A U.S. astronaut is about to undertake a risky repair of the space shuttle Discovery's delicate heat shield during a spacewalk. The space agency NASA says the task is easy, but that the crewman must take extreme care not to damage the surface that protects the shuttle from a fiery re-entry.

Astronaut Steve Robinson is going where no one has ever gone before in orbit - to a shuttle's underside, which is covered with thousands of fragile ceramic tiles as a barrier to the scorching heat caused by high-speed friction with the atmosphere.

Slivers of heat-resistant fabric filling the thin gaps between the tiles have popped up a couple of centimeters in two places. Engineers worry that the protrusions might change the aerodynamics of Discovery's re-entry Monday and make some parts of the orbiter a few hundred degrees hotter than normal. They do not know if the shuttle can withstand the extra heat.

So astronaut Robinson's task is to stand on the end of the robot arm mounted on the International Space Station, where Discovery is docked, and either pull out or trim the gap filler. He must be very careful not to strike and damage the delicate tiles with his space suit or tools, as he explained to reporters.

Discovery Mission Specialist Steve Robinson can be seen suspended from Discovery's robotic arm during a space walk

"Like most kinds of repairs, it is conceptually very simple, but it has to be done very, very carefully," Mr. Robinson says. "The tiles, as we all know, are fragile and a crewmember out there is a pretty large mass, so I'll have to be very careful. But the task is extremely simple and we predict that it won't be too complex."

The tiles have been a concern during this shuttle mission because a photographic and laser survey of the orbiter showed minor damage in a few, presumably from pieces of debris that fell from Discovery's external fuel tank during launch. This was the same problem that doomed the shuttle Columbia in 2003 when debris punctured its wing during liftoff, halting missions for two-and-a-half years while NASA pursued solutions to the problem. Now, NASA has grounded flights again to work on it.

Deputy shuttle program manager Wayne Hale says the tile gap filler issue is as serious as falling fuel tank debris.

"We have classified the gap fillers as an in-flight anomaly that we are going to resolve before we fly again," he says.

As Discovery's crew prepared for the exterior repair venture, they received a telephone call from President Bush, who thanked them for being risk takers for the sake of exploration.

"We look forward to successful completion of this mission. Obviously, as you prepare to come back, lots of Americans will be praying for a safe return," Mr. Bush says.

If astronaut Robinson's effort to remove the gap filler fails, NASA says the shuttle mission has time for another spacewalk to pursue a different solution.