NASA administrator Michael Griffin's gamble to launch Discovery despite objections from some of his own experts paid off, as the shuttle landed safely Monday from a 13-day mission focused on highlighting safety three years after the Columbia tragedy.

Officials wanted to show a marked improvement in flight safety to pave the way for the resumption of regular missions to complete the ISS before the shuttle's retirement in 2010. The Discovery mission was just the second since the Columbia disaster in February 2003.

"Completing the space station and flying out the shuttle program is the first step in the exploration plan outlined by President (George W.) Bush," Griffin told reporters at the Kennedy Space Center in Cape Canaveral, Florida after Discovery's landing.

NASA plans to replace the shuttle, which is limited to low Earth orbit, with a capsule capable of returning astronauts to the moon and eventually sending them to Mars.

Griffin has argued that the two missions since the Columbia accident were the first step toward resuming regular flights to the ISS, which would serve as a stepping stone for missions to Mars.

Even before returning to Earth, Discovery's astronauts expressed confidence during media interviews that the mission would signal the space program's revival following the Columbia trauma.

Discovery pilot Mark Kelly said his twin brother Scott Kelly, a fellow astronaut, had summed it up in a few words: "We're back baby!"

The Discovery mission was punctuated by three successful spacewalks to test new shuttle repair techniques created after the Columbia tragedy and replace a cable reel for the ISS's construction equipment transporter.

The repairs were needed in order to resume construction of the half-finished ISS.

NASA plans to launch the Atlantis shuttle next month in the first of 16 missions aimed at completing the ISS by 2010.

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

But the next four years must be trouble free, otherwise the shuttle could be grounded earlier than planned.

"We don't have any slack, we have just enough shuttle flights left to do the job. So we can't afford to mess up," Griffin said.

Prior to the July 4 launch, all eyes had been on the shuttle's troublesome fuel tank, whose perplexing loss of foam insulation doomed Columbia.

NASA officials described the tank's performance as "outstanding" during Discovery's liftoff, as it shed small pieces of debris as officials had expected and too late during ascent to be threatening.

The shuttle "is safer than it ever has been," said John Logsdon, who served on the Columbia Accident Investigation Board.

Columbia's demise was caused by debris that pierced its heat shield during liftoff, dooming its return to Earth with seven astronauts aboard.

A large chunk of foam insulation peeled off the fuel tank during the first post-Columbia launch last year, prompting NASA to ground the three-spaceship fleet until now to make further modifications.

Griffin gave the green light for Discovery's launch even though his chief safety officer and head engineer had called for a six-month delay to the mission to make even more changes to the foam insulation.

But the two officials finally backed the launch after NASA said the six shuttle astronauts could take refuge in the International Space Station (ISS) and wait for a rescue mission if Discovery suffered serious damage.

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