When the historic flight ends, Discovery will become the first space shuttle to enter retirement as the US program winds down after 30 years, leaving a major gap in American space flight.

Discovery was initially scheduled for launch in November 2010 but cracks emerged on the external fuel tank just ahead of the launch, postponing the mission until now.

"Over the last few months the team has been very busy effecting repairs to the external tank and making it stronger than ever," said NASA test director Steve Payne.

"We have also resolved a problem with a hydrogen vent system leak and are now ready to fly again," he told reporters.

Payne said there was one small interior leak on a reaction control system regulator, but described the issue as "minor" and one that NASA has seen before, so it was not expected to impact the launch planned for 4:50 pm (2150 GMT) on Thursday.

"Our launch countdown is proceeding smoothly and is on schedule," said Payne.

Shuttle launch weather officer Kathy Winters expected Thursday to be mainly sunny and breezy, with only a 20 percent likelihood of unfavorable weather for the launch at Kennedy Space Center in Florida.

"Overall, the weather looks good," she said.

Discovery first flew in 1984. Final flights for the other two remaining in the fleet, Atlantis and Endeavour, are scheduled for later this year.

Tanking is set to begin Thursday morning at 7:25 am (1225 GMT) and should take about three hours to complete, Payne said.

The six-member crew for the mission, STS-133, is to arrive at the launch pad at around 1:30 pm (1830 GMT).

The mission will be led by commander Steven Lindsey, pilot Eric Boe and astronauts Alvin Drew, Michael Barratt, Steve Bowen and Nicole Stott.

Astronaut Tim Kopra was scratched from the crew list after a bicycle accident in January. He was replaced by Bowen.

The 11-day mission aims to deliver spare parts and install a new module to the International Space Station.

The Permanent Multipurpose Module will provide room for extra storage and space for experiments.

Discovery will also bring Robonaut 2, which NASA has described as "the first dexterous humanoid robot in space."

Astronauts will first test how it works in microgravity before figuring out how upgrades could graduate the robot to a full-fledged space assistant.

Discovery is slated to return to Earth on March 7, and when it is retired the shuttle will have flown more missions than any other spacecraft, NASA said.

"I can tell you it has generated a lot of interest," Payne said.

"People are starting to realize that they either see one now or they don't get to see one at all, so we have had some pretty good crowds come the last couple of times we expect an equally large crowd for this one," he said.

"It is always impressive to watch so I am sure we will have a full house."

When the US space shuttle program officially ends later this year, the Russian space program's Soyuz capsule will be the sole method for transporting astronauts to and from the ISS.

Endeavour is set for its final takeoff on April 19 and a last mission for Atlantis is scheduled for June 28, though funding for Atlantis remains in question.

There were initially five space shuttles in the fleet -- Challenger exploded shortly after liftoff in 1986 and Columbia disintegrated on its way back to Earth in 2003.

The sixth shuttle, Enterprise, did test flights in the atmosphere but was never flown into space. It is already on display at a museum outside Washington.

Earlier this year, the US company SpaceX succeeded in sending its Dragon space capsule into orbit and back, but it will likely be several years before a private US spaceflight can carry crew and cargo to the ISS.

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