SPACE CENTER, Houston – Employing the kind of NASA ingenuity seen during Apollo 13, an astronaut prepped for an emergency repair job on Discovery’s exterior today with forceps, scissors, and a hacksaw fashioned out of a blade and a little duct tape.

Stephen Robinson’s mission was to remove two short pieces of filler material that were sticking out of the shuttle’s belly. NASA feared the material could lead to a repeat of the 2003 Columbia tragedy during Discovery’s reentry next week.

Astronauts have never ventured beneath an orbiting shuttle before, and have never attempted repairs to the fragile thermal skin in space.

“No doubt about it, this is going to be a very delicate task, but as I say, a simple one,” Mr. Robinson said yesterday.

The plan was carefully worked out on the ground over the past four days. It called for maneuvering Mr. Robinson underneath Discovery – a no man’s land, up to now – on the end of the linked international space station’s 58-foot robot arm.

The hope was that Mr. Robinson could simply pull the stiff fabric out with his gloved hands. If a gentle tug did not work, he was to pull a little harder with forceps. And if that didn’t work, he was supposed to use a hacksaw put together in orbit with a deliberately bent blade, plastic ties, Velcro, and the handyman’s favorite all-purpose fix-it: duct tape.

The scissors were considered a last resort because they are clumsier to use and would not provide as close a cut.

Paul Hill, the lead flight director for the mission, said yesterday that one way or another, the two dangling strips of ceramic-fiber material need to come off before Discovery returns to Earth on Monday. Engineers fear the spacecraft could overheat to dangerous levels if it descended through the atmosphere with the material protruding.

The material – used to fill the extremely narrow gaps between thermal tiles to keep them from rubbing together – was hanging about an inch out of the shuttle’s belly in two spots.

“We’re going to go outside and eliminate this risk, and I expect that we’re going to pull this gap filler right out on the first try,” Mr. Hill said.

The overriding concern was that Mr. Robinson might bump his helmet or a tool against the thermal tiles and, by scraping or chipping them, make matters far worse. He was under orders to keep his body at least a foot away from the shuttle’s surface at all times. Just in case, NASA planned to have a tile repair kit positioned out in the open payload bay.

“The tiles, as we all know, are fragile, and a crew member out there is a pretty large mass, so I’ll have to be very, very careful,” Mr. Robinson said. “There won’t be any yanking going on at all.”

He is accustomed to using hacksaws and the like. “I’ve got some old airplanes at home that I’ve had for many, many years, so I’m pretty comfortable with using tools – very carefully,” Mr. Robinson, who has a Ph.D. in mechanical engineering, said.

A blowtorch would seem to be ideal for burning off the fabric, since the thermal tiles are designed to withstand the 2,300-degree heat of re-entry. The idea occurred to one of the astronauts who practiced the pulling and sawing technique on the ground, David Wolf. But he noted that there is no blowtorch on the shuttle, and no one knows how it would behave in space anyway.

Discovery’s astronauts as well as flight controllers had misgivings when they were first notified about the repair plan. They wondered whether it was justified – space shuttles have landed safely before with protruding gap fillers, though not necessarily ones this long – and whether it might prove too risky.

Mr. Hill said most of his team, initially, “really did not want to do this … we did not want to have to put a crew member near the thermal protection system if we didn’t have to.”

Discovery astronaut Andrew Thomas said the crew members’ concerns were alleviated once they learned it was a straightforward and relatively easy procedure tested as much as possible on the ground.

Teams of aerodynamic and thermal experts spent four days developing and refining the plan, and experienced spacewalkers like Mr. Wolf practiced the techniques underwater at Johnson Space Center. Among the items studied exhaustively, besides the actual pulling and cutting of the fabric: the stability of Mr. Robinson’s work platform on the end of the robot arm, his body position, and the robot arm’s clearances to the docked spacecraft.

“We feel very comfortable we have a very doable task,” Mr. Wolf said. “Delicate is the word that we want to stay with while we’re at the bottom of the orbiter.”

President Bush called the astronauts yesterday to wish them Godspeed, and said he and other Americans will be praying for their safe return.

Today’s spacewalk – the third of the mission – had been scheduled even before the filler material was discovered. The main task was to install a giant toolbox on the space station.

Late yesterday, deputy shuttle program manager Wayne Hale said the filler material problem, like the fuel tank foam loss, must be resolved before a shuttle flies again. He also hinted the astronauts might have yet another repair job, on yet another spacewalk.

Engineers are assessing whether something needs to be done about a thermal blanket below the commander’s window that was nicked during launch, most likely by debris.