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CAPE CANAVERAL, Fla. -- Just a few hours from liftoff, NASA fueled space shuttle Endeavour for one last ride into orbit Friday as hundreds of thousands of visitors began to converge on the coast for prime viewing spots.

The launch team began loading more than a half-million gallons of fuel into Endeavour at dawn, moments after royal wedding vows were exchanged across the ocean between Prince William and Kate Middleton in London.


Commanding Endeavour on NASA's next-to-last shuttle flight is Navy Capt. Mark Kelly, who is married to Rep. Gabrielle Giffords. She was to watch the launch from Kennedy Space Center. Giffords was shot in the head in January and left rehab behind to attend the afternoon liftoff.

President Barack Obama also will attend with his wife and two daughters – the first time in NASA history that a sitting president and his family will have witnessed a launch.

Launch time was 3:47 p.m. Forecasters put the odds of good weather at 70 percent; low clouds and stiff crosswind were the main concern.

Endeavour is bound for the International Space Station.

For its last hurrah, it's carrying one of the most expensive payloads in NASA's 30-year shuttle history: a $2 billion particle physics detector that will seek out antimatter and dark energy across the universe. Many in and outside NASA say the experiment, if successful, could validate science operations at the decade-old orbiting lab.

Kelly and his all-male crew – all six of them space veterans – saw their families for the last time, face to face, Thursday. They awoke a little before sunrise Friday as launch controllers gathered for the fueling operation.

"Watched 'Patriot' with the crew. Bonding before a big day," pilot Gregory Johnson said in a Twitter update late Thursday.