US astronauts prepared early yesterday for the first spacewalk of the shuttle Endeavour mission following discovery of damage in the spacecraft's protective shield.
The two spacewalkers, mission specialists Rick Mastracchio and Dave Williams, were to spend the night at a special airlock to prevent decompression sickness.
The walk, which will be undertaken with the purpose of installing and activating a new truss aboard the International Space Station (ISS), was set to begin at 12:31pm.
On Friday, NASA detected an apparent gouge on shuttle Endeavour's heat shield during a routine inspection, after the orbiter docked with the station.
A piece of ice struck the shuttle shortly after Wednesday's liftoff from Cape Canaveral, Florida, leaving what appears to be a 19cm2 gouge near the hatch of one of the shuttle's landing gears, mission manager John Shannon said.
Small white marks were also visible on other thermal tiles surrounding the gouged area, he told a news conference.
He said NASA was trying to determine the extent of the apparent damage, adding: "What this means, I don't know at this point."
The possible damage was detected on Friday after ISS crew members took 296 pictures of the shuttle's underside while it performed a backflip during its approach to the station. The pictures were analyzed by NASA experts on Earth.
Today astronauts are to use a camera attached to a robotic arm to closely inspect the area of concern and a laser to determine exactly the depth of the gouge, Shannon said.
If repairs are deemed necessary the Endeavour mission will be extended by an additional space walk, he said, adding that materials to patch up the thermal shield were available to the astronauts.
The ice presumably was formed by Florida's humid air coming in contact with the fuel tank's cold surface -- it holds supercold liquid hydrogen fuel, something the insulation layer is supposed to prevent.
NASA has carefully inspected the orbiter's protective thermal tiles in the missions that followed the shuttle Columbia disaster of February 2003.
Columbia's heat shield was pierced by a piece of insulating foam that peeled off its external fuel tank during liftoff, causing the shuttle to disintegrate into a ball of fire as it re-entered Earth's atmosphere, killing all seven astronauts on board.
Endeavour brought to the ISS the first teacher in space and a new truss segment to expand the orbiting laboratory, which NASA considers a key part of its space exploration mission.
Endeavour's seven astronauts floated inside the station to a warm welcome by the three ISS crew members, with hugs and hand shakes, NASA television images showed.
The crew is scheduled to do three space walks during the mission, which includes replacing a defective gyroscope on the ISS and installing an external stowage platform.