Atlantis streaked toward a landing in the California desert Monday after the astronauts replaced one of the shuttle's main computers, which quit during the mission's last full day in space.
The problem posed no threat to the four-day flight, but ground controllers decided to take no chances and had the crew cut short their experiments and install a new one."We just don't have the confidence to run with it for entry," said flight director Ron Dittemore.
After the repair job, the five astronauts began stowing their gear for the fiery dash through the Earth's atmosphere and landing at Edwards Air Force Base, Calif. Touchdown in the Mojave Desert was scheduled for 1:43 p.m. MDT, and the forecast called for favorable weather.
The computer problem was the only glitch reported during the mission, during which the shuttle sent the $550 million Magellan probe on a 15-month, 800 million-mile voyage to Venus to map its surface with supersharp radar.
The astronauts' wake-up call Monday morning consisted of a barking dog followed by the Beatles' song "A Hard Day's Night," a reference to the work they did on the computer the night before.
"Good morning. It's time to come home," Mission Control communicator Ken Cameron told the crew.
A relatively small crowd of 100,000 was expected at the landing site because it is on a weekday, NASA spokeswoman Nancy Lovato said. An estimated 460,000 people watched Discovery's landing March 18 - a Saturday.
Flight director Bill Reeves said this morning that the forecast at Edwards was for winds between 9.2 mph and 15.4 mph, meaning Atlantis probably would land on a dry lake bed to test its crosswind handling.
He said if winds dropped below 9.2 mph, the shuttle would probably land on a concrete runway. If the landing takes place on the lake bed, commander David Walker will test the nosewheel steering by driving off the center line and back at 161 mph.
"It's been a nice, clean flight - a good vehicle and good crew," Reeves said. "We're looking forward to a good landing."
The other crew members are pilot Ronald Grabe and mission specialist Norman Thagard, Mark Lee and Mary Cleave.
Sunday's failure occurred in one of four operating computers that control and monitor all systems aboard the spacecraft and actually fly it most of the time. The shuttle could land with only one computer.
The crew abandoned plans to melt and crystalize a metal sample in a furnace and to photograph lightning and polluted areas on Earth.