Shuttle Flight to Be Cut Short as Unit Fails : Space: Mission Control favors landing at Edwards today if winds subside. Crew not considered in danger due to backup navigational devices.

NASA ordered the six astronauts aboard the shuttle Atlantis to shorten their military mission by three days after a vital navigational unit failed Saturday. A landing was planned for today.

Officials at Mission Control told shuttle commander Frederick D. Gregory that his first landing opportunity would be at 12:58 p.m. at Edwards Air Force Base in California.

NASA said the astronauts were in no danger.

Flight directors were assessing the weather at Edwards, where high winds were forecast. They said they would wait until Monday to bring Atlantis home if the weather does not cooperate today.

The navigational unit, one of three aboard the spaceship, failed Saturday morning. NASA decided to shorten the 10-day flight following unsuccessful repair attempts by the crew.

Atlantis was not supposed to return until Wednesday.

National Aeronautics and Space Administration flight rules dictate that a shuttle must come home early when one of the navigational units fails. The rules also stipulate that the spaceship try for the expansive, dry lake bed at Edwards if possible.

Flight directors said they would consider Kennedy Space Center, the original touchdown site, only as a last resort because it has only a single concrete runway surrounded by marsh.

NASA officials said there was no danger in coming back with only two working navigational devices, called inertial measurement units, or IMUs. Only one of the units is needed during flight.

“We don’t want another one to go down and just have one available,” NASA spokesman Jim Cast said.

It would be extremely difficult, if not impossible, for a shuttle to land without any navigational unit, Cast said. The IMUs provide information on the ship’s speed and position to on-board computers.

“You wouldn’t know how fast you were going and you wouldn’t know what the attitude (position) was,” Cast said. “I don’t think it would be impossible, but I frankly don’t want to speculate right now because we’ve never done it and we don’t want to do it.”

NASA has shortened shuttle missions because of equipment trouble only once before. Columbia was forced to return to Earth three days earlier than planned on the second shuttle flight in November, 1981, because of a fuel cell failure.

Inertial measurement units have failed on two previous flights. Neither of those missions had to be shortened because the trouble occurred late in the flight, Cast said.

The astronauts calmly went about their business Saturday after being notified that they would be coming back sooner than expected. They wrapped up military surveillance and medical experiments before stowing their equipment.

Atlantis blasted into orbit last Sunday. The crew successfully released a missile-warning satellite the next day.