NASA officials were elated about the launch, although television images showed that pieces of debris fell off the shuttle's troublesome external fuel tank minutes after the start of just the second flight since the Columbia accident.

Officials confirmed that up to six pieces fell off, but it was too soon to know whether it was the foam insulation that has perplexed NASA since the Columbia disaster or if it struck Discovery.

NASA had cleared the Discovery launch despite finding a small crack in foam insulation on the fuel tank on the eve of the launch. Officials said the fissure posed no threat to the shuttle.

But the day started with a successful Discovery launch after two weekend launch attempts were delayed due to storm clouds.

"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 Center's launch control center 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 (1838 GMT) in a spectacular cloud of smoke on US Independence Day.

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 liftoff.

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.

Up to three pieces of debris peeled off nearly three minutes after Discovery's blastoff and up to two others five minutes into the flight, said shuttle program manager Wayne Hale.

But he cautioned that it was "very raw, preliminary data" and that officials would analyze the images.

Hale, however, told reporters the debris came off the fuel tank later than officials fear it could damage the shuttle's heat shield.

"Both of those (losses of debris) are interesting because they are after the time we're concerned about aerodynamic transport causing damage to the shuttle tiles," he said.

Foam fell off Columbia's fuel tank 70 seconds after liftoff, piercing its heat shield. The shuttle broke apart while returning to Earth in February 2003, killing the seven astronauts aboard.

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 officials had expected some foam to come off during Discovery's liftoff, but that it would not be of a size that would endanger the shuttle.

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.

The astronauts, led by Lindsey, will test new procedures to boost safety as well as deliver critical equipment and supplies to the ISS.

They will also drop off European Space Agency astronaut Thomas Reiter of Germany, who will join the ISS's two other crew members for a long-term stay, and will undertake two space walks.

NASA Administrator Michael Griffin stuck with his controversial decision to launch Discovery despite concerns over potentially damaging debris peeling off the orange-hued fuel tank yet again.

The agency said the seven astronauts could take refuge at the ISS and wait for a rescue mission should the shuttle suffer irreparable damage.

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