Space Center, Houston ? NASA 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 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.

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.

Discovery is scheduled to return to Earth next Monday.