Two astronauts stepped outside the international space station Thursday for a spacewalk to spruce up the orbiting outpost's newest room — a $1 billion US Japanese lab.

In the early part of their excursion, space shuttle Discovery crew members Michael Fossum and Ronald Garan Jr. set up two TV cameras around the Kibo lab's robotic arm. Japanese flight controllers were to test the cameras later. The spacewalkers also finished removing thermal covers from the robot arm.

Later, they planned to do some advance work for a nitrogen-gas tank replacement scheduled for their third and final spacewalk this weekend.

The scheduled six-and-a-half 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 the last shuttle crew delivered it 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 could 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 — the Japanese word 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.

At 11.2 metres in length, Kibo is the largest of the nine rooms now at the space station, 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.