Technical failures delayed Discovery's launch in November 2010, but sunny weather and smooth preparations gave engineers every indication that the 39th mission for the historic shuttle would proceed on schedule.

The shuttle is set to lift off from Florida's Kennedy Space Center on Thursday at 4:50 pm (2150 GMT), carrying a six-member crew of astronauts, a new closet module for the orbiting International Space Station, and a robot.

"Everything is on track and going beautifully with the countdown," said mission management team director Mike Moses. "We're really looking forward to a very action-packed, successful mission."

After Discovery returns to Earth on March 7, it 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.

Once all three remaining shuttles in the American fleet are retired later this year, the sole method of transport for astronauts to and from the ISS will be the Russian space program's Soyuz capsule.

Cracks on Discovery's external fuel tank emerged just before launch in November, causing engineers to puzzle for many weeks over the cause and how to fix it.

In January, engineers agreed that installing small metal strips, called radius blocks, on the 22-foot-long U-shaped aluminum brackets, called stringers, would reinforce their strength.

Shuttle launch director Mike Leinbach said the teams preparing Discovery this time had found "no problems at all" as they count down toward lift-off.

"We're not tracking any issues and it looks like Discovery will fly this time," said Leinbach.

The rotating service structure around Discovery was to be rolled away on Wednesday evening, revealing the shuttle on the launch-pad for the first time.

The loading of the external fuel tank was to begin early Thursday morning at 7:25 am (1225 GMT). The astronauts were expected to board the shuttle at around 1:35 pm (1835 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 crew plans to deliver the Permanent Multipurpose Module, with extra storage space and an area for experiments, as well as some spare parts and the Express Logistic Carrier, an external platform for large equipment.

The shuttle is also to bring the first humanoid robot to the ISS. The Robonaut 2, or R2, is a joint project of General Motors and NASA and will stay behind when Discovery leaves as a permanent resident of the ISS.

The weather forecast -- clear skies, sunshine and a mild breeze -- was considered exceptionally good with only a 20 percent chance of conditions that could delay the launch, NASA weather officer Kathy Winters 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.

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.

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