This image, taken Sunday, shows the damage to thermal protective tiles on the shuttle's underside. ((NASA TV/Associated Press))

A laser inspection by Endeavour's astronauts has revealed that a nine-centimetre-long gouge the shuttle suffered during its launch penetrates all the way through thermal tiles on its belly.

The unevenly shaped gouge, which is just over five cm wide, straddles two adjacent tiles and the corner of a third. The inspection, which was carried out on Sunday using the Canadian-made robotic arm Canadarm, showed the damage went through the 2.5-centimetre-thick tiles, exposing the felt material sandwiched between the tiles and the shuttle's aluminum frame.

Mission managers expect to decide Monday, or Tuesday at the latest, whether to send astronauts out to patch the damage.

Engineers are trying to determine whether the area can withstand the searing heat of atmospheric re-entry at the end of the flight; theywill conduct heating tests on similarly damaged samples.

Four other damaged areas on the shuttle's underside were also scanned, but it was determined that those smaller gouges pose no threat, said John Shannon, chairman of the mission management team.

"We have really prepared for exactly this case, since Columbia," Shannon said. "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 U.S. space agency says it now knows the gash was caused by a piece of fuel-tank insulation foam that broke off during the launch from Cape Canaveral, Fla., on Aug. 8.

Columbia was destroyed in 2003 when hot atmospheric gases seeped into a gash in its wing caused by a foam strike during liftoff.

Foam has come loose on other flights, Shannon said, and NASA is looking at how to redesign the craft to mitigate this problem, he said.

"It's a little bit of a concern to us because this seems to be something that has happened frequently," Shannon said.

On Saturday, Canadian astronaut Dave Williams and American astronaut Rick Mastracchio successfully installed a roughly two-tonne beam to the backbone of the space station during a spacewalk of more than six hours.

The Canadarm was used in that procedure as well, helping astronaut Charles Hobaugh lower the beam into place as Mastracchio and Williams floated nearby, offering guidance.