Discovery's crew were preparing the shuttle for its return voyage, the day after making a perilous repair to the underbelly of the spacecraft. But mission control continued to study the risks posed by a small piece of thermal blanket that has been knocked loose below the cockpit.

On Wednesday, US astronaut Stephen 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.

Dangling at the end of a 58 foot (17.7 metre) long robotic arm and equipped with a hacksaw and forceps, Robinson easily pulled out two small pieces of ceramic-coated fabric used as gap fillers between the craft's heat-resistant tiles, using only his gloved hands.

"I am just putting it in my trash bag," he said, triumphantly holding the first piece up for cameras.

Seven astronauts from the Discovery and two from the International Space Station were Thursday transferring supplies and equipment, waste and discarded gear to and from the station, in preparation for the shuttle's return to Earth, NASA said.

On Friday, the Raffaello cargo module used in the transfer operations will be stowed inside the Discovery's payload bay, and the shuttle will be prepared for its undocking Saturday from the space station and its descent and landing on Monday.

Mission control team at Johnson Space Center were still, however, considering what to do about the thermal blanket below the shuttle cockpit.

Though it does not risk overheating during re-entry to Earth's atmosphere since it is not located on the underside of the shuttle, NASA engineers were worried the blanket might tear loose and cause damage.

NASA said it was conducting tests in a wind tunnel to determine the impact the piece of blanket would have on the shuttle's outer structure.

Should a fourth space walk be necessary, it would take place no sooner than Saturday, said NASA, which was to hold a press briefing at 1700 GMT.

Wednesday's operation lifted one of the doubts about Discovery's safety. Engineers feared the protruding gap fillers -- two among thousands on the bottom of the shuttle -- would cause overheating sufficient to detroy the craft in the same way as the Columbia, the shuttle that burned up on reentry on February 1, 2003, killing all seven members of its crew.

The shuttle travels at about 26,500 kilometers an hour (16,700 miles per hour) as it reenters the atmosphere, pushing temperatures on its surface to 1,370 degrees Celsius (2,500 Fahrenheit).

The contour of the vessel's belly has to be smooth to make sure it does not overheat.

A crack in Columbia's thermal shield caused by a piece of foam that hit the wing during liftoff was blamed for its destruction. Discovery is the first shuttle mission since the Columbia tragedy.

US astronaut Robinson was accompanied on the 361-minute excursion outside the shuttle by Japanese astronaut Soichi Noguchi.

Besides removing the gap fillers, which took about 40 minutes, they installed a tool platform and a materials exposure science experiment on the orbiting space station, and retrieved a faulty heat dissipator.

NASA has expressed confidence that shards of foam that came off the external fuel tank during Discovery's liftoff on July 26 did not damage the shuttle in the way that the Columbia was damaged.

All future shuttle flights, however, have been grounded until the foam issue is resolved, NASA decided last week.

The New York Times said Thursday it obtained an internal NASA report issued in December that warned of deficiencies in the way the foam was applied by hand to the shuttle's fuel tank, despite some 200 million dollars NASA had spent after the Columbia disaster to address the problem.

"There will continue to be a threat of critical debris generation," said the report, adding that the "variable could reasonably be eliminated, and yet it continues."

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