"It was an enormously successful flight ... we're back on track," NASA chief Michael Griffin said following completion of the 13-day mission that paved the way for the resumption of regular shuttle flights in the wake of the 2003 Columbia disaster.

The crew and National Aeronautics and Space Administration leaders were delighted to see the space vehicle did not have the beat-up appearance of previous shuttle flights.

"This is the cleanest orbiter anyone remembers seeing," said Griffin.

US President George W. Bush issued a statement hailing the crew for "a job well done" and NASA for their dedication "to putting our space program back on track and implementing our nation's vision for human and robotic space exploration."

NASA officials heaved a collective sigh of relief as Discovery came to a full stop on the 4,572-meter (15,000-foot) Kennedy Space Center (KSC) runway on Florida's Atlantic seashore Monday morning.

"The final entry was beautiful," said Commander Steven Lindsey.

The last moments of a shuttle mission are among the most critical. It was upon re-entering the Earth's atmosphere that Columbia burst into flames on February 1, 2003, killing the crew of seven.

NASA hoped Discovery's successful mission will help ease concerns over the shuttle program that have persisted since the Columbia disaster, and pave the way for a resumption of regular flights in August.

Like last year's first post-Columbia flight, the latest shuttle voyage was largely aimed at improving the safety of the shuttle missions that are critical to plans to complete construction of the International Space Station (ISS) by 2010.

The orbiting space station is a cornerstone in US ambitions to send manned flights to the Moon again, and eventually to Mars.

During the mission, astronauts Mike Fossum and Piers Sellers performed three spacewalks to test shuttle repair techniques and fix equipment needed to resume construction of the ISS.

The crew delivered critical supplies and removed discarded material and experiment results from the orbiting laboratory.

They also dropped off European Space Agency astronaut Thomas Reiter of Germany, who became the space station's third resident.

Lindsey said the mission's two main objectives were accomplished. "We are ready to go assemble the station; we are ready to start flying shuttles on a more regular basis."

"I like to think of this as a begining, a transition flight."

But Griffin added a note of caution.

"This is as good a mission as we've ever flown ... but we won't get overconfident," he said, pointing out there were still 16 flights to go before the three-shuttle fleet is retired.

For the next flight, the Atlantis shuttle is tentatively scheduled to launch on a new mission from Kennedy Space Center in late August.

The Discovery mission was the second since the Columbia disaster and the first since the fleet was grounded a year ago because the orbiter's external tank had shed a large chunk of foam on take-off.

A similar piece of foam insulation had peeled off on Columbia's doomed flight, piercing the shuttle's heat shield and causing the vehicle to break apart as it returned to Earth.

The tank shed some debris this time around when Discovery blasted off from KSC on July 4, but NASA found no damage on the shuttle's heat shield.

NASA officials said they would carefully analyse data from the flight to continue improving the shuttle's safety.

"I think we need to learn the lessons from Columbia," said Lindsey. "We want to continue being vigilant."

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