CAPE CANAVERAL, Fla. (Reuters) - U.S. Space shuttle Discovery landed smoothly in Florida on Monday at the end of a 13-day mission meant to show the fleet is fit to fly safely, three years after the fatal Columbia accident.
Double sonic booms thundered over central Florida as the shuttle glided through partly cloudy skies heading toward a three-mile-long (five km) runway at the Kennedy Space Center.
Commander Steve Lindsey gently steered the shuttle through a series of turns to slow the craft down before the winged spacecraft landed at 9:14 a.m. (1314 GMT).
"Welcome back Discovery. Congratulations on a great mission," said astronaut Steve Frick from NASA's Mission Control at the Johnson Space Center in Houston.
NASA administrator Michael Griffin told a post-landing news conference, "It was an enormously successful flight."
But he sounded a cautionary note about what was a make-or-break mission for the U.S. space agency, even as he described Discovery as "the cleanest" or most damage-free shuttle on its return to Earth.
"Obviously, this is as good a mission as we've ever flown but we're not going to get overconfident," Griffin said.
Griffin said before Discovery's launch that he would move to pull the plug on the shuttle program if it were marred by any further accidents or serious technical problems.
Discovery's flight was the first in a year and only the second since the 2003 Columbia disaster. NASA has spent more than $1.3 billion on safety upgrades since the accident, which killed seven astronauts and brought construction of the $100 billion International Space Station to a halt.
The agency's chief concern has been to fix foam insulation on the shuttle's external fuel tank, which triggered Columbia's demise. The doomed spacecraft was hit by a piece of falling foam during launch, opening a hole that let in super hot atmospheric gases during re-entry 16 days later.
NASA had hoped to resume space station construction last year, but Discovery's tank shed large pieces of foam during its July 2005 liftoff, forcing the grounding of the fleet while space agency experts worked on the problem.
When the shuttle was launched again on July 4, its tank lost only small pieces of foam, none a threat to the spaceship or its crew. Discovery reached the space station two days later, transferring a new crew member to the outpost and delivering more than 2.5 tons of supplies.
Piers Sellers and Michael Fossum made three spacewalks, including a repair of the station's transporter so construction of the half-built space station can resume. NASA hopes to launch its first station assembly mission since the Columbia accident around Aug. 28.
The shuttle dropped off German astronaut Thomas Reiter at the station, giving it a full three-person crew for the first time in three years.
Spacewalkers Sellers and Fossum also tested techniques to reach and repair heat shield damage should it occur. The work is critical for a proposed fifth and final servicing mission to the Hubble Space Telescope.
Griffin said a decision about whether to service Hubble will be made by this fall.
The space agency plans to fly 16 shuttle missions, including two more this year, to finish the space station before the fleet is retired in 2010.
"We don't have any slack," Griffin said. "We can't afford to mess up."
(Additional reporting by Tom Brown in Cape Canaveral)