Astronauts aboard the space shuttle Discovery are preparing to make an unprecedented repair to the orbiter, after it was discovered that heat resistant fabric was sticking up between some of the shuttle's protective tiles. 

Discovery astronauts are carrying out the repair during a scheduled space walk. Once they get in position underneath the shuttle they will either try to pull out or trim off two pieces of gap filler that are protruding a couple of centimeters from between heat protective tiles on the ship's underside.

Mission engineers are concerned the bulging fabric could cause dangerous overheating when Discovery re-enters the Earth's atmosphere, leading to a disaster like the one that destroyed the space shuttle Columbia in February 2003 and killed all seven crew members.

Wednesday's space walk repair mission will be the first in the history of the shuttle program.

At first, lead flight director Paul Hill says everyone, including the astronauts, was nervous about a repair because a slip could damage critical protective heat tiles.

"The crew's in a good place and ready to go for tomorrow," Mr. Hill says. "But it took them some time to evolve there, just like it did us. But they didn't have the same head start like we did. So, it took them a little while to get there."

U.S. astronaut Steve Robinson will be making the repair while standing on the end of a robot arm attached to the international space station. In a telephone hook-up with the astronaut, he talked about the delicate task ahead.

"I'm pretty comfortable with using tools very carefully. But no doubt about it, this is going to be a very delicate task, but a simple one," he says.

Mission control sent the crew a video tape demonstrating how to remove the gap filler.

Tuesday, President Bush spoke to the crew.

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

If the repair does not work, NASA officials say they will come up with an alternate repair plan which they will attempt to carry out Thursday or Friday.