Cape Canaveral (Florida): NASA managers may have to decide between three imperfect choices as they try to select a landing site for the shuttle Discovery.
The forecast calls for low clouds and showers at Kennedy Space Center, where space shuttle is slated to land.
Crosswinds are expected at the primary back-up landing site at Edwards Air Force Base in California.
The weather looks good at a seldom-used back up site in New Mexico, but its lack of equipment would give NASA logistical headaches it would rather avoid.
As the astronauts prepared for their last full day in space on Thursday, NASA managers on the ground faced a series of complicated trade-offs about where to land the shuttle.
"More than likely, we will evaluate the conditions on a case-by-case basis on Friday and pick the lesser of evils," said Phil Engelauf, chief of the flight directors' office.
On Thursday, Discovery's seven astronauts planned to test the shuttle's flight control system, stow away items, deploy a tiny satellite and give their last media interview from space.
Engineers also were expected to give a green light for Discovery to come home after studying images Discovery's crew took of the shuttle on Wednesday to make sure there was no damage to its heat shield.
"Hopefully, there is nothing to see," Discovery commander Mark Polansky radioed Mission Control.
NASA wants the shuttle on the ground no later than Saturday because the fuel that generates electricity on the spacecraft will run out.
The space agency has seven chances to land the shuttle on Friday and Saturday at any of the three sites.
On both days, the weather is expected to be unacceptable at Kennedy, NASA's preferred site since it would save the agency the time and cost of shipping the shuttle to Florida from the backup sites in California and New Mexico.
Crosswinds at Edwards on Friday are expected to be just about 20 miles per hour, more than three miles per hour above the 17 mph limit permitted under safety rules.
NASA may consider relaxing that restriction, though, since Discovery's new wheels and tires can land with winds at 23 miles per hour.
The weather at Edwards on Saturday is expected to improve to an acceptable level.
While the shuttle-landing site in White Sands, N Moffers the best hope in terms of weather, it presents NASA with the most logistical hurdles.
Its lack of equipment is the main reason why it only has been used once for a shuttle landing, in 1982.
If the shuttle lands in New Mexico, NASA would have to fly heavy equipment there, including a crane to hoist the shuttle atop a jetliner for the trip back to Florida and devices that control the shuttle's temperature and electrical systems on the ground.
Those steps would delay the shuttle's return to Florida by more than a month, compared with a week from Edwards Air Force Base.
NASA managers also were concerned that below-freezing weather expected over the weekend at White Sands could cause leaking thrusters and could freeze fuel and water lines.
They are also concerned that fine sand could dirty the shuttle. During the New Mexico landing in 1982, sand on the runway contaminated the orbiter, and the brakes were damaged.
Discovery had been scheduled to land Thursday, but an unplanned fourth spacewalk during the shuttle's visit to the international space station pushed back the landing by a day.
During Discovery's successful eight days at the space station, astronauts rewired the orbiting outpost, installed a new $11 million section, retracted a stubborn solar array and rotated out a space station crewmember.
"We are really looking forward to getting back, seeing everybody having a great holiday," Polansky said adding, "I think this is a great way to bring in the end of the year."
NASA kept a close eye on worrisome weather forecasts at its primary space shuttle landing sites, where low clouds and high winds threatened to interfere with Discovery's scheduled landing.
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