NASA (search) said Monday it will send out a spacewalking astronaut 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.

Engineers simply do not know enough about the problem and its consequences to leave it unattended, so the fabric strips will be pulled out or cut Wednesday "to set our minds at rest," said deputy shuttle program manager Wayne Hale.

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

It will be a spacewalking first: Astronauts have never ventured under an orbiting shuttle before, and have never attempted to fix their ship's thermal shielding in flight.

Mission managers spent three days trying to decide whether the dangling strips of stiff ceramic-fiber cloth could cause dangerous overheating during re-entry and lead to another Columbia-type disaster. The possibility of exceeding the shuttle's certified limits was very real, Hale said, calling it "a place we don't want to go."

Both pieces are sticking out about an inch; they're supposed to fill the thin gaps between thermal tiles.

It will be a largely unrehearsed operation, with the risk that the astronaut repairman, Stephen Robinson, might accidentally damage Discovery's fragile thermal shield and make matters worse.

The plan calls for Robinson to perform the operation on the end of the international space station's 58-foot robot arm, which would bend and wrap around the side of Discovery (search) to enable him to reach all the way underneath.

Robinson will first try to tug the protruding 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 (search) 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.

Removal of the two gap fillers will not jeopardize the shuttle in any way, Hale said. The risks of the spacewalk are fairly well understood, and have been alleviated as much as possible, he said.

Space shuttles have returned in the past with drooping gap fillers, almost all of them shorter. The trouble is, NASA does not know if the strips started out longer and simply burned off during re-entry, or perhaps fell out altogether. In that sense, the space agency was unwittingly playing Russian roulette with every flight, Hale said.

This was the first time the problem was noted in orbit because of all the photography and laser imaging devoted to Discovery, a post-Columbia requirement. It also was a "new NASA" analyzing the problem, 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," he said.

The belly of the shuttle normally sees 2,300-degree temperatures during re-entry, and the dangling pieces of cloth could crank the heat up there by hundreds of degrees, said Chuck Campbell, an engineer specializing in thermal dynamics.

Discovery lifted off with a crew of seven July 26 on the first shuttle flight since the Columbia disaster 21/2 years ago. Columbia broke up on re-entry after the wing was pierced during liftoff by a chunk of foam insulation that fell off the big external fuel tank.

With so much focus on flyaway foam and possible nicks and gouges to the shuttle's thermal shielding, protruding gap fillers were not "one of the big-ticket items" studied before Discovery's flight, said mission operations representative Phil Engelauf.

But it ended up consuming teams of experts who had nothing to rely on but previous shuttle flight data.

"Nobody else flies in these regimes. Nobody else flies Mach 22 at 216,000 feet," Hale said.

Earlier Monday, Robinson and Noguchi stepped outside Discovery and replaced a broken gyroscope on the 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.

Until Discovery arrived last week, the space station had been flying with only two good gyroscopes, the bare minimum needed to keep the complex stable and pointed in the right direction. During the mission's first spacewalk on Saturday, the spacewalkers rewired a third gyroscope and got it working again, and so Monday's swap left the station with four functional gyroscopes.

Engineers suspect a lubrication problem with the bearings may be plaguing the gyroscopes, but will know for sure once the broken unit is returned to Earth aboard Discovery next Monday.