Astronaut Stephen Robinson said his mission to fix the space shuttle Tuesday will be "very delicate" but "simple."

He'll be lifted to the space shuttle Discovery's underside to get rid of two ceramic-fabric strips hanging out from the heat-shield tiles. Controllers think they could cause a potentially dangerous overheating when the shuttle races through the atmosphere on its way back to Earth next Monday.

Tuesday, trainers on Earth walked Robinson through the steps needed to make a very delicate snip to remove pieces of filler material dangling from the shuttle.

Shuttle Commander Eileen Collins said she's "not that worried about it." She said it's never been practiced, but the crew has the flexibility to pull it off.

Robinson will be dangled into position at the tip of the space station's robot arm. He'll try to tug the pieces out with his gloved fingers. If that doesn't work, he'll use a hacksaw to cut them off.

Robinson will be getting some tips Tuesday from spacewalk trainers back in Houston.

It'll be the first time NASA has tried an in-flight shuttle repair. But the agency decided to give it a shot after spending days debating whether the strips could eventually lead to another Columbia-type disaster. The shuttle program's deputy manager called that "a place we don't want to go."

Tuesday's a day of putting away supplies for the crew of Discovery, along with preparations for the spacewalk.

It's a risky maneuver but a NASA manager said it's one that has to be done.

It's an operation that one NASA official says is designed to "put our minds at rest."

Mission Control's spacewalk officer said there is a danger Robinson's repair could damage the shuttle's thermal protection system. But she said overall, she's not that worried.

Bush Calls Astronauts

Late in the crew's workday Tuesday, President George W. Bush gave the shuttle and space station crews a call to talk about the mission.

"I want to thank you for being risk takers for the sake of exploration," Bush said. "We look forward to seeing the successful completion of this mission."

Commander Eileen Collins replied the astronauts enjoy what they're doing and believe in the mission.

Before he signed off he added, "We are with you and wish you all the best."