The scheduled 6½ hour spacewalk, the second by Fossum and Garan in three days, began about 30 minutes ahead of schedule. Kibo was delivered by the shuttle earlier this week.
An extra task was added to the spacewalk: tucking in some thermal insulation around one of Kibo's docking ports so there won't be any problems when another part of the lab - essentially a storage shed - is attached on Friday. The storage room has been temporarily located on another section of the space station since it was delivered by the last shuttle crew in March.
"Hopefully it's not going to take very much more time," Emily Nelson, a space station flight director, said of the extra task.
Meanwhile, the inside of Kibo was also getting a makeover.
The shuttle and space station crews on Thursday moved in more equipment racks so the new lab can be fully brought to life. Some of the racks contain equipment for the lab's power and data needs while others contain scientific experiments.
The door to Kibo - Japanese for hope - was swung open Wednesday, a day after its installation at the international space station.
It was a momentous occasion for Japanese astronaut Akihiko Hoshide, who hung a banner over the threshold and led the procession inside.
Hoshide noted that Kibo was empty, for now, but quoted an engineer back on Earth who told him, "It looks really empty but it's full of dreams."
"Enjoy your new module," radioed Japanese Mission Control near Tokyo.
The 10 inhabitants of the linked shuttle Discovery and space station took advantage of all the empty space inside the bus-sized lab and twirled, performed back flips and bounced on the walls. Then they started hauling in racks for science experiments.
Nelson said the new lab will look more full as it's loaded with equipment Thursday.
"So it will look maybe a little bit smaller although it's always going to look like this incredibly long module," she said.
At 37 feet in length, Kibo is the largest of the nine rooms now at the space station. It surpasses the two other labs, belonging to NASA and the European Space Agency, by nine feet and 14 feet, respectively, and an expansion is planned.
Besides the storage shed to be installed on Friday, a third section - essentially a porch for experiments - will be launched next spring. That's when full-scale science operations are expected to begin inside Kibo.
The other good news Wednesday was that the space station's toilet finally was working normally again.
Russian space station resident Oleg Kononenko put in a new pump that was delivered earlier this week by Discovery, after it was rushed to the launch site from Moscow. Flight controllers gave the go-ahead for the toilet's use once it became apparent it worked.
"We fully expect it's now fixed and we don't have to worry about it anymore," Nelson said.
For two weeks, the three men living aboard the space station had to manually flush the Russian-built toilet with extra water several times a day. It was a time-consuming job and a waste of water, not to mention an unpleasant chore.
NASA was reviewing a faulty sensor on the laser camera system that is part of a 50-foot inspection boom the shuttle uses to conduct a detailed inspection of the spacecraft's wings and nose. But NASA said the sensor would not affect the inspection that is planned for next week after the shuttle leaves the space station.
