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Discovery heads back to launch pad

Space shuttle Discovery is returning to the launch pad now that repairs to its external fuel tank are complete.

The 5.5km (3.4mi) trip from the Vehicle Assembly Building (VAB) to Launch Pad 39A on the crawler-transporter is expected to take at least six hours, according to NASA. Since December, Discovery has been housed inside the VAB undergoing first tests and then repairs after a series of scrapped launches in November 2010.

The stress of loading extremely cold liquid oxygen and hydrogen into the tank combined with a set of stringers not as fracture-resistant as earlier batches led to cracks in the 21ft-long (67.4m) support beams. The fix was simple, NASA says -- fitting pieces of metal called radius blocks, over both edges of the stringers where they attach to the external tank's thrust panel area - but time consuming and extensive.

The planned 1 November take-off was repeatedly scrubbed, first to repair leaking helium and nitrogen gas lines on the Shuttle's orbital manoeuvring system pod, which took longer than expected. Electrical problems discovered during a routine engine power-up and check-out and weather added further delays. The cracked stringers were discovered during preparation for the 5 November launch attempt.

While the crew waited for a diagnosis on the stringers and the eventual fix, astronaut Timothy Kopra was injured in bicycling accident. He has been replaced by astronaut Stephen Bowen. To cut down on pre-flight injuries, NASA forbids astronauts assigned to missions from to participation in potentially dangerous activities such as skiing, parachuting and acrobatic flying, but bicycling is not on the list, the US space agency says.

The mission will deliver a pressurised logistics module called Leonardo to the 10-year-old International Space Station. Leonardo, built by the Italian space agency, will be permanently attached to the space station and used to transfer cargo to and from the station. The Shuttle will also carry Robonaut 2, a 136kg (300lb) humanoid robot developed by NASA, making its first trip into space for testing.

The mission, now slated for 24 February, will be Discovery's 39th and final trip to space.

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