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Two spacewalking astronauts were set to spend the day replacing a broken space station gyroscope.
A chunk of insulating foam smacked the shuttle at liftoff last week, creating a 3 1/2-inch-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 3-D images of the gash and a few other damaged areas that NASA officials say pose no threat.
Mission managers expect to decide Monday, or Tuesday at the latest, whether to send astronauts out to patch the gouge. Engineers are 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 were to remove the gyroscope that failed in 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 on Wednesday and 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're 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 2,300 degrees Fahrenheit 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.
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 This time, the foam came off a bracket on the external fuel tank 58 seconds after 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.
It's possible some ice was attached to the foam, which would have made the impact even harder. The debris that came off is believed to have been grapefruit-sized.
These brackets have lost foam in previous launches, a concern for NASA, Shannon said. A switch to titanium brackets, eliminating foam, will not occur before next year.
Shannon said he did not know whether the recurring foam problem would delay the next shuttle flight, currently scheduled for October.
Endeavour has been docked at the space station since Friday. It will remain there until Aug. 20 for a record 10-day stay. Mission managers on Sunday approved the prolonged visit based on the successful testing of a new power transfer system flying on Endeavour. The system is drawing power from the station and converting it for use aboard the shuttle.
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On the Net:
NASA: http://spaceflight.nasa.gov/
[Associated Press;
by Liz Austin Peterson]
AP aerospace writer Marcia Dunn contributed to this report from Cape Canaveral, Fla.
Copyright 2007 The Associated Press. All rights reserved. This
material may not be published, broadcast, rewritten or
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