Favorable weather awaits Columbia astronauts as they attempt to return to Earth with a steering system handicapped by a cooling problem, NASA said Saturday.
The shuttle was scheduled to touch down at the Kennedy Space Center in Cape Canaveral at 12:09 this morning, with commander Rick Searfoss and pilot Scott Altman at the controls. There was a second opportunity to land later in the day.
Forecasters predicted mostly sunny skies and winds close to, but beneath, the shuttle's cross-wind limit. The space agency, however, will watch conditions more closely than usual because of a cooling system failure on one of the shuttle's three auxiliary power units (APU), which generate hydraulic power, said John Shannon, the NASA flight director who will supervise Columbia's return to Earth.
The breakdown was discovered Saturday during routine pre-landing tests. Two of the generators are required to supply the hydraulics needed to move the shuttle's braking rockets, move its wing flaps and tail rudder, lower the landing gear and provide runway braking and steering.
"I don't consider it to really be any additional risk," Shannon said. "We have a high degree of confidence we could land the shuttle with a single APU. A third APU is just a good backup."
Shuttle crews landed safely after a similar problem in 1991, with one APU leaking in 1996 and with one turning sluggish during another mission two years ago.
Columbia's problem is thought to be a mechanical failure in the nozzle of a device that sprays water on hydraulic fluid lines. The auxiliary power units are used only during the launching and landing and the same generator overheated after the April 17 liftoff. Engineers suspected the cooling system iced up.
The shuttle crew also performed a plumbing chore Saturday, draining nearly 10 gallons of liquid from the shuttle's blocked septic tank into a rubber bag carried inside the living quarters. The narrow line used to periodically jettison the fluid into space has been blocked since midweek.
The six-man, one-woman shuttle crew is wrapping up a 16-day mission dedicated to researching the effects of spaceflight on the brain and nervous system. Twenty-six major investigations were conducted, many on a cargo that included more than 2,000 rodents, fish and insects.
Meanwhile on Saturday, the Discovery was moved from its hangar at the Kennedy Space Center to a coastal launch pad. Discovery is slated to lift off June 2 for a trip to Russia's Mir space station. Its crew will retrieve Andy Thomas, the seventh and final American astronaut assigned by NASA to live and work aboard the orbital outpost.