NASA engineers worked feverishly yesterday to decide whether Endeavour's crew would have to perform risky repairs to a gouge on the space shuttle's belly later this week.

A chunk of insulating foam smacked the shuttle during liftoff last week, creating a 9-cm-long gouge that penetrates all the way through the thermal shielding on the ship'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 later yesterday, or today at the latest, whether to send astronauts out to patch the gouge. Engineers are trying to determine whether the affected area can withstand the searing heat of atmospheric re-entry at flight's end. Actual heating tests will be conducted on similarly damaged samples.

The space shuttle 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.

"We have really prepared for exactly this case, since Columbia," said John Shannon, chairman of the mission management team. "We have spent a lot of money in the program and a lot of time and a lot of people's efforts to be ready to handle exactly this case."

The crew awoke yesterday to the funky strains of Billy Preston's "Outta Space."

"There's nothing like a good old school 1970s jam to get you up in the morning," astronaut Alvin Drew said.

Meanwhile, two astronauts prepared for the mission's second spacewalk, a six-hour effort yesterday 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 tomorrow and Friday, and they could add the gouge repairs to their to-do list. Depending on the extent of the damage, astronauts can apply protective paint, screw on a shielding panel, or inject filler.

The damaged thermal tiles are located near the right main landing gear door. In a stroke of luck, they are beneath the aluminum framework for the right wing, which would offer extra protection during the ride back to Earth.

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 August 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.