Shuttle astronauts complete first of three spacewalks

Astronauts completed a nearly seven-hour spacewalk Tuesday,the first of three scheduled during the shuttle Discovery's stay at the International SpaceStation.

U.S. astronaut Robert Curbeam and a European Space Agency astronaut Christer Fuglesang installed an addition to the orbiting space lab.

The two guided mission specialists Joan Higginbotham and Sunita "Suni" Williams as they used a robotic arm from inside the station to install a two-tonne, $11 million addition along the space lab's truss.

The spacewalk started several hours after NASA engineers studying Discovery's heat shield for damage declared it safe.

Sensors haddetected it was struck by something while the crew slept Monday night.

The crew was installing a girder-like truss that will be used to permanently add another truss and solar arrays.

It was the first of three spacewalks during the 12-day mission that left Earth on Saturday. The $11-million US addition will act as a spacer between a pair of the station's power-generating solar arrays and as a channel through which lines of electricity, data and cooling liquid will run, NASA said.

As the linked vessels passed over central Europe, mission specialists Curbeam and Fuglesangleft throughthe hatch at 3:31 p.m. ET in a 14-step process that took about 21 minutes.

Curbeam and Fuglesang positioned themselves on opposite sides of the P4 segment on the end of the space station. One astronaut was on the side facing the Earth and one was on the side looking out into space.

From the space station, mission specialist Joan Higginbotham manoeuvred the 1,800-kilogram addition into position using the orbiting lab's Canadarm 2.

Curbeam and Fuglesang released four locks securing the segment and Higginbotham slowly eased the piece into place.

"How does that look?" Higginbotham asked about 1 hours into the mission about the time it takes to complete one orbit of Earth.

"Awesome!" Curbeam responded, prompting Higginbotham to "push in" the arm to start mating the new truss to the station.

"We're going to go to our world-famous procedure," she said as the astronauts prepared to temporarily pin the new P5 piece onto the station in a so-called "soft-dock."

"We don't want to say we're extremely happy," Higginbottom said, hinting that she didn't want to jinx the mission, as the astronauts began securing the trusses together.

"We'll do it for you," mission control responded.

The astronauts then began affixing the new truss with permanent bolts before they were to run the power, data and heater lines through it.

In addition to continuing the assembly of the station, the Discovery crew will rewire it to use solar arrays installed on the last mission. They will also switch a space station crew member.

Shuttle declared OK

About two hours into the spacewalk, mission control notified the Discovery crew that the shuttle's heat shield was OK after a sensor on Monday detected a "very low" impact while the crew was sleeping.

Mission control advised theastronauts they would not need to take any morepictures of the area using a camera on the end of the station's robotic arm.

Tiles on the shuttle's underbelly appeared to be dinged, so NASA engineershad been examining images to see how deep the nicks were.

Managers were also looking at images showing an orange cellophane-like material used to keep nitrogen in place during launch that was sticking out of the shuttle's left external fuel tank door. The material sometimes burns off by the time shuttles land but engineers want to make sure it isn't blocking the tank door's seal.

After praising the shuttle's flawless docking with the station on Monday, lead flight director Tony Ceccacci said the hard work was just beginning.

"We have seven challenging docking days ahead of us," Ceccacci said.

Permanent power

The first part of Tuesday's spacewalk may sound familiar to people on Earth who have sought help trying to fit into a tight parking spot.

The later spacewalks are challenging because power to the station will be shut off in two phases, with astronauts rewiring one half of the station one day and the other half two days later.

This process will put the station on a permanent power system, from the temporary one it had been using. But NASA has never done this so managers are not sure everything will power up after the rewiring.

Astronauts can undo their work if that's the case.

The space agency has been especially alert to damage to the shuttle's heat shield since the Columbia tragedy in 2003. A piece of foam broke off Columbia's external fuel tank during liftoff and gashed a wing, allowing hot gases to penetrate the spacecraft during its return to Earth. All seven astronauts died.