The US space agency decided to cut off two ceramic fibre strands hanging from the belly of the shuttle to end any "uncertainty" about whether it would be in danger on re-entering the Earth's atmosphere, said Wayne Hale, deputy head of the US shuttle programme.

Either Japanese astronaut Soichi Noguchi or American Steve Robinson will carry out the repairs during their third spacewalk of the mission on Wednesday, officials said.

NASA experts are worried that the shuttle could overheat because of instability that the gap fillers, dangling from between thermal tiles on the underside of the shuttle, could cause.

"Given that large degree of uncertainty, life could be normal during entry or some bad things could happen," Hale told a news conference.

"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.

NASA engineers have spent the past three days "working very hard" to assess the risk from the gap fillers, which are only about an inch (2.5 centimetres) long.

He said the teams had "put together a very simple plan with good safety precautions and mitigations of any hazards that will allow a crewmember to go out and remove those two gap fillers."

To accomplish the task, one of the astronauts -- NASA has not yet said which -- will be carried underneath the shuttle on the station's robotic arm and attempt to either pull out the danglers or cut them off.

If it doesn't work, "we can always go back out on Thursday or Friday with improved techniques and try again," Hale said.

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

NASA engineers believe that the operation is unlikely to damage the surrounding thermal tiles but even so, "that risk has got to be mitigated and controlled. Obviously you don't want to make it worse," Hale said.

He said the shuttle could safely re-enter the atmosphere with the errant gap filler removed, even though for one of the two pieces, "there's a possibility that heating could get into that gap."

"For one flight we are well within our safety margins," he said.

The ceramic fiber fillers are used to keep hot gas from flowing into gaps between the thermal outer protection tiles.

The National Aeronautics and Space Administration has been ultra-sensitive about Discovery's safety because it is on the first mission since Columbia broke up as it re-entered the atmosphere on February 1, 2003.

A piece of foam came off Columbia 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 but NASA is confident that apart from the gap fillers, Discovery can come back safely.

NASA's chief administrator admitted Sunday that the US space agency had "goofed" on key safety checks prior to Discovery's launch Tuesday.

"Our judgment at the time was that it was okay. As everyone has said without any attempt to hide it ... we goofed on that one," Michael Griffin said.

NASA announced the repairs just after Noguchi and Robinson finished a seven hour and 14 minute mission to replace a faulty gyroscope on the International Space Station where Discovery is now docked.

Noguchi rode a 15.25 metre (50 feet) long robotic arm on the space station to get to one of the four gyroscopes that steer the station and keep it on its pre-determined orbit.

Noguchi was attached to the robotic arm to collect the broken gyroscope, take it back to Discovery's cargo bay, and ride back with a new one. "Oh, the view is priceless," he said as he moved toward the gyroscope. "I can see the moon."

"You've got a ticket to ride," Robinson joked.

NASA said the walk had been a complete success. Hale called it an "outstanding day".

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