Two astronauts have begun a spacewalk outside the International Space Station (ISS) as Nasa decides whether to repair a gouge on the space shuttle Endeavour.
A chunk of insulating foam hit the shuttle during lift-off, creating a 9cm (3.5in) square gouge that penetrates the shuttle's thermal shielding.
Nasa managers expect to decide on Monday or Tuesday whether to send astronauts out to patch-up the damage.
The gash was seen as Endeavour neared the ISS prior to docking on Friday.
Astronauts Dave Williams and Rick Mastracchio began the mission's second spacewalk on Monday, floating out of the space station hatch to replace one of the four gyroscopes that help control the orbiting outpost's orientation in space.
The astronauts will replace the broken gyroscope with equipment they brought aboard Endeavour. The gyroscope that broke in October will be stored at the station so it can be brought back to Earth during a later mission.
Astronauts plan 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 apply protective paint, screw on a shielding panel, or squirt in filler.
Over the weekend, astronauts used the shuttle's robotic arm and extension boom, tipped with a laser and a camera, to determine the exact size and depth of the damaged area.
A piece of foam that broke off the external fuel tank is thought to be responsible.
Three-dimensional images and other information collected have been sent back to mission control in Houston where the conditions of re-entry into the atmosphere are to be simulated in a laboratory.
Nine pieces of foam are thought to have broken away during launch.
Mission management team chairman John Shannon said a mould of the gouge would be reproduced in thermal tiles and tested in a laboratory simulating the extreme heat and friction encountered on re-entry to the Earth's atmosphere.
The tests should allow engineers to determine whether repairs were needed, he said, quoted by AFP news agency.
"This is something we would rather not deal with but we have really prepared for exactly this case," he said.
Foam damage has been a major concern for Nasa since the Columbia disaster in 2003 when a briefcase-sized chunk of foam insulation broke off during launch and pierced the shuttle's heat-protection tiles.
This caused the shuttle to disintegrate on re-entry into the atmosphere, killing all seven crew.
The mission was scheduled to last for 11 days but was extended to 14 thanks to a new piece of equipment that allows the shuttle to tap into the power grid of the ISS.
On Saturday, two astronauts attached a new beam to the ISS on the mission's first spacewalk.
Mastracchio and Williams attached the 1.58-metric ton beam, or truss, to extend the space station's length to 108m (354ft).
Each of the spacewalks is scheduled to last six-and-a-half hours.
The mission is the second of four that Nasa plans this year to finish the $100bn (£49.1bn) space station before the shuttle fleet retires in 2010.