SPACE CENTER, Houston – NASA announced yesterday that an astronaut will perform a spacewalk to fix two worrisome pieces of filler material protruding from Discovery’s belly – a high-stakes operation to repair a problem that could threaten the shuttle during re-entry.

Wayne Hale, the deputy shuttle program manager, told a news conference that engineers simply did not know enough about the problem to leave it unattended, so they decided to conduct the spacewalk tomorrow to “set our minds at rest.”

“At the end of the day, the bottom line is there is large uncertainty because nobody has a very good handle on the aerodynamics at those altitudes and at those speeds,” Mr. Hale said. “Given that large degree of uncertainty, life could be normal during entry or some bad things could happen.”

NASA managers have spent the last few days trying to decide whether the cloth strips could cause overheating during re-entry and lead to another Columbia-type disaster.

The ceramic-fiber cloth that is used to fill the gaps between Discovery’s thermal tiles is sticking out an inch or less in two spots, and some engineers worry the material might disturb the air flow enough during re-entry to cause a dangerous heat buildup beneath the spaceship.

Astronauts have never ventured under an orbiting shuttle before, and have never attempted to fix their ship’s thermal shielding in flight. “This is the kind of new shuttle program, the new NASA,” Mr. Hale said. “If we cannot prove that it is safe, then we do not want to go there. This exceeded our threshold and we needed to take action.”

It will be a largely unrehearsed operation, with the risk that the astronauts might accidentally damage Discovery’s fragile thermal shield and make matters worse. “The biggest thing that everybody’s concerned about is doing no harm,” said Mission Control spacewalk officer Cindy Begley.

Nevertheless, she said she was not overly concerned about the potential repair. “It’s not actually that bad,” she said. All spacewalks are risky, she said, adding, “It’s just new stuff we’ve never done before.”

The plan called for the operation to be performed by Stephen Robinson on the end of the space station’s 58-foot robot arm, which would bend and wrap around the side of Discovery to enable him to reach all the way underneath.

Mr. Robinson will first try to tug the dangling strips out with his gloved fingers. If that does not work, he will use a hacksaw to cut them off while holding the material taut with forceps.

Discovery’s other astronauts and Mission Control would see him the whole time via robot-arm cameras, but he would be out of sight of his spacewalking partner, Soichi Noguchi, who would be busy elsewhere doing other things. NASA decided two astronauts would be too many for the work site and might cause too much banging around.

NASA had planned to send Messrs. Robinson and Noguchi out anyway tomorrow to perform some final space station work.

Earlier yesterday, Messrs. Robinson and Noguchi stepped outside Discovery and replaced a broken gyroscope on the international space station. They pulled out a 660-pound, washing machine-size gyroscope that stopped working three years ago, and installed a new one in its place. After two attempts to hook it up, they got power flowing to the unit.