While two spacewalking astronauts spend the day replacing a broken space station gyroscope, NASA engineers on the ground are trying to decided whether it will be necessary to perform risky repairs to a gouge on Endeavour's belly.

A chunk of insulating foam struck the shuttle 53 seconds after liftoff last week, gouging a 3-inch-long hole that penetrates all the way through the thermal shielding.

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.

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 during their six-hour spacewalk. 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 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.