HOUSTON - The 2-ton, $11 million addition astronauts have delivered to the international space station may be one of the smaller pieces of the structure, but even supporting actors are vital.

The addition will act both 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.

Two astronauts were scheduled to install the addition on Tuesday, in the first of three harrowing spacewalks during the 12-day mission that left Earth on Saturday.

The space shuttle Discovery crew are continuing the assembly of the station. Besides installing the addition, Discovery will also rewire the station to make use of solar arrays that were installed on the last mission. They will also rotate a space station crew member.

Meanwhile, NASA engineers were looking over images of areas on the space shuttle that caught their attention Monday.

A sensor on the shuttle's left wing detected a "very low" impact while the crew slept the night before, so astronauts took pictures of the area using a camera on the end of the station's robotic arm.

John Shannon, chairman of the mission management team, said late Monday he didn't think specialists were seeing anything of concern in the pictures so far.

Also, tiles on the shuttle's underbelly seemed to be dinged, so specialists were examining images to see how deep the nicks were. Shannon said he was not worried about the tiles, saying astronauts could repair them if they were chipped.

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, Shannon said, but engineers want to make sure it isn't blocking the tank door's seal.

NASA is expected to know by Tuesday whether further inspections are warranted or whether it can declare the shuttle's heat shield healthy.

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.

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.

Spacewalk veteran Bob Curbeam and first-timer Christer Fuglesang, the first Swede in space, will be guiding mission specialists Sunita "Suni" Williams and Joan Higginbotham as they use the station's robotic arm to install the addition. The clearance at times will be less then 3 inches.

The spacewalking pair will then tighten bolts and connect utility cables to complete the installation process.

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.

Williams, who will perform the third spacewalk with Curbeam, became only the third woman to take up longterm residence at the space station. She replaces German astronaut Thomas Reiter, who will return with the Discovery crew in a little more than a week.

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.

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Associated Press writer Mike Schneider in Cape Canaveral contributed to this report.

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On the Net:

NASA: http://www.spaceflight.nasa.gov

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