HOUSTON (AP) – For the second straight day, spacewalking astronauts toiled on a brand-new 17½-ton addition to the international space station, removing protective covers and loosening latches and bolts that held the hardware down during the jolting ride into orbit.

The 7-hour, 11-minute spacewalk brought the total amount of time dedicated to assembling and maintaining the half-built space station to 431 hours and 54 minutes.

The installation of the new piece – a $372 million, 45-foot truss that holds electricity-generating solar panels – marks the first time since the Columbia disaster more than 3½ years ago that construction on the orbiting outpost has resumed.

Space shuttle Atlantis astronauts Dan Burbank, an American, and Steve MacLean, a Canadian, grunted through the last hour or so of the first spacewalk of their career, as they fought to loosen a few stubborn bolts.

“We had numerous battles with the hardware,” said lead space station flight director John McCullough, “but that’s the reason we have people working out there.”

He added: “It’s almost like giving birth today, the fighting that we had to go through and the labor pains. I hate to use the analogy because it’s nothing like that.”

The spacewalking duo continued connecting the solar panel and getting them ready to be rotated for the first time. The panels have a joint that allows them to move with the sun and maximize the amount of solar power they generate.

Mission Control in Houston remotely rotated the joint 180 degrees to get it positioned toward the Earth so the solar panels, now folded in long boxes, have room to unfurl Thursday. The panels will supply a quarter of the space station’s power when the outpost is completed by 2010.

Astronauts Joe Tanner and Heidemarie Stefanyshyn-Piper, who conducted the first spacewalk of the 11-day mission on Tuesday, planned to finish the installation job during the third and final spacewalk on Friday.

Burbank and MacLean had to remove more than a dozen insulation covers and 243 bolts while wearing bulky spacesuit gloves, and the work wasn’t easy. One bolt got stuck, and the two astronauts had to struggle together to get it loose. Tanner, inside the space station, helped out.

“We sure appreciate you answering that age-old question from Mission Control: How many astronauts does it take to unscrew a bolt?” Mission Control joked. “Apparently, it takes three: two outside and one inside.”

While MacLean was removing a cover on a crucial rotary joint, a bolt disappeared into the void. Another bolt floated off during Tuesday’s spacewalk.

Space debris can be dangerous if it punctures space station walls or spacesuits and can jam crucial mechanisms. However, spacewalkers have a long history of losing things in space, and NASA spokesman Grey Hautaluoma said the latest piece of space junk probably just floated away.

MacLean ran into another small problem when an extension on his pistol-grip power tool broke while he was trying to remove a restraint on the rotary joint.

“Son of a gun,” he muttered, then gathered the pieces in a trash bag so they wouldn’t float away and went to a toolbox to retrieve another.