Barring technical difficulties with the spacecraft, the shuttle Discovery will land today, but officials don't know where.
The weather forecast is unfavorable for the craft's primary and secondary landing sites, the Kennedy Space Center in Florida and Edwards Air Force Base in the Mojave Desert. Unless the outlook changes, the craft will come down at the mission managers' third choice, White Sands Space Harbor in New Mexico.
NASA officials want to land Discovery today -- or Saturday at the latest -- because the craft is running low on supplies, particularly the cryogenic oxygen used to generate electricity in the shuttle's fuel cells.
The mission has already been extended one day so that astronauts could take an unscheduled fourth spacewalk to fold up an older solar panel array.
White Sands has been used only once before -- in March 1982, when the shuttle Columbia returned from its third flight into orbit.
The safety of the landing "is no concern at all at White Sands," said deputy shuttle program manager John Shannon. "As a landing site, White Sands is phenomenal. It is extremely flat and extremely hard."
The problem is the lack of equipment there to service the shuttle and load it atop a specially modified Boeing 747 for the flight back to Kennedy. Getting the proper equipment to White Sands will delay the agency's ability to prepare the craft for its next flight.
Thursday afternoon, engineers were loading backup equipment from Kennedy aboard two C-17 aircraft to transport it to New Mexico. Among other cargo, the planes carried a power unit to protect the craft from the freezing temperatures, electrical generators and equipment to vent remaining cryogenics from Discovery's tanks.
Sixty technicians are also going along to service the shuttle.
A crane will have to be erected at the site to load the shuttle on top of the 747, which could delay its return to Kennedy by six to eight weeks, Shannon said. Normally, it takes about a week to fly a shuttle back from Edwards.
When Columbia landed at White Sands, it was damaged by a sandstorm that blew gypsum into openings and scratched surfaces.
"Last time they parked in a really bad spot," Shannon said.
Since then, NASA has constructed a new tow way to a service pad where the shuttle will be safe.
The first opportunity for landing at Kennedy today will be at 12:51 p.m. PST, but the forecast is for rain and an unacceptably low cloud cover. The first opportunity at Edwards is at 2:27 p.m. PST, but the forecast there calls for unacceptably high crosswinds.
The forecast for Saturday at Kennedy is no better, but the crosswinds at Edwards are expected to have abated by then. Weather is expected to be good at White Sands both days.
In addition to packing up for their return, the shuttle astronauts used a camera on the shuttle's robot arm Thursday to inspect the heat tiles for damage one more time. None being found, the craft was cleared for reentry.
(BEGIN TEXT OF INFOBOX)
If the weather is bad at Florida's Kennedy Space Center, NASA managers have two other options for landing the space shuttle. Landing sites of 113 shuttle flights since 1981:
Edwards Air Force Base...50
White Sands, N.M....1