The aging shuttle has flown more than any other in the fleet during its 27-year career, and will become the first of three to retire as the United States winds down its storied space shuttle program this year.

The weather at Florida's Kennedy Space Center was mild and looked promising for a landing at 11:57 am (1657 GMT) Wednesday, NASA said.

If that window does not work, there will be another opportunity at 1:34 pm (1834 GMT), NASA said.

Astronauts completed an inspection of the shuttle's heat shield Tuesday and found no problems, said chair of NASA mission management team Leroy Cain, who heaped praise on the performance of the aging spacecraft.

"The entire space shuttle system just performed outstanding on this entire mission," he told reporters.

"We have had an awesome display here of the capabilities of the team and the hardware," he added.

Discovery's mission was initially scheduled to last 11 days but was extended to 13 so that astronauts could work on repairs and install a spare room at the orbiting lab.

The Italian-built Permanent Multipurpose Module, formerly known as Leonardo, is now on the underside of the lab and adds 21 by 15 feet (6.5 by 4.5 meters) of extra room for storage and experiments.

Astronauts also brought the first humanoid robot to the space station, though it spent most of its time still wrapped in packing materials and will not become fully operational for some time.

During the mission, President Barack Obama called the six Discovery astronauts and their six crewmates who were already aboard the ISS to say he was proud of their cooperation in space.

"I wanted to call and say how personally proud I am of you and all that you are accomplishing. We are always inspired by the images of you guys at work," Obama said.

But not every moment was quite so glamorous.

A robotic arm breakdown during the first of two spacewalks left US astronaut Steve Bowen stranded outside the ISS clutching a broken ammonia pump for about 30 minutes, but no harm was done and the crew laughed as they described the situation to reporters later.

When the shuttle lands, it will have spent a total of 365 days in space over the course of its career, logging about 150 million miles (241 million kilometers).

"We have one major milestone remaining and that is to get the crew of Discovery, and Discovery, back safely," said Cain.

Hours after Discovery touches down, NASA plans to roll out the shuttle Endeavour to launch pad 39A in preparation for its final journey to the orbiting space lab on April 19.

Endeavour is to be commanded by astronaut Mark Kelly, whose lawmaker wife Gabrielle Giffords is recovering from a gunshot wound to the head, after a gunman went on a deadly rampage at a political meeting she was holding at a grocery store.

Kelly has said his wife is undergoing a grueling schedule of rehabilitation exercises and he hopes she will be well enough to attend the launch at Kennedy Space Center in April.

The shuttle Atlantis is scheduled for its final flight on June 28, which would mark the last shuttle mission ever.

After that, the sole method of transport to and from the ISS will be via Russia's Soyuz space capsules, which can carry three astronauts at a time.

Discovery has broken new ground multiple times since it first launched in 1984.

It transported the Hubble Space telescope, was the first shuttle to be commanded by a female astronaut and was the first to rendez-vous with the Russian Mir Space Station.

The shuttle was also the first to return to space after two major disasters, the Challenger explosion in 1986 and the Columbia disaster in 2003 when the shuttle broke up on its return toward Earth.

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