Spacewalking astronauts worked on the outside of Japan's shiny new science lab Thursday, installing cameras and removing covers.

Dressed head to toe in white, Michael Fossum and Ronald Garan Jr. looked like puffy dolls against the 37-foot-long, 14-foot-wide lab, which is now the biggest room at the international space station.

It was their second spacewalk in three days at the shuttle-station complex, orbiting 210 miles above Earth.

"I feel like I'm on a camping trip trying to pack up a wet tent on a Sunday morning," Fossum said as he wrestled with some of the lab's insulation.

He and Garan removed thermal covers from the lab's robot arm and added them to a variety of attachment points.

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As the spacewalkers toiled outside, their eight colleagues hauled more experiment racks into the billion-dollar lab, called Kibo, Japanese for hope, and flight controllers near Tokyo monitored the power systems.

"Lots of people at work in there," astronaut Kenneth Ham informed the spacewalkers.

"No, there's not. I don't see anybody," one of the spacewalkers said.

"They got tired of your banging on the roof," Ham answered.

Even with all the racks moving in, Kibo was still noticeably bigger than the eight other rooms at the space station.

"We have not seen that much space in space since Skylab," Mission Control told the astronauts in a written message.

Skylab was NASA's first space station, back in the 1970s.

Space shuttle Discovery's astronauts delivered and installed Kibo earlier in the week. There are now three labs at the orbiting complex, supplied by NASA, the European Space Agency and, now, the Japanese Space Exploration Agency.

On Friday, the astronauts will attach a storage shed to Kibo that was dropped off by another shuttle crew in March. And on Saturday, they will test drive Kibo's 33-foot robot arm.

The two TV cameras that were set up on the lab's exterior Thursday will be instrumental in those robot-arm operations.

One last spacewalk is planned for Sunday, to replace an empty nitrogen-gas tank at the space station. Fossum and Garan got a head start on that work Thursday.

Some of their chores ended up being downright strenuous. As the spacewalk hit the five-hour mark, the two joked that they would skip the workout at the gym and eat whatever they wanted for dinner.

Just before the seven-hour spacewalk ended, Fossum checked the solar wing rotating joint on the space station's left side.

He found streaks of white grease but no metal shavings like those that are clogging an identical joint on the right side.

Flight director Annette Hasbrook said the left joint looked to be in fine shape and noted that the leaked grease actually may be preventing a buildup of friction between the moving parts.