Astronauts confident

Nasa wants the astronaut to make an unprecedented spacewalk under the orbiter to remove the dangling material known as gap fillers.

Engineers are concerned the strips may cause part of the shuttle to overheat as it re-enters the atmosphere.

"I'll have to be careful but the task is very simple," said Robinson.

"We predict it won't be too complex," he told reporters from orbit.

We have one unknown left and that's the gap filler that we hope to pull out tomorrow; and after we've done that I think we're going to have a very clean entry Discovery Commander Eileen Collins

Nasa has identified two gap fillers that are sticking proud of heatshield tiles near Discovery's nose section, one by about 2.8cm (1.1in) and the other by about 2.2cm (0.9in).

The pieces of ceramic fabric, which act like a grouting for the tiles, were probably shaken out during the vibrations of launch, the agency said.

'Easy decision'

The plan is for Robinson to go after the gap fillers during the third planned spacewalk of Discovery's mission to the International Space Station (ISS).

He will be manoeuvred into position by the ISS's Canadarm2 robotic arm.

Made of ceramic-coated cloth, the gap fillers act as seals between Discovery's heat-resistant tiles Astronaut Stephen Robinson will attempt to remove a section of protruding filler either by hand, or using a hacksaw See tile areas of concern

"The idea is pull out this thin gap filler either by hand or with a pair of forceps; we'll use a hacksaw only if necessary," Robinson explained.

Wayne Hale, the shuttle's deputy programme manager, said Nasa could not be certain what effect the protruding gap fillers would have as the shuttle heats up during re-entry.

The concern is such protrusions on the otherwise smooth belly of the shuttle could disturb the air flow during re-entry, causing turbulence that raises temperatures on heatshield tiles downstream.

"When we looked at the unknown versus what we do know about [spacewalks], it was a very easy decision," said Mr Hale.

"The bottom line is there is large uncertainty because no one has a very good handle on aerodynamics at those altitudes and those speeds. Given that large degree of uncertainty, life could be normal during entry or some bad things could happen."

Helmet watch

A team of managers, engineers and aerodynamicists has been working to address the issue for the past three days. They have been experimenting with the different techniques Robinson could use in his spacewalk, which is currently scheduled for Wednesday.

Nasa will want to be sure the spacewalk does not make matters worse - by damaging tiles in the process of pulling out the fillers.

"The thing I'll be watching most closely is the top of my helmet... because I'll be leaning in toward the orbiter... so that's what I'll be most careful with," Robinson said.

Commander Eileen Collins said she had few worries about the re-entry set for Monday, 8 August.

"We have done a full inspection," she told reporters. "We have one unknown left and that's the gap filler that we hope to pull out tomorrow; and after we've done that I think we're going to have a very clean entry."

She said Nasa had worked hard to investigate the effects of particular types of damage to the orbiter and was producing a range of in-orbit repair techniques.

"We have come to a point where we understand what is safe to come home with and what isn't; and for areas that we're not sure about, there will be ongoing work for that."

Surface survey

Nasa is being ultra-cautious over Discovery's safety.

This is the first orbiter flight since Columbia was lost with its crew in February 2003.

A suitcase-sized piece of foam fell from Columbia's external tank during launch, punching a hole through heatshield panels in the shuttle's left wing.

The damage did not present problems in orbit, but as the vehicle tried to re-enter the Earth's atmosphere, super-heated gases entered the wing and tore the ship apart.

Discovery has suffered minor tile damage to its protective tiles at launch because of flying foam debris, but Nasa says laser scans together with still-image and video surveys have shown up no major problems.

President George W Bush spoke to the Discovery and ISS crews on Tuesday.

"I just wanted to tell you all how proud the American people are of our astronauts. I want to thank you for being risk-takers for the sake of exploration," he said in the link-up.

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

SHUTTLE DAMAGE Discovery has sustained minor damage to tile sections close to the front landing gear doors on the underside of its nose This is one of the parts of the shuttle that experiences greatest heating as it re-enters the Earth's atmosphere Nasa is confident the nick in the tile will not present a problem but it wants the protruding gap filler removed