Two spacewalking astronauts yesterday were scheduled to replace a broken space station gyroscope as NASA engineers worked feverishly to decide whether Endeavour's crew would have to perform risky repairs to a gouge on the ship's belly later this week.
A chunk of insulating foam smacked the shuttle at liftoff last week, creating a 9cm-long gouge that penetrates all the way through the thermal shielding on the shuttle's belly.
Teacher-turned-astronaut Barbara Morgan and other crew members spent much of Sunday using a laser boom attached to the shuttle's robot arm to create 3D images of the gash and a few other damaged areas that NASA officials say pose no threat.
Mission managers were expected to decide yesterday, or today at the latest, whether to send astronauts out to patch the gouge. Engineers were trying to determine whether the marred area can withstand the searing heat of atmospheric re-entry at flight's end. Actual heating tests will be conducted on similarly damaged samples.
"This is something we would rather not deal with but we have really prepared for exactly this case," said John Shannon, chairman of the mission management team.
Meanwhile, the crew prepared for the mission's second spacewalk, a six-hour effort to replace one of the gyroscopes that help control the space station's orientation.
Astronauts Dave Williams and Rick Mastracchio will remove the gyroscope that failed last October and replace it with one Endeavour carried to the station. The broken gyroscope will be stored at the station so it can be brought back to Earth during a later mission.
Endeavour's crew plans to conduct two more spacewalks tomorrow and on Friday, and they could add the gouge repairs to their to-do list. Depending on the extent of the damage, astronauts can slap on protective paint, screw on a shielding panel, or squirt in filler goo.
The damaged thermal tiles are located near the right main landing gear door. In a stroke of luck, they are right beneath the aluminum framework for the right wing, which would offer extra protection during the ride back to Earth.
This area is subjected to as much as 1,200oC during re-entry. A hole, if large and deep enough, could lead to another Columbia-type disaster. Columbia was destroyed in 2003 when hot atmospheric gases seeped into a hole in its wing and melted the wing from the inside out. A foam strike at liftoff caused the gash.
This time, the foam came off a bracket on the external fuel tank 58 seconds after last Wednesday's launch. It fell down onto a strut on the tank, then bounced up, right into Endeavour's belly. Ice apparently formed before liftoff near the bracket, which helps hold the long fuel feed line to the tank and caused the foam to pop off when subjected to the vibrations of launch.