The National Aeronautics and Space Administration is debating whether to use one of two remaining spacewalks to conduct unprecedented repairs to patch up damage on the shuttle, which has been docked to the station since August 10.

In the meantime, US astronaut Rick Mastracchio and Canadian colleague Dave Williams floated outside the ISS to replace the defective gyroscope, one of four devices controlling the orbiting laboratory's position.

It was the second spacewalk of the mission, which was extended by three days to 14 days to add a fourth spacewalk. The third spacewalk is scheduled to take place Wednesday, while the fourth would be conducted on August 17.

Astronauts used a camera on a robotic arm on Sunday to scan the shuttle for a closer look at the damage on Endeavour's underbelly.

The damage took place when a piece of debris slammed the shuttle's underbelly, creating the 8.75 by 5.0 centimeter (3.5 by 2.0 inch) gouge near a landing gear's hatch.

NASA officials said they were conducting simulations on Earth, applying extreme heat on a sample of shuttle shield that was purposely damaged to determine whether Endeavour can safely land despite its gash.

Officials said they would decide Wednesday whether an astronaut would need to conduct on-orbit repairs before Endeavour undocks from the ISS on August 20.

"If we have to do it, I feel fully confident that we could execute it with a minimum impact to the mission," shuttle mission manager John Shannon told reporters.

If repairs are needed, an astronaut would stand on a robotic arm to be positioned in front of the damaged thermal tile and apply material that would keep heat from penetrating the shuttle.

The astronaut could apply heat-reflecting black paint, a paste to fill in the hole, or a metal plate to completely cover the damaged thermal tile.

"All the astronauts on board have practiced on the ground the repair techniques, so we have no concern for the crew," Shannon said.

NASA has kept a close eye on the shuttle's heat shield on every mission since the Columbia disaster that killed seven astronauts in February 2003.

Columbia's heat shield was pierced by foam insulation that peeled off its external fuel tank during liftoff. The damage allowed hot gases to burn through the shuttle during re-entry into Earth's atmosphere, destroying the orbiter.

Since then, NASA extensively films and photographs the shuttle during launch to detect any debris hitting the vessel. Astronauts also use robotic arms to scan the shuttle while on orbit.

A film of Endeavour's August 8 liftoff from Cape Canaveral, Florida, shows a piece of foam -- possibly covered with ice -- hitting the shuttle's underbelly.

Endeavour is carrying seven astronauts, including 55-year-old teacher-turned-astronaut Barbara Morgan, on a mission to continue construction of the ISS.

During the first spacewalk on Saturday, Mastracchio and Williams installed a new truss segment to expand the station, whose construction NASA wants to complete by 2010, when it will retire its three-shuttle fleet.

The shuttle is scheduled to land on August 22.

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