U.S. astronaut Steve Robinson and Soichi Noguchi of Japan spent about seven hours in the frigid vacuum of space outside the shuttle Discovery. They replaced a failed space-station gyroscope, a rapidly spinning mechanism that helps the station maintain its stable position. They repaired another faulty gyro during their first outing Saturday.

The loss of both units had left the station with only two working gyroscopes, the minimum needed to orient the outpost. The loss of a third would have required periodic firings of the station's steering jets to maintain stability, using precious fuel.

Shuttle flight director Mark Ferring says the restoration of the two backups is a major accomplishment.

"I think the biggest thing we have done on this mission for the space station is to make sure we have enough gyroscopes so we do not have to use our fuel to maintain our pointing on orbit," he said. "With such a massive vehicle, if we had to use fuel for that, we would only be able to go for a certain amount of months before we would run out of that resource."

Now, there is one maintenance spacewalk left on Discovery's visit to the station. NASA is considering using much of the time to have the crewmen make unscheduled, emergency repairs to the heat shielding on Discovery's underside.

Two thin slices of ceramic-coated fabric that fill the cracks between the shuttle's ceramic tiles are protruding two to three centimeters beyond the surface. NASA engineers say they could upset the smooth flow of air beneath the shuttle as it glides back toward Earth next week at speeds of 18 to 20 times the speed of sound. They believe that air turbulence created by these tiny projections could cause up to a 25 percent increase in already superheated temperatures at locations on the orbiter near them.

Shuttle official Phil Engelauf says engineers are debating whether this would threaten the orbiter with a calamity like the one that befell the shuttle Columbia, which burned up during re-entry in 2003 because launch debris punctured a hole in its wing.

"There are not a lot of vehicles that fly in the flight regime that the shuttle operates in," he said. "There is a fair amount of conservatism prevailing that until we can satisfy ourselves, maybe the better course of action is to go out and remove these, if that is the right thing to do. Again, that is getting discussed and debated."

A spacewalk to repair the shuttle's surface would be unprecedented. It would require astronaut Steve Robinson to stand on the end of the robot arm underneath the shuttle, where no crewmember has ever been in orbit.

NASA officials say if the decision is made to perform the task, they would take every precaution to ensure that his tool belt and tether line do not strike and damage Discovery's fragile ceramic barrier that protects it from the blistering heat of re-entry.

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