Astronauts begin spacewalk as NASA analyzes shuttle damage

WASHINGTON (AFP) - NASA on Saturday attempted to gauge the extent of ice damage on the space shuttle Endeavour's heat shield as two astronauts set off on their first spacewalk of the shuttle mission.

Mission specialists Rick Mastracchio of the United States and Canadian Dave Williams headed outside the International Space Station to install and activate a new beam, on the first of three planned spacewalks of the mission, to which a fourth could be added.

Over a planned six and a half hours the two astronauts were to attach the Starboard 5 truss segment, brought up on the Endeavour, to the Starboard 4 segment, with the help of Charles Hobaugh, the shuttle's pilot, who was operating the station's robot arm holding the truss from inside.

They also have to retract a radiator on the Port 6 truss, which will be moved during a future mission.

Meanwhile, 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.

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 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.
 
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 would mean extending the 11-day mission to 14 days.