Atlantis was to have touched down at the Kennedy Space Center inFlorida on Wednesday afternoon. Officials diverted the shuttle tothe desert lakebed at Edwards because they consider it a safer placeto land.

"They're looking at weather, too, so they may delay untilMonday, but the flight rule dictates that they come home," said JackRiley, a NASA spokesman at the Johnson Space Center in Houston.

Each shuttle has three IMUs and is capable of flying with onlyone, but NASA's policy is to end the flight if only two are working.

The IMU had been turned off early in the flight to save power, aroutine practice on long missions. It failed about 14 hours afterthe astronauts turned it on to aid with tests of some other onboardsystems, Fluegel said.

Out of 44 total shuttle flights, this is only the fourth thatNASA has been forced to shorten.

The second mission of Columbia in November, 1981, was cut bythree days because of a power generator failure.

Shuttle Discovery returned a day early in September, 1985, afterhaving cargo problems. And Atlantis landed 90 minutes early inDecember, 1985, to beat bad weather in California.