On what was scheduled to be their last full day in space, crew members aboard space shuttle Discovery woke up Thursday to the strains of The Road Less Travelled.

"It's great to hear some good jazz from Houston's own incomparable Joe Sample. He'll help us ease into the last few days of this mission," astronaut Joan Higginbotham said by radio to mission control.

Discovery's seven astronauts planned to spend the day testing the shuttle's flight control system, stowing items, deploying a tiny satellite and giving their last media interviews from space.

NASA managers in Houston, meanwhile, faced a series of complicated trade-offs about where to land the spacecraft. They may have to decide among three imperfect choices as they try to select a landing site.

The forecast calls for low clouds and showers at Kennedy Space Center, where Discovery is slated to land Friday.

Crosswinds are expected at the primary back-up landing site at Edwards Air Force Base in California. The weather looks good at another back-up site in New Mexico, but it is rarely used and lacks equipment, meaning a landing there would give NASA logistical headaches.

"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, engineers 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.

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 a few kilometres-per-hour above the limit permitted under safety rules. NASA may consider relaxing that restriction, though, since Discovery's new wheels and tires can land in higher winds than its older ones. The weather at Edwards on Saturday is expected to improve to an acceptable level.

While the shuttle landing site in White Sands, N.M., offers 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 the landing ahead by a day.

During Discovery's successful eight days at the space station, astronauts rewired the orbiting outpost, installed a new $11-million US section, retracted a stubborn solar array and rotated out a space station crew member.