Discovery's astronauts had misgivings about the risky spacewalk mission to the shuttle's fragile belly but agreed to try the repair because it seemed relatively easy to do, crewmembers said on Tuesday.
On Wednesday, Steve Robinson will make space history when he ventures out to Discovery's smooth underside to try to remove two loose fibre strips sticking out from between the crucial heat-resistant tiles.
"Like most kinds of repairs, it's conceptually very simple, but it has to be done very, very carefully," Robinson said during a news conference from space on Nasa 's first shuttle flight since the 2003 Columbia disaster.
The protruding strips are small - only an inch (2.5 cm) long - but Nasa fears they could affect aerodynamics and dangerously add to the intense heat on the shuttle as it re-enters Earth's atmosphere on August 8.
The primary danger is that the tiles, which can withstand hellish heat, are easily broken if touched.
Robinson, who, along with Soichi Noguchi, has already made two spacewalks on the mission, will become the first astronaut in the 24-year shuttle program to venture out to the belly of an orbiting spacecraft.
Nasa is undertaking the risky repair because it does not want to lose another shuttle and crew because of heat damage, as it did with Columbia.