Spacewalking astronaut Steve Robinson plucked a couple of loose fiber strips from Discovery's belly today in an unprecedented repair to the shuttle's heat shield.
"I'm grasping it and I'm pulling it and it's coming out very easily. Beautiful. Nice," Mr Robinson radioed as he pulled the material out from between the heat resistant tiles on the shuttle's underside.
"It looks like this big patient is cured."
With fellow spacewalker Soichi Noguchi watching from a perch on the International Space Station, to which Discovery is docked on the first shuttle flight since the 2003 Columbia disaster, Mr Robinson was lowered on the station's robot arm to an area below Discovery's nose.
Moving deliberately, he used his gloved fingers to tug out the two strips with surprisingly little effort. Mr Robinson carried a small hacksaw, scissors and forceps in case he could not pull the strips out.
The strips, made of ceramic covered cloth, are thought to have come loose from their adhesive bond and, though protruding only an inch (2.5 cm), NASA engineers feared they could change the aerodynamics enough during landing on August 8 to cause dangerous heat damage to the shuttle.
Mr Robinson was the first astronaut in the 24-year shuttle program to spacewalk to a shuttle's underside and, once there, the first to fix the heat shield during flight.
He did not appear to damage any of the shuttle's fragile tiles. The thin strips in question are each about six inches long and are placed between the tiles to cushion them.
In a press conference from space yesterday, Discovery astronauts said they initially had misgivings about Mr Robinson's spacewalk because they were not convinced it was necessary. But, they said, they supported it because it seemed an easy thing to do.
NASA admitted it did not know if the protruding strips were a danger to the shuttle, but after 2 1/2 years of work and $1 billion spent on safety upgrades since the Columbia disaster, the agency was taking no chances it could lose another shuttle to heat damage.
Agency officials said today they are even considering another spacewalk on Friday to fix a protrusion in an insulating blanket outside the shuttle commander's window.
Columbia broke apart while returning to Earth on February 1, 2003, because superheated gases penetrated its structure through a break in the wing heat shield. The seven astronauts on board were killed.
Columbia's wing had been struck by a briefcase-sized piece of insulating foam that shook loose from the external tank at launch.
Videos showed loose foam swirling from Discovery on its July 26 launch from Florida, which prompted NASA to ground the shuttle fleet until the foam problem is finally fixed.
The loose strips on Discovery are not believed to have been caused by impacts.
The landmark repair job came at the end of the third spacewalk this mission by Mr Robinson and Japan's Noguchi. Before the fix, they attached an external storage platform to the $95 billion space station.