A piece of ice struck the shuttle shortly after Wednesday's liftoff from Cape Canaveral, Florida, leaving what appears to be a three square inch 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 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.

Astronauts on Sunday will 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, he added, the Endeavour mission would be extended by an additional space walk. He said 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.

The US space agency 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. Seven astronauts died.

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.

On Saturday, they will operate a robotic arm to attach the new segment to the ISS and conduct the mission's first space walk.

The Endeavour crew includes 55-year-old Barbara Morgan, the first teacher in space 21 years after the Challenger explosion in 1986 killed fellow educator Christa McAuliffe and six astronauts.

Three space walks are scheduled during the mission, which includes replacing a defective gyroscope on the ISS and installing an external stowage platform. The 11-day mission may be extended to 14 days with a fourth space walk.

Morgan will operate robotic arms on the ISS and the shuttle to unload and install new equipment and supplies on the space station.

"When we first came to orbit it took a little getting used to," she said in a video transmission from Endeavour. "I felt like I was upside down the whole time."

She joked about the weightlessness that makes items casually drift out of sight after being put aside.

"We'll have to do a treasure hunt later."

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