Space shuttle Endeavour is ready to go, says NASA, in what will be its last launch and the second-to-last for any shuttle. It says it's confident that the power problems which have held up the launch for more than two weeks are now fixed, and that the shuttle is now fueled and ready to go. There's a 70 percent chance that the weather will be favorable, and liftoff is scheduled for 8.56am EDT.
The mission commander is Mark Kelly, whose wife, Arizona congresswoman Gabrielle Giffords, is still recovering from a gunshot wound to her head. Gregory Johnson, Greg Chamitoff, Drew Feustel, Mike Finke and Roberto Vittori form the rest of the crew.
"I am really excited and charged up for this mission! Slept great," commented Johnson on Twitter.
As part of its STS-134 mission to the International Space Station, Endeavour is carrying the $2 billion AMS-2 particle detector, which will detect cosmic rays and, NASA hopes, help answer questions about dark matter and dark energy.
It will also deliver spare parts to the ISS, including including two S-band communications antennas and a high-pressure gas tank.
After the 16-day mission, Endeavour will be retired to a museum in Los Angeles. It's flown 23 times since its first launch in 1992. The last shuttle mission, by Atlantis, is due to take place this summer.
NASA says it expects as many as 45,000 guests to attend the launch, with many more watching from the surrounding area.