HOUSTON -- NASA announced Monday that an astronaut will perform a spacewalk Wednesday to fix two worrisome pieces of filler material sticking out between heat tiles on the shuttle Discovery.

The material sticks out about an inch in two locations.

NASA isn't sure how big a danger that might be during re-entry because the shuttle's belly has never been checked during flight.

Wayne Hale, the deputy shuttle program manager, told a news conference Monday that "the bottom line is, there is large uncertainty."

"Nobody has a very good handle on the aerodynamics at those altitudes and those speeds," Hale said.

Several strips of ceramic cloth are sticking out between thermal tiles on the underside of the spacecraft. The cloth fills the gaps between the thermal tiles that protect the shuttle from the intense heat generated from friction when it re-enters the atmosphere.

In this case, they've decided to repair or cut off the insulation. An astronaut will be moved under the ship with a robotic arm for the first time.

On Monday, the astronauts made a spacewalk to replace a gyroscope that helps position the international space station.

Soichi Noguchi was able to bring the old gyroscope back into the shuttle's cargo bay by himself. He carried the massive piece of machinery in his hands, thanks to the weightless of space.

The new gyroscope will be brought on line Tuesday after testing, NASA said.

The space station just needs two of its four gyroscopes to function.