Astronaut Stephen Robinson will carry out the unprecedented repair Wednesday to cut two ceramic fibre strands that engineers fear could cause Discovery to overheat as it re-enters the Earth's atmosphere.

Another member of the crew said the seven astronaut had some "misgivings" about events and President George W. Bush telephoned them to praise the "risk-takers" of space exploration.

Robinson will at first try to pull off the strands but if that does not work cut them off with a hacksaw.

"I am pretty comfortable with using tools very carefully," Robinson said in a press conference from Discovery. "But no doubt about it, this is going to be a very delicate task. But as I say a simple one."

NASA experts are worried that the shuttle could overheat because of instability that the gap fillers, which are about one inch (2.5 centimetres) long, dangling from between thermal tiles on the underside of the shuttle, could cause during the descent to Earth.

Japanese astronaut Soichi Noguchi will accompany Robinson on the spacewalk, the third of the Discovery mission, which is the first since Columbia disintegrated during re-entry on February 1, 2003, killing all seven crew on board.

"We are all going to be very busy" during the repair walk, Discovery Commander Eileen Collins told reporters. "I think we're all going to be involved somehow."

On Tuesday, the Discovery crew went over the repair operation and prepared the makeshift hacksaw for Robinson, who said care had to be taken with every movement to avoid scraping the delicate tiles.

"The hacksaw is really a contingency device," Robinson said, "the idea is just to pull out this thin gap filler either by hand or with a pair of forceps, and we'll use the hacksaw only if necessary."

Andy Thomas, another member of the crew, said the astronauts originally had "misgivings" about going underneath the shuttle. Such a repair has never been attempted in the quarter century history of shuttle flights.

"We were concerned about it - we were concerned about the implications of it," he told reporters from the shuttle.

President Bush praised the astronauts' courage in a special telephone linkup.

"I want to thank you for being risk-takers for the sake of exploration," Bush said.

"Thanks for being such great examples of courage for a lot of our fellow citizens," Bush went on. "As you prepare to come back, a lot of Americans will be praying for a safe return."

Bush concluded: "Thanks for taking my phone call - now get back to work."

The National Aeronautics and Space Administration has been ultra-sensitive about Discovery's safety because of the cause of the Columbia disaster. A piece of foam came off during its liftoff and damaged a wing, which led to superheated gases getting into the shuttle and causing it to break up. All seven astronauts were killed.

Foam also came off during Discovery's liftoff last week but NASA is confident that apart from the gap fillers, Discovery can come back safely.

NASA said it had ordered the repair operation because of the "uncertainty" about leaving the errant pieces of ceramic-covered fabric which stop the shuttle's delicate thermal tiles from crunching together during the high speed return into the Earth's atmosphere.

The shuttle travels at about 26,500 kilometers (16,700 mph) into the atmosphere expanding and contracting in the extreme heat and cold.

"Given that large degree of uncertainty, life could be normal during entry or some bad things could happen," deputy shuttle programme director Wayne Hale told a news conference late Monday.

"We examined our options to set our minds at rest and make sure that we didn't stay up late at night worrying about bad things happening," Hale added.

Hale said that if the hacksaw did not work another spacewalk could be organised for Thursday or Friday with a new technique.

The shuttle is scheduled to leave the space station Saturday and return to Earth early on August 8.

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