Pilot James Kelly fired up the craft's steering jets, taking Discovery away from the International Space Station (ISS) in what NASA described as "the first step on the road home to the Kennedy Space Center."

Earlier, the seven Discovery crew, including a Japanese astronaut, exchanged hugs and handshakes with the Russian and the American who hosted them in space for eight days.

"What we're telling them here is we thank them for being such great hosts," said Discovery Commander Eileen Collins.

"We thank them for being part of such a successful mission," she said. "These are memories we'll have forever," she told the two ISS residents, before Discovery undocked from the orbiting space lab.

Kelly then flew the shuttle around the ISS to photograph the space outpost and check it for any wear and tear.

After the maneuver, Discovery headed to the orbital position needed for Monday's reentry into Earth's atmosphere

The crew had been given the green light for their return after NASA decided loose fibers on the orbiter's thermal blanket should not compromise the shuttle's safety as it reenters the atmosphere.

But once it is back, Discovery will be grounded with the rest of the fleet until nagging problems with the shuttle's thermal insulation have been fixed, though NASA managers still hope they can launch another flight on September 22.

Foam insulation had fallen off the shuttle's external fuel tank as the craft blasted into orbit on July 26.

The same problem doomed Columbia, after debris hit the orbiter's left wing, causing a crack that eventually allowed superheated gases to penetrate the structure upon re-entry into the atmosphere.

A key goal of Discovery's 13-day mission was to test improvements made to the shuttle since Columbia burst into flames on February 1, 2003.

"We've got every objective accomplished on this mission. We now have data we've never had before," said Michael Griffin, administrator of the National Aeronautics and Space Administration.

Collins agreed.

"We've been happy to finally put the icing on the cake through this mission, we are so happy things have worked out so well, see you next week, see you on the ground," she said.

"It's been a wild ride," she said.

The crew still faces some nerve-racking moments when it starts reentry into the atmosphere.

"You only get one shot at it," said Paul Hill, lead shuttle flight director.

"De-orbit is not a risk-free activity," he said.

"During de-orbit, I'm sure I'll have a thought or two about Rick Husband and his crew," he said of the seven astronauts who perished aboard Columbia.

The shuttle is scheduled to land at the Kennedy Space Center, on Florida's Atlantic coast, at 4:47 am (0847 GMT) Monday.

NASA could leave the shuttle in orbit an extra two days if the weather turns bad. It also could order in California.

Before the descent, officials will check data to confirm Discovery is in good shape. New detectors pinpoint any abnormal temperature rise on reentry.

The mission initially had be scheduled to last 12 days, but an extra day was added on so the crew could transfer as much material and provisions as possible to the ISS, amid uncertainty over the date of the next shuttle flight.

The crew also retrieved waste and equipment to clear out space in the cramped orbiting lab.

During the mission Japanese astronaut Soichi Noguchi and his US counterpart Stephen Robinson conducted three spacewalks.

On Wednesday Robinson became the first astronaut ever to carry out a space walk beneath the shuttle during orbit, to extract two protruding pieces of fiber that risked overheating during the shuttle's reentry.

In another spacewalk, Noguchi and Robinson tested repair techniques adopted after the Columbia tragedy.

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