Space station spinner replaced as NASA mulls repair

HOUSTON (Reuters) - Spacewalking astronauts wrestled a 600-pound (272-kg) gyroscope into place on the International Space Station on Monday, while NASA managers assessed a small but deep pit in shuttle Endeavour's heat shield.

In the second of four spacewalks planned for the shuttle mission, astronauts Rick Mastracchio and Dave Williams replaced one of the space station's four gyroscopes, which spin like tops to keep the outpost properly positioned in orbit, without the use of gas-burning rocket thrusters.

During the spacewalk, mission managers discussed what to do -- if anything -- about a deep cut in two of Endeavour's heat-resistant belly tiles, discovered shortly before the shuttle parked at the station on Friday.

"This is not a catastrophic loss of orbiter case at all," John Shannon, the deputy shuttle program manager told reporters. "This is a case where you want to do the prudent thing for the vehicle."

Endeavour's astronauts used a laser scanner and imager attached to an extension on the shuttle's robot arm on Sunday to take three-dimensional pictures of the gouge.

It was caused by a piece of insulating foam, possibly mixed with ice, that broke off the shuttle's fuel tank during launch Wednesday and slammed into the underside of the orbiter.

NASA has made several tank modifications since the fatal 2003 Columbia accident, which was traced to a similar, but far more serious, impact with falling tank insulation. Changes to the part of the tank that shed foam during Endeavour's launch have not yet been implemented.

NASA has three more flights planned with its current tank design before the modification is ready.

In addition to deciding if fixing Endeavour's heat shield during the mission is necessary to stave off more serious damage from re-entry, which could delay future flights on Endeavour, NASA faces a larger question of whether additional modifications to the tank must be made before any shuttle is cleared for flight.

The agency is pressing to complete 11 more construction missions to the space station, two resupply flights and a final servicing call to the Hubble Space Telescope before the shuttles are retired in three years.

Gyroscopes, like the one replaced by the Endeavour crew on Monday, are among the items that so far have no rides to the station after the shuttle's retirement.

As soon as Mastracchio and Williams bolted in the new gyroscope on Monday, ground controllers tested electrical connections and pronounced the new equipment in good health.

"Alllllright! That's great news," shouted Endeavour astronaut Tracy Caldwell who was overseeing the spacewalk from aboard the station.

Mastracchio and Williams headed back inside the station's airlock at about 6 p.m. EDT/2200 GMT, completing a 6 1/2-hour spacewalk. The next outing is scheduled for Wednesday.

Endeavour is due back at the Kennedy Space Center in Florida on August 22.