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 will 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.