Two spacewalking astronauts unhooked a 35,000-pound (15,000-kilogram) girder from the international space station Sunday, starting the delicate process of moving the giant solar power tower to another part of the orbiting outpost.

Spacewalkers Scott Parazynski and Daniel Tani started their 6 1/2-hour jaunt by disconnecting cables and unscrewing bolts that connected the girder to the space station's backbone.

They then guided astronauts Stephanie Wilson and Doug Wheelock as they used the station's robotic arm to detach the huge truss.

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"Don't drop it!" joked one of the spacewalkers.

The robotic arm operators inside the station will move the girder to a location where it can be temporarily parked. Installation is set for Tuesday during the mission's third spacewalk.

Parazynski and Tani also planned to add equipment to the outside of Harmony, a school bus-sized chamber that was delivered by Discovery and installed during the mission's first spacewalk. The crew entered the room for the first time on Saturday.

Besides spacewalking handrails, the astronauts plan to install a fixture on Harmony that will allow the station's robotic arm to move the compartment from its current temporary location to its permanent home.

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The space station's crew will relocate Harmony after Discovery leaves in another week.

The European Space Agency's science laboratory, named Columbus, will hook onto Harmony as early as December. The Japanese Space Agency's lab — called Kibo or in English, Hope — will latch onto Harmony early next year.

Harmony also will function as a nerve center, providing air, electricity and water for the space station.

Meanwhile, Tani is scheduled to inspect a rotary joint for the station's solar wings that is acting up and check for possible sharp edges on a rail for the robot arm.

NASA had to cut a spacewalk short during Endeavour's August mission after one of the astronauts noticed a quarter-inch-long rip in the thumb of his glove. Another glove was damaged during an earlier flight, and Mission Control said sharp edges on the rail may be to blame in both cases.

While they worked, Tani caught a glimpse of Ireland through the clouds and had a chance to wave at his friends and family there. Tani met his wife while golfing in Cork.

"Can't wait to get back there and share all my stories with them," he said.