The 8:56 am (1256 GMT) launch was expected to draw as many as 500,000 onlookers to the area around Kennedy Space Center, with the US shuttle program set to end later this year after the final flight by Atlantis.

The six-member crew of astronauts including five Americans and one Italian, Roberto Vittori, will deliver a potent physics experiment to probe the origins of the universe during the 16-day mission, which will include four spacewalks.

The weather forecast was 70 percent favorable for the launch window and the US space agency was "not working any issues" that would interfere with the liftoff, NASA spokesman Allard Beutel said.

Endeavour is toting the Alpha Magnetic Spectrometer-2, a two-billion-dollar, 15,000 pound (7,000 kilogram) particle detector which will be left behind to scour the universe for hints of dark matter and antimatter over the next decade.

"It's a bit like searching for a needle in a haystack," said French scientist Jean-Pierre Vialle, part of the international team that worked on the AMS-02 project.

"But if we find it, it will show beyond a doubt that stars made of antimatter exist in some part of the universe. That would be a major revelation."

The STS-134 mission, initially set to begin April 29, was delayed hours before liftoff when technicians discovered a power failure in a heating line that served to prevent fuel from freezing in orbit.

But NASA completed exhaustive repairs last week and engineers were optimistic that the repaired shuttle would fly this time.

US Congresswoman Gabrielle Giffords of Arizona, the wife of shuttle commander Mark Kelly, arrived safely in Florida after traveling there with family of another astronaut and will watch the launch from Kennedy Space Center, her office said.

"Gabrielle & Mark said goodbye during afternoon visit before launch," said a status update on her Facebook page on Sunday.

Giffords, who was allowed a break by her rehabilitation doctors in Houston to watch the planned April 29 launch, is recovering from a gunshot wound to the head, after she was attacked in January during a meeting with local voters.

The 30-year US space shuttle program formally ends later this year after the last flight of Atlantis, leaving Russia's space capsules as the sole option for world astronauts heading to and from the orbiting research lab.

Endeavour's delay has pushed back Atlantis's planned liftoff from June 28 to mid-July, but no final date has been set.

After the final shuttle missions, the three spacecraft in the flying fleet and the prototype Enterprise will be sent to different museums across the country.

Discovery, the oldest in the group, was the first shuttle to retire after its last journey to the ISS ended in March. Endeavour is the youngest, and flew its first space mission in 1991.

With US shuttle program closing, the world's astronauts will rely on Russia's space capsules for transit to the ISS at a cost of 51 million dollars per seat until a new American spacecraft can be built by private enterprise, possibly by 2015.

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