August 1, 2005
Shuttle Astronauts Work on Space Station Gyroscope
HOUSTON -- Two spacewalking astronauts, one of them perched on a 50-foot robot arm, worked to replace a failed gyroscope on the International Space Station on Monday, while NASA pondered whether shuttle Discovery's heat shield needs an unprecedented repair.
Japan's Soichi Noguchi wrestled to pull the 660-pound 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."You've got a ticket to ride," fellow spacewalker Steve Robinson joked.
They were to replace the broken gyroscope, which has not worked since June 2002, with a new one in a 6 1/2 hour spacewalk, the second of at least three they will perform.
NASA said on Sunday 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's arrival. Robinson and Noguchi repaired the other malfunctioning gyroscope in their first spacewalk on Saturday.
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.
Discovery is making the first shuttle flight since the Columbia disaster on Feb. 1, 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 Discovery after 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 percent when it re-enters Earth's atmosphere for landing scheduled on Aug. 8.
Heat shield damage was responsible for the loss of shuttle Columbia and its seven astronauts.
The gap filler problem has cropped up on previous missions and the shuttles landed without problems but deputy shuttle program manager Wayne Hale said NASA engineers were assessing the risk of trying it again versus performing a repair that would involve trimming or removing the loose material.
"I think the jury is out at this point on whether we'll do anything," Hale said.
But, he said, "The Columbia accident made us realize that we had been playing Russian roulette with the shuttle crews."
The Columbia disaster was caused by a piece of foam insulation that broke off the ship's fuel tank during launch and smashed into the ship's wing.
After the accident, NASA adopted new procedures, spent $1 billion on safety upgrades and built equipment to inspect the shuttle while it is in orbit.
Videos showed pieces of tank foam flying off during Discovery's launch, which prompted NASA to ground the shuttle fleet until the problem is solved.
The shuttle suffered minor damage to its heat shield, but the protruding material is not believed to have been caused by foam impacts.