With a gentle tug of his gloved right hand, Discovery (search) astronaut Stephen Robinson removed two worrisome pieces of filler material from the shuttle's belly Wednesday in an unprecedented space repair job that drew a big sigh of relief from NASA.

But he may have to go out again to fix yet another trouble spot.

Robinson was barely back inside the shuttle and out of his spacesuit when Mission Control (search) informed the crew there was a chance that a fourth spacewalk may be needed Friday to deal with a torn thermal blanket below a cockpit window.

The concern is that a roughly 1-foot section of the blanket could rip away during re-entry, whip backward and slam into the shuttle, perhaps causing grave damage. Engineers expect to know by Thursday afternoon whether the danger is real and whether any blanket trimming is required.
There was no immediate response from the exhausted but exhilarated astronaut.

It took Robinson just seconds to pull out each short dangling strip of ceramic-fiber cloth, which engineers had feared might cause the shuttle to overheat during its descent through the atmosphere and lead to another Columbia (search)-type disaster.

Robinson never had to pull out his forceps or his makeshift hacksaw, which he took along just in case the material was stuck between the thermal tiles and he needed to employ more force.

It was a delicate operation: Robinson had to be careful not to bump into the shuttle's fragile thermal tiles and make things worse.

Standing on the end of the international space station's 58-foot robot arm, he tugged out the first piece as the two linked spacecraft passed over Massachusetts. By the time he had pulled out the next fabric strip 10 minutes later, he had crossed the Atlantic and was zooming over the French coast.

"That was the ride of the century!" Robinson exclaimed.

"Steve, we trained for four years. You're going to spend the next four years signing autographs," said his spacewalking partner, Soichi Noguchi.

Robinson, a 49-year-old mechanical engineer and musician who took his childhood space-cadet lunchbox into orbit with him, became the first person to venture beneath an orbiting shuttle and the first person to repair a shuttle's fragile thermal skin in space.

His crewmates inside the shuttle kept an eye on him via the robot-arm camera. His spacewalking partner watched from 75 feet away, though he lost sight of him at one point.

But Robinson described what he was seeing and doing the entire time, so his colleagues would know he was safe.

"I'm pulling. It's coming out very easily," Robinson called. "The offending gap filler has been removed."

The second piece slid out even more easily, with just a gentle tug of Robinson's right thumb and index finger.

"I was absolutely relieved and I think you could probably hear the sigh of relief throughout the building" after the first piece came out, flight director Paul Hill said. "And when he pulled the second one out, it was a huge relief and it definitely felt like the rest is downhill from here."

The mood aboard Discovery also improved dramatically. Space station flight director Mark Ferring said he could hear "a palpable change in the tone" of the astronauts' voices.

The spacewalk ended after six hours. Robinson and Noguchi also installed a massive toolbox filled with spare parts on the space station.

NASA had spent four days analyzing the potential threat of the so-called thermal tile gap fillers and what to do about them.

Officials insisted it was absolutely safe to simply remove the fillers. Their primary purpose in those two spots was to prevent the silica glass fiber thermal tiles from rubbing against each other and chipping during liftoff.

With the gap filler problem behind them, teams of engineers and thermodynamic experts turned their attention to the torn, crumpled blanket beneath the commander's side window. Blanket samples were rushed from Cape Canaveral, Fla., to California for wind tunnel testing.

The blanket is covered with a quiltlike fabric and stuffed like a pillow, and serves as insulation.

The insulation would blow harmlessly away if the blanket came apart; the concern is where the top layer of fabric might go and how much damage it might do at high descending speeds, despite its less than 1-ounce weight, said deputy shuttle program manager Wayne Hale.

"Worst case, we could do some structural damage and that's obviously not something that we want to incur," Hale said Wednesday night.

The blanket was apparently ripped by debris during the July 26 liftoff, the first shuttle flight since Columbia disintegrated on re-entry 21/2 years ago. It is a type of blanket problem never seen before, Hale said.

"I think in the old days, we would not have worried about this nearly so much," Hale said, referring to NASA's pre-Columbia days. He said he believes the likelihood of a repair is low, but noted: "We're just pounding this flat. We're not going to leave any stone unturned at this stage, to make sure the crew's safe during entry."

Discovery and its crew of seven are scheduled to return to Earth on Monday.