The Discovery shuttle rocketed seven astronauts into orbit on Tuesday on a pivotal mission for US space ambitions amid persistent concerns about safety since the 2003 Columbia tragedy.
"Discovery, straight as an arrow, speeding towards a date with the International Space Station (ISS) on Thursday," said NASA launch commentator Bruce Buckingham.
NASA employees at the Kennedy Space Centre's launch control centre applauded and hugged each other as ground control announced the shuttle had reached orbit about nine minutes after blasting off from Cape Canaveral, Florida under blue skies.
The 115th shuttle mission finally launched on time at 2:38 pm (0008 IST, Wednesday) in a spectacular cloud of smoke on US Independence Day, after two weekend launch attempts were scuttled by concerns over lightning-producing clouds.
Discovery jettisoned its two rocket boosters two minutes into the flight and split from the fuel tank as it reached orbit some nine minutes after blastoff.
"I cannot think of a better place to be here on the Fourth of July and on Independence Day to begin to launch," Commander Steven Lindsey said just before the final countdown.
"We hope that very soon we give you an up close and personal look of 'the rockets red glare,'" Lindsey said, quoting the US national anthem.
The two female and five male astronauts smiled broadly before boarding the shuttle, waving small American flags as they headed to the bus that took them to the launch pad before the first ever Independence Day lift-off.
The astronauts were "psyched" but calm before the flight and watched "The World's Fastest Indian," a biopic starring Anthony Hopkins as a New Zealander who set the land speed record with his motorcycle in 1967, said Scott Kelly, the twin brother of Discovery pilot Mark Kelly.
"It seemed appropriate," Kelly, an astronaut who flew in a shuttle mission in 1999, told reporters after the launch.
Scott Kelly said he watched the "fantastic" liftoff with his parents.
"It's always emotional to see the shuttle launch, especially on the Fourth of July," he said.
NASA cleared the mission despite finding a small crack in foam insulation on the shuttle's troublesome external fuel tank on the eve of the launch. Officials said the fissure posed no threat to the shuttle.
The second shuttle mission to the ISS since the Columbia disaster will show whether modifications made to the fuel tank since the accident have succeeded.
Columbia was doomed by a piece of foam insulation that came loose and pierced its heat shield during liftoff, causing the shuttle to break apart into a ball of fire as it returned to Earth on February 1, 2003. Seven astronauts died.
Foam also peeled off Discovery's tank in the first post-tragedy launch last year, but the debris missed the shuttle. Nevertheless, NASA grounded the 25-year-old fleet until now to make further modifications.
NASA placed more than 100 cameras around Discovery's launch pad for Tuesday's launch to detect any loose debris. The ISS will then take pictures of the vessel's heat shield while it performs a backflip during approach.
Up to six pieces of debris fell off Discovery
Up to six pieces of debris that could be foam insulation fell off Discovery's troublesome external fuel tank shortly after liftoff, a top NASA official said.
"About two minutes and 47 seconds give or take (after the launch), we saw three perhaps four pieces come off," said shuttle programme manager Wayne Hale, adding that it was unclear whether it was foam or "something else."
"We also saw another piece or two come off at about four minutes 50 seconds," he told reporters at the Kennedy Space Centre on Tuesday.