HOUSTON (Reuters) - Two shuttle Discovery astronauts completed a seven-hour spacewalk on Saturday to restore a space station steering device and test shuttle heat shield repairs that might give crews a better chance of avoiding a Columbia-like disaster.
NASA extended the shuttle's visit to the outpost by a day because the next trip will likely be postponed.
"You did a great job today," Mission Control's Mike Massimino radioed to the crew as rookie spacewalkers Stephen Robinson and Soichi Noguchi tucked back inside the shuttle's airlock after nearly seven hours outside the ship.
"They were moving as if they've been out there their whole lives," added lead spacewalk coordinator Cindy Begley. "I'm just more than happy with the performance today."
The astronauts finished all the tasks assigned for their first spacewalk and had time for extra work, such as retrieving a pair of science experiments and photographing a loose insulation blanket near the commander's window.
NASA is meticulously scrutinizing and documenting the smallest of nicks and blemishes in the shuttle's protective heat shield to understand how much damage the ship can withstand and still safely return to Earth.
So far, engineers have reviewed and cleared 90 percent of the shuttle's heat shield and found it to be in good shape to endure the tremendous heat and forces of atmospheric re-entry, with final clearance expected on Monday, deputy shuttle program manager Wayne Hale said.
NASA lost shuttle Columbia on Feb. 1, 2003, because its wing had been damaged during liftoff by a chunk of foam insulation that fell off its fuel tank. As the shuttle plunged through the atmosphere for landing, superheated gases blasted into the wing and destroyed the ship, killing the seven astronauts aboard.
NASA spent more than two years redesigning the fuel tank so it would not shed foam. But sensors and radar analysis suggest that bits of foam debris that broke off when Discovery launched this week had struck the wings, prompting NASA administrator Mike Griffin to say more work must be done before the shuttles will fly again.
The next mission to the station had been targeted for September, but with postponement likely, NASA on Saturday approved station managers' request for the Discovery crew to remain at the outpost an extra day. The astronauts would use the time to transfer more equipment to the station and possibly help fix some station hardware, such as the treadmill.
While NASA has no remedy for a hole the size of the one that downed Columbia, engineers are developing materials that future crews might be able to use to patch small but potentially threatening areas.
Robinson and Noguchi spent the first part of their spacewalk using a caulking gun, putty knives and other tools to fill cracks in sample heat-resistant tiles and wing panel pieces that had been deliberately damaged for the test.
The astronauts worked only on the sample materials in the payload bay and did not touch any of the minor damage caused during liftoff to Discovery's heat shield.
Robinson got to work first, squeezing out thick black beads of a heat-resistant product called NOAX, then working the material into cracks and gouges in the sample wing panel.
"I see just a little bit of bubbling," he said. "It's about like pizza dough."
Noguchi then used another device similar to a shoe polish applicator to dab a material, known as an emittance wash onto damaged heat-shield tile samples.
Engineers will study the repaired sample tiles and wing panels after the shuttle returns to Earth.
The spacewalkers then turned their attention to space station repairs, installing a new Global Positioning System antenna, reviving a backup gyroscope used for orienting the outpost and setting up power cables and mechanical connections for new equipment to be installed during two other spacewalks on Monday and Wednesday.
During the outing, Discovery's robot arm operators Jim Kelly and Charlie Camarda repositioned a 50-foot (15-metre) boom so laser scanners could take more high-resolution images of the shuttle's wing panels.
John Shannon, a flight operations manager, said the additional inspections were intended to help engineers learn what to look for on future flights.
"These are not areas that are giving us great concern for the health of the vehicle," he said.
Sensors had also detected that several large pieces of foam insulation had broken off Discovery's tank during launch but NASA officials do not believe they hit the orbiter.
With the mission extended one day, Discovery's landing is now targeted for 4:47 a.m. (0847 GMT) Aug. 8 at the Kennedy Space Center in Florida.