Cape Canaveral: The space shuttle Endeavour blasted off Monday toward the International Space Station on the penultimate flight for the US shuttle program.
"We want to thank all the tens of thousands of employees who have put their hands on this incredible ship," shuttle commander Mark Kelly said moments before liftoff at 8:56 (1256 GMT).
The six-member crew of astronauts including five Americans and one Italian, Roberto Vittori, is delivering a potent physics experiment to probe the origins of the universe during the 16-day mission, which will include four spacewalks.
As many as 500,000 onlookers crowded into coastal viewing spots in Brevard County, the area around Florida`s Kennedy Space Center, with the US shuttle program set to end later this year after the final flight by Atlantis.
The STS-134 mission, initially set to begin April 29, was postponed when technicians discovered a power failure in a heating line that served to prevent fuel from freezing in orbit. NASA completed exhaustive repairs last week.
The shuttle is set to dock at the ISS Wednesday at 6:15 am (1015 GMT), and will stay there until May 30, returning to the United States on June 1, the US space agency said.
The Alpha Magnetic Spectrometer-2, a two-billion-dollar, 15,000 pound (7,000 kilogram) particle detector, 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."
US congresswoman Gabrielle Giffords of Arizona, the wife of shuttle commander Kelly, watched the launch from Kennedy Space Center, her office said.
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.
She and her husband swapped wedding rings ahead of the launch, the US television channel ABC reported.
The 30-year US space shuttle program formally ends later this year with the 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 final journey to the ISS ended in March. Endeavour is the youngest, and flew its first space mission in 1991. STS-134 marks its 25th and final mission.
Endeavour is the sixth US space shuttle ever built, and was commissioned after the Challenger exploded in 1986.
The other original members of the fleet include Enterprise, a prototype that never flew in space, and Columbia which disintegrated on its return to Earth in 2003.
A total of 14 astronauts were killed in the Columbia and Challenger disasters.
With the 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 per seat until a new American spacecraft can be built by private enterprise, possibly by 2015.