Discovery and seven astronauts blasted off Tuesday on America's first manned space shot since the 2003 Columbia disaster, ending a painful, 21/2-year shutdown devoted to making the shuttle less risky and NASA more safety-conscious.

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IMAGES: See The Launch
VIDEO: See The Launch

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At stake were not only the lives of the astronauts, but also America's pride in its technological prowess, the fate of the U.S. space program and the future of space exploration itself.

"Our long wait may be over. So on behalf of the many millions of people who believe so deeply in what we do, good luck, Godspeed -- and have a little fun up there," launch director Mike Leinbach told the astronauts right before liftoff.

Space program employees and relatives of both the Discovery and Columbia crews watched nervously as the shuttle rose from its pad at 10:39 a.m., climbed into a hazy midsummer sky, pierced two decks of clouds, and headed out over the ocean in the most scrutinized launch in NASA history. Two chase planes and more than 100 cameras documented the ascent from every possible angle to capture any sign of flying debris of the sort that doomed the last flight.

The multitude of images will not be fully analyzed -- and NASA will not give a final verdict on whether Discovery is safe to return to Earth -- until halfway through the 12-day flight.

The fuel-gauge problem that thwarted a launch attempt two weeks ago did not resurface before liftoff, to NASA's great relief, and the countdown was remarkably smooth. The space agency had been prepared to bend its safety rules to get the shuttle flying.

During the mission, commander Eileen Collins and her crew will deliver supplies to the international space station and test new techniques for inspecting and patching the shuttle in orbit.

The 114th shuttle liftoff came after painful self-examination on NASA's part, extensive safety modifications to the spacecraft and many months of hurdles and setbacks. A launch attempt July 13 was scrapped after one of four critical hydrogen-fuel gauges in Discovery's giant orange external tank failed just two hours before liftoff.

Hundreds of engineers chased the problem, which had cropped up three months earlier in a fueling test. In the end, they could not fully explain the trouble but fixed some bad electrical grounding inside the shuttle in hopes that might solve it.

The space agency said it was prepared to relax a rule, instituted after the 1986 Challenger explosion, that required that all four gauges be working for launch.

NASA Administrator Michael Griffin said the shuttle was as safe as NASA could make it, but was still a risky venture.

"Some things simply are inherent to the design of the bird and cannot be made better without going and getting a new generation of spacecraft. That's as true for the space shuttle as it is for your toaster oven," he told The Associated Press on the eve of launch.

Columbia was brought down by a suitcase-size piece of foam insulation that broke off the big external fuel tank during liftoff and caused a gash that allowed hot gases into the wing during the return to Earth 16 days later on Feb. 1, 2003. But NASA could barely make out the blow in the photographs of the launch because the few available images were poor.

The space agency added more and better surveillance cameras for Discovery's launch and sent up a pair of camera-equipped planes to chase the flight. Pictures will also be taken from space, by the astronauts themselves and spy satellites. Also, once Discovery arrives at the space station on Thursday, the two residents will photograph the shuttle as it completes a slow flip.

NASA's chief acknowledged a lot is riding on the flight: the shuttle program, the space station program, President Bush's plan to send astronauts back to the moon and on to Mars -- and seven lives.

"It's about hope, it's about imagination, it's about the future, and when you take away a great space program, you take away a lot of people's future," Griffin told the AP. "What's riding on this flight is people's hope for the future."

Thousands descended on Cape Canaveral for the launch, including first lady Laura Bush, Florida Gov. Jeb Bush, her brother-in-law, and members of Congress, as well as relatives of the 14 fallen Columbia and Challenger astronauts. They sang the national anthem just minutes before liftoff.

In addition to Collins, the crew members are pilot Jim Kelly; Soichi Noguchi of Japan; Stephen Robinson; Andrew Thomas; Wendy Thomas; and Charles Camarda.

Griffin's message to the lost crews' families: "We have left no stone unturned that we know of to make this flight and every other one as safe as we can do it."

In all, nearly 50 safety improvements were made to the shuttle in the wake of Columbia tragedy. The fuel tank was extensively redesigned, with less foam insulation than before but extra heaters to prevent a dangerous buildup of ice once it is filled with super-cold liquid propellant. NASA feared falling ice could be as lethal as chunks of foam.

Also, dozens of motion and temperature sensors were embedded in the wings to detect any blows from debris.

The space agency also revamped the way it makes decisions and listens to dissenting views, especially from lower-level employees. Columbia accident investigators blamed the catastrophe in part on a broken safety culture, or a tendency to downplay risks and discourage engineers from speaking up.

Unlike the Columbia astronauts, who had no knowledge of the gaping hole in their spacecraft's wing and no realistic way of plugging it, the Discovery crew has a variety of inspection and repair techniques on board. But they are all untested. And even the best of the bunch could not fix a hole the size of the one that destroyed Columbia.

The astronauts will try out the repair kits on deliberately broken samples of thermal tiles and panels. They will practice working with goo and other patching materials and different types of brushes, putty knives and a caulking gun.

They also will spend their first full day in space using a remote-control, 50-foot boom to inspect the shuttle's most vulnerable areas -- the wings and nose cap -- for any cracks and holes.

If any serious damage is found, NASA will have to choose between attempting repairs or, more likely, moving the shuttle crew into the space station for at least a month to await rescue by space shuttle Atlantis, which is already being readied for liftoff. Both scenarios are extremely risky.

Faced with so many uncertainties, the liftoff was a relatively solemn affair. NASA did not plan to hold the usual post-launch party.

Griffin said he had no intention of celebrating until he hears Collins announce "wheels stop" at touchdown. "We'll be thrilled with every successful step we complete," he said earlier this month, "but you won't see people really let go until they have landed safely."

Discovery is hauling an almost three-year back order of supplies and replacement parts to the half-built space station and its two residents. Construction has been on hold since the last shuttle visit in late 2002.