The two astronauts attached the Starboard 5 truss segment to the Starboard 4 segment, with the help of shuttle pilot Charles Hobaugh, who from inside the ISS operated the station's robot arm holding the 3.37 x 4.24-meter (11 x 14-foot) truss.

They also retracted a radiator on the Port 6 truss, which will be moved and attached to the Starboard 5 truss during a future mission.

"The two spacewalkers completed all their tasks," a NASA television commentator said after the two astronauts drifted back inside the ISS at 2145 GMT.

Back on Earth, meanwhile, National Aeronautics and Space Administration experts analyzed shuttle pictures taken during a routine inspection Friday just before the orbiter docked with the station. The pictures showed what appeared to be a 19 square centimeter (three square inch) gouge on shuttle's heat shield near the hatch of one of the shuttle's landing gears.

A piece of ice that struck the shuttle 58 seconds after Wednesday's liftoff from Cape Canaveral, Florida is believed to have caused the gouge, mission manager John Shannon said.

Small white marks were also visible on other thermal tiles surrounding the damaged area, he told reporters.

He said NASA was trying to determine the extent of the apparent damage. "What this means, I don't know at this point," he said.

The gash was detected Friday after ISS crew members took 296 pictures of the shuttle's underside while it performed a back flip during its approach to the station.

Astronauts on Sunday will use a camera attached to a robotic arm on the ISS to closely inspect the area of concern, and a laser to determine exactly the depth of the gouge, Shannon said.

The ice presumably was formed by Florida's humid air coming in contact with the cold surface of the fuel tank, which holds super-cold liquid hydrogen fuel. An insulation layer on the tanks is supposed to prevent icing.

The US space agency has carefully inspected the orbiter's protective thermal tiles during each of the missions that have followed the shuttle Columbia disaster of February 2003.

Columbia's crucial protective heat shield was pierced by a piece of insulating foam that peeled off its external fuel tank during liftoff. The breach resulted in the shuttle disintegrating into a ball of fire as it re-entered Earth's atmosphere, killing all seven astronauts on board.

The Endeavour's mission was initially planned for 11 days, with three space walks.

Saturday's tasks included replacing a defective gyroscope on the ISS and installing an external stowage platform, as well as the new truss segment carried up on the Endeavour.

NASA will decide Sunday whether to extend the mission by three days, and add a space walk, after they test a new system that transfers electricity from the ISS to the orbiter, prolonging the life of the shuttle's batteries.

Separately, if NASA decides that the damaged shuttle tiles need fixing, an additional space walk is possible.

After Friday's docking, Endeavour's seven astronauts floated into the station to a warm welcome by the three ISS crew, with hugs and handshakes, NASA television images showed.

Among the shuttle crew was 55-year-old Barbara Morgan, the first school teacher in space. Her presence came 21 years after the shuttle Challenger launch explosion in 1986 killed another woman intended to become the first teacher-astronaut, Christa McAuliffe. Morgan had been McAuliffe's understudy at the time.

earlier related report
NASA on Saturday attempted to gauge the extent of ice damage on the space shuttle Endeavour's heat shield as two astronauts prepared for their first spacewalk of the shuttle mission. The walk, set to begin at 12:31 pm (1631 GMT), is mainly to install and activate a new truss on the International Space Station, and could be followed by another spacewalk to repair the damage.

Experts at the National Aeronautics and Space Administration on Earth were analyzing pictures taken of the shuttle during a routine inspection Friday after the orbiter docked with the station, that showed the apparent three square inch (19 square centimeter) gouge on shuttle's heat shield.

A piece of ice that struck the shuttle shortly after Wednesday's liftoff from Cape Canaveral, Florida is believed to have caused the 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 damaged 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 gash 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 two spacewalkers, mission specialists Rick Mastracchio of the United States and Canadian Dave Williams, spent the night in a special airlock to prevent decompression sickness during the walk.

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 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 cold surface of the fuel tank, which holds supercold liquid hydrogen fuel. An insulation layer on the tanks is supposed to prevent icing.

The US space agency has carefully inspected the orbiter's protective thermal tiles during each of the missions that followed the shuttle Columbia disaster of February 2003.

Columbia's crucial protective heat shield was pierced by a piece of insulating foam that peeled off its external fuel tank during liftoff. The breach resulted in the shuttle disintegrating into a ball of fire as it re-entered Earth's atmosphere, killing all seven astronauts on board.

After Friday's docking Endeavour's seven astronauts floated into the station to a warm welcome by the three ISS crew, with hugs and handshakes, NASA television images showed.

Among the shuttle crew was 55-year-old Barbara Morgan, who has become the first schoolteacher in space. Her presence came 21 years after the Challenger launch explosion in 1986 killed another woman intended to become the first teacher-astronaut, Christa McAuliffe. Morgan had been McAuliffe's understudy at the time.

"When we first came to orbit it took a little getting used to," Morgan 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."

Three space walks are scheduled during the mission, which includes replacing a defective gyroscope on the ISS and installing an external stowage platform, as well as the new truss segment carried up on the Endeavour.

Adding a fourth space walk for repairs could mean extending the 11-day mission to 14 days.

earlier related report
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 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 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.

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, 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 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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