As well as the unprecedented repair operation, the space walkers will also mount a tool platform and a material science experiment to the ISS, and retrieve a faulty heat dissipator on the orbiting space station.

Japanese astronaut Soichi Noguchi and mission specialist Stephen Robinson suited up and stepped outside the Discovery-ISS complex at 0848 GMT, for what could be a seven-hour space walk.

Attached to a foot restraint at the end of the shuttle's robotic arm -- which for this operation will be attached to the ISS, Robinson will try to remove the protruding tile gap filler with a pair of forceps.

Should the fibres resist the tugging, Robinson will try again with a makeshift hacksaw, NASA said.

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

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

Although the ceramic fibre strands stuffed between the shuttle's thermal tiles are not needed for protection during reentry -- they prevent acoustic vibrations during liftoff, by protruding they break the smooth contour of the shuttle's underside, which is essential for a problem-free re-entry, NASA said.

The shuttle could overheat because of instability that the gap fillers, dangling about one inch (2.5 centimetres) from between thermal tiles on the underside of the shuttle, could cause during the fiery descent to Earth.

A crack in the shuttle's thermal shield caused by a falling piece of foam during liftoff caused the destruction of the Columbia during its reentry and the death of its seven astronauts on February 1, 2003.

In all, Wednesday's space walk could last as long as seven hours, NASA said. It is the third and possibly final space walk in the Discovery's mission, which is the first since the Columbia disaster.

Andy Thomas, an Australian 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 and what was motivating it," he told reporters from the shuttle on Tuesday.

He said NASA engineers were also analyzing a thermal blanket below the Discovery's cockpit which apparently was hit by debris during liftoff on July

"I have people who will come back and report within 48 hours, so I will get a status report tomorrow," Hale said, adding that there was some concern the blanket might tear off during re-entry.

"It's not a concern if it came off above Mach 6 (speed) (since) this would not be a heating impact but a low mark impact," Hale added.

NASA said the mission management team would convene later Wednesday to analyze all the data on the blanket and determine if any further cours of action -- perhaps an additional space walk -- may be required.

President George W. Bush on Tuesday telephoned the Discovery crew to praise their courage.

"I want to thank you for being risk-takers for the sake of exploration," Bush said in a special telephone linkup.

"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.

Foam also came off during Discovery's liftoff on July 26 but NASA is confident that apart from the gap fillers, Discovery can come back safely.

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

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

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