The foam problem resurfaced during last July's flight of Discovery and again Monday, keeping the space agency debating safety all the way up to the eve of liftoff.

Discovery thundered away from its seaside pad at 2:38pm (1838 GMT).

Commander Steven Lindsey, an Air Force fighter pilot, was at the controls and aiming for a linkup in two days with the international space station.

"Discovery's ready, the weather's beautiful, America is ready to return the space shuttle to flight. So good luck and Godspeed, Discovery," launch director Mike Leinbach said just before liftoff.

It was unclear for a while on Monday whether Discovery would fly.

Crack

A slice of foam, no bigger than a crust of bread, fell off an expansion joint on Discovery's external fuel tank after Sunday's delay. Shuttle managers concluded on Monday night after intensive engineering analysis that the remaining foam on that part of the tank was solid.

Engineers said the piece - 3 inches (7.6 centimetres) long and one-tenth of an ounce - was too small to pose a threat even if it had come off during launch and smacked the shuttle. Inspectors devised a long pole with a camera on the end to get an up-close look at the joint where the foam came off, and found no evidence of further damage.

Nasa made sure there was no excessive ice buildup at that spot; ice could be even more damaging than foam at liftoff.

Space shuttle Columbia exploded, killing all crew members

"You could mail 10 of these things with the cost of a single first-class stamp," Nasa Administrator Michael Griffin said earlier on NBC's Today. "We're talking about a very, very minor piece of foam here. ... This is not an issue."

The fallen foam added to the tension surrounding this mission.

Nasa's chief engineer and top-ranking safety official objected two weeks ago to launching Discovery on the 12-day station delivery mission, without first eliminating the lingering dangers from foam loss, considered probable and potentially catastrophic.

They were overruled by shuttle managers and, ultimately, by Griffin. He stressed the need to get on with building the half-done, long-overdue space station before the shuttles are retired in 2010 to make way for a moonship, per President Bush's orders.

Crew

Riding aboard Discovery is German astronaut Thomas Reiter, who will move into the space station for a half-year stay. He carried a small German flag out to the launch pad.

Two astronauts, an American and a Russian, are already living on the station; Reiter will expand the size of the station crew to three for the first time since 2003. Staffing was scaled back in the wake of the Columbia disaster because of the lack of shuttle supply flights.

Besides Lindsey and Reiter, Discovery is carrying pilot Mark Kelly; Michael Fossum and Piers Sellers, who will conduct at least two spacewalks at the station; and Lisa Nowak and Stephanie Wilson.