Shuttle astronauts work on space station


Two spacewalking astronauts, one of them perched on a 50-foot robot arm, are working today on a dramatic bid to replace a failed gyroscope on the International Space Station.

The repair mission was taking place as Nasa pondered whether shuttle Discovery'sheat shield needs an unprecedented repair.

Japan's Soichi Noguchi wrestled to pull the 660-pound (300 kg) gyroscope from the station, then held on while Wendy Lawrence, inside the spacecraft, worked controls to swing the robot arm on which he stood back to Discovery.

They were to replace the broken gyroscope, which has not worked since June 2002, with a new one in a spacewalk lasting nearly seven hours, the second of at least three they will perform.

Nasa said yesterday a fourth spacewalk might be needed to trim or remove loose material sticking out from heat-resistant tiles on Discovery's belly, an operation astronauts have never done before.

The gyroscope replacement is one of the critical tasks of this mission because gyroscopes keep the 200-ton space station correctly positioned. It has four of the units, which look like large toy tops, but only two were working before Discovery'sarrival.

The $95 billion station can maintain position with just two working gyroscopes but if only one is functioning, the station crew would have to fire rocket thrusters, which burn precious fuel, to keep it steady.

Discoveryis making the first shuttle flight since the Columbiadisaster in February 2003, and is the first shuttle to link up with the space station since November 2002.

The spacewalking astronauts were scheduled to venture out once more on Wednesday to install a storage platform outside the station. If Nasa decides it is needed, they may try the heat shield repair on that spacewalk or perform an extra spacewalk on Friday.

Video inspections of Discoveryafter it launched last week found that two strips of material known as "gap fillers" between heat-resistant tiles on the shuttle's belly are protruding about an inch.

Nasa managers fear the protrusions could change aerodynamics and increase heat on the shuttle by as much as 25 per cent when it re-enters Earth's atmosphere for landing scheduled next week. Heat shield damage was responsible for the loss of Columbiaand its seven astronauts.