Poor weather may force the Discovery space shuttle to land at an alternative site to the Kennedy Space Center.
The shuttle Discovery has been cleared to return to Earth, but poor weather over the Kennedy Space Center may force it to an alternative landing site.
The spacecraft was scheduled to put down at 2056GMT (1556EST) on Friday at the Florida base, but forecasts of low cloud and rain may shift it elsewhere.
Nasa controllers have the Edwards Air Force Base, in California, and White Sands Space Harbor, in New Mexico, on standby to receive the vehicle.
The US space agency is considering two back-up sites - at the Edwards Air Force Base, in California, and White Sands Space Harbor, in New Mexico.
If the rain and clouds lift at Kennedy, Discovery's first landing opportunity is at 2056 GMT (1556 local time).
Nasa said it would decide on the final landing site on Friday.
The shuttle has been on a mission to rewire the International Space Station.
Jessica Rye, a Nasa spokeswoman at the Kennedy Space Center, said: "So far, the weather conditions are acceptable in White Sands for Friday, but the weather in Florida is always unpredictable.
"The decision will be made 90 minutes before landing, so there is no way to know it before Friday."
Mission objectives
Astronauts are making final landing preparations, which has included time spent checking the spacecraft's heatshield for damage.
Such inspections were prompted by the Columbia disaster in 2003. A briefcase-sized chunk of foam damaged Columbia's wing on lift-off, allowing superheated gasses to penetrate the heatshield as the vehicle returned to Earth.
Discovery has met all its construction objectives Discovery was expected to return on Thursday from its visit to the International Space Station (ISS), but a fourth unplanned spacewalk during its mission has delayed the landing by a day.
In the early hours of Tuesday, US astronaut Robert Curbeam, with Sweden's Christer Fuglesang, made the excursion to fold away a stuck solar panel on the ISS.
Nasa has declared the space station construction work a success.
"We finished all the mission objectives, so we are very pleased. It's a great day for the International Space Station programme and the shuttle programme," said Kirk Shireman, deputy manager of the ISS programme.
The crew had three main objectives during their eight days spent docked to the space station: installation of a two-tonne truss to the girder-like ISS structure; the rewiring of the power and thermal systems; and the retraction of an old solar array.
At least 14 more missions are needed to finish the $100bn (£50bn) ISS.
