The space shuttle Discovery has undocked from the International Space Station (ISS) after an eight-day stay on the orbiting outpost.
Poor weather may force the Discovery space shuttle to land at an alternative site to the Kennedy Space Center.
Early on Tuesday, astronauts fixed jammed solar panel on the ISS during an unscheduled fourth spacewalk.
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.
A German astronaut on ISS has switched places with a member of the seven-person Discovery crew and will travel back to Earth on the orbiter.
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.
Discovery detached from the space station at 2210 GMT.
Nasa said it would decide on the final landing site on Friday.
"We finished all the mission objectives so we are very pleased. It's a great day for the ISS programme and the shuttle programme," said Kirk Shireman, deputy manager of the ISS programme.
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 mission had three main objectives: 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 to allow a new one to collect solar energy.
"The decision will be made 90 minutes before landing, so there is no way to know it before Friday."
Extra spacewalk
Mission objectives
In the early hours of Tuesday, US astronaut Robert Curbeam, with Sweden's Christer Fuglesang, folded the stuck solar panel away, to complete a record-breaking fourth spacewalk on a single mission.
Astronauts are making final landing preparations, which has included time spent checking the spacecraft's heatshield for damage.
The two astronauts returned to the ISS at 0138 GMT after more than six and a half hours.
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's launch left a fiery trail in the Florida skyThe unscheduled spacewalk means that the shuttle will return to Earth a day later than planned, on Friday at Florida's Kennedy Space Center.
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.
Nasa approved the spacewalk after astronauts failed to fix the panel despite rewiring electrical systems.
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.
The 35m (115ft) panel needed to be folded up to be moved to another part of the ISS.
Nasa has declared the space station construction work a success.
Staff in Mission Control applauded when the final section fell into the box.
"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.
In Saturday's spacewalk, which was to have been the final one of the mission, Mr Curbeam left the ISS ahead of Sunita Williams, who was beginning her first mission.
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.
Construction task
At least 14 more missions are needed to finish the $100bn (£50bn) ISS.
After several attempts at fixing the jammed solar panel on the ISS, Nasa decided to halt the spacewalk as the astronauts' spacesuits were running short of supplies.
New solar panels, fitted in September, will take over the job of powering the ISS, boosting its power by about 50%.
The mission is taking place to finish the $100bn (£50bn) station. At least 14 more missions are needed.
Discovery blasted off from Florida in the first night launch of a space shuttle in four years, after being delayed for two days because of bad weather.
Discovery's crew consists of Commander Mark Polansky, pilot William Oefelein and mission specialists Mr Curbeam, Joan Higginbotham, Nicholas Patrick, Ms Williams and the European Space Agency astronaut Christer Fuglesang.
A British-born astronaut, Nicholas Patrick, is among Discovery's crew. For five of the seven astronauts, this is their first shuttle flight.
Ms Williams will stay on at the ISS when Discovery heads home, taking German Thomas Reiter back to Earth.
