NASA hopes to decide Monday whether to order an unrehearsed spacewalk to make the first exterior "repair" of the space shuttle in orbit. An astronaut would try to eliminate a potential reentry hazard by removing two protruding bits of heat shielding on the belly of the shuttle Discovery.

Wayne Hale, the shuttle's deputy project manager, said he thought such a spacewalk would be a relatively "easy thing," but "we are not making light" of a problem that NASA officials earlier had appeared to dismiss.

"The risk here of going underneath the vehicle is, we hope, relatively remote," Hale said during a Johnson Space Center news conference Sunday. "But it is surely something you have to think about. That is part of the calculation."

Even if planners ultimately determine a repair is not needed, the potential danger from the two "gap fillers" offered another cause for alarm for a shuttle fleet already grounded because Discovery's external fuel tank shed large pieces of foam insulation during launch last Tuesday.

A similar, but larger, piece of foam damaged space shuttle Columbia's heat shielding during launch, causing the orbiter to disintegrate on reentry Feb. 1, 2003. NASA spent the next 21/2 years designing a new tank. Discovery's launch was supposed to herald its successful debut.

Hale said he expects to hear Monday from aerodynamics experts studying how much effect the protruding gap fillers could have in increasing the heat from atmospheric turbulence during reentry. "I expect we'll have some decisions for you," he said.

The analysis "may very well show" that the fabric does not pose a reentry problem, he added. In that case, NASA shuttle directors would tell commander Eileen Collins and her six fellow crew members simply to come home to Kennedy Space Center in Florida as planned on Aug. 8.

The shuttle crew, along with space station commander Sergei Krikalev and flight engineer John Phillips, spent a quiet Sunday transferring equipment and supplies, and preparing for a second planned spacewalk Monday to replace a broken gyroscope on the station. Hale said if a spacewalk were ordered for the gap fillers, it would most likely be an added task for the final scheduled spacewalk, on Wednesday.

Protruding gap fillers have nothing to do with debris damage. They are the space shuttle equivalent of shims -- thin pieces of treated fabric or ceramic inserted between thermal tiles that have separated beyond design tolerances.

Not all the tiles in the quilt-like pattern on the shuttle's underside need gap fillers, but there are thousands of them on every orbiter. They have been used since the dawn of the program nearly 25 years ago to "caulk" openings in the shielding that protects the orbiter during reentry.

Steve Poulos, NASA's orbiter project office manager, said engineers preparing for Discovery's flight were "fully aware" of the tendency of the gap fillers to come loose, probably due to the expansion and contraction of the heat shield during launch.

He said Discovery is stocked with the necessary tools -- including pliers, scissors and a hacksaw-like implement -- for removing an offending gap filler by slicing it off. Both Discovery spacewalkers, Soichi Noguchi and Steve Robinson, have used the tools during training.

Hale acknowledged that his "gut feeling" when he first heard about and then reported on Discovery's protruding gap fillers last week was "knee-jerk." He said he thought at the time, "We've seen this before, and we can live with this."

But Paul Hill, Discovery's flight director, said earlier Sunday that engineers were seriously contemplating an unplanned spacewalk to get rid of the gap fillers. "We have time," he said.

Hill said experts were trying to determine the feasibility of putting an astronaut aboard a crane from the international space station or Discovery, or even using a 50-foot sensor boom extension to give the spacewalking repairman better access to the affected areas.

Robotics and spacewalk experts are "nervous about doing that because we've never done it before," Hill said of using the boom -- an extension that would be attached to the 50-foot shuttle crane.

Also unprecedented is the repair itself, "if you want to call it that," Hale said. "We do repairs and maintenance on the space station all the time, on the Hubble Space Telescope, on satellites, but I don't recall ever doing anything like this on the orbiter itself."

Hale said the preferred method of removing the gap fillers would be to pull them out, a relatively easy task. "They're usually very loose," he said. If that did not work, the spacewalker would simply trim them.

Protruding gap fillers are a potential hazard during reentry because they can disturb the air flow as a returning shuttle nudges into the upper atmosphere traveling more than 18 times the speed of sound.

Exactly how this process works is not totally understood. The shuttle is the only crewed vehicle in the world that reaches such speeds. "Wind tunnels max out at Mach 6 or 7 times," Hale said, "so there's not much data."

A smooth belly during reentry forms a turbulence-free "boundary layer" that allows the orbiter to glide through the thickening atmosphere, even as its underside temperature rises to 2,300 degrees Fahrenheit.

Protruding gap fillers mar the smooth surface, "tripping" the boundary layer to create hot spots of turbulence that radiate toward the stern of the shuttle, much like ripples from a pole stuck in a rushing stream.

Over the years, shuttles have displayed scores of protruding fillers. Discovery, fitted with a new suite of post-Columbia imaging devices, is the first orbiter in which astronauts can scan the underbelly while still in space.

The suspicion is that the ends of protruding gap fillers may burn away during reentry, leaving stubby pieces that stick out perhaps a quarter of an inch. The two on Discovery -- made of a felt-like fabric with a heat-resistant coating -- are on the forward part of the orbiter. One is about 0.9 of an inch long and the other 1.1 inches long.

NASA engineers noticed and reported on the gap fillers almost immediately after launch. But Hale as recently as Saturday minimized their importance, saying he was confident that ongoing tests would show that they posed no danger for reentry.

He was far less dismissive Sunday. "We have a very large group of people looking at this, and considering it at great length," he said. "We'll want to get the technical answers first" on whether a spacewalk is either necessary or advisable. "The jury is still out."

Flight engineer John Phillips, top, works with Discovery pilot James Kelly to install the human research facility rack in the space station laboratory.