Monday's conclusion of Discovery's near-flawless 13-day mission marked a milestone for the US space program that has been plagued by continuing problems and safety concerns since Columbia broke apart in a fiery reentry on February 1, 2003.

"It was an enormously successful flight ... we're back on track," said NASA chief Michael Griffin after Discovery's smooth touchdown at the Kennedy Space Center on Florida's Atlantic Coast.

The crew and National Aeronautics and Space Administration leaders were delighted to see the Discovery 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 hailed the crew of six 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 experts will analyse data from the flight with a view to further improve the safety of orbital flights as they ready for the projected launch of Space Shuttle Atlantis at the end of next month.

The next flight should mark the resumption of scheduled shuttle missions NASA hopes will enable crews to complete construction of the partly-finished International Space Station (ISS.)

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

Like last year's first post-Columbia flight, the Discovery's voyage aimed at improving the safety of shuttle missions and delivering critical supplies to the ISS.

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

"We are ready to go assemble the station; we are ready to start flying shuttles on a more regular basis," said Discovery Commander Steven Lindsey.

"I like to think of this as a begining, a transition flight," he said after landing the shuttle at the end of its 8.8 million-kilometer (5.5 million-mile) journey.

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.

The latest 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 killing all seven occupants.

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.

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