In another trouble-free day of the mission, Ron Garan and Mike Fossum checked off a list of maintenance tasks large and small over six hours and 33 minutes outside the orbiting space station, heading back inside at 2028 GMT.

In the most dramatic moment of the walk, Garan -- his feet locked into the end of the ISS's massive Canadarm robotic arm like a puppet at the end of a rod -- was slowly swept from one side of the ISS to the other, carrying a 239 kilogram (528 pound) depleted nitrogen tank in his arms.

"Light as a feather, huh?" Garan said as the arm, manipulated from inside the ISS by NASA astronaut Karen Nyberg, began moving him through the dark of space.

"Enjoy the ride," Nyberg said.

"That looked like fun," she added after the traverse was over.

Besides replacing the nitrogen tank with a full one, the two spacewalkers removed a thermal cover from Kibo's robotic arm, brought up from Earth on the shuttle and installed earlier this week. They also tested cameras anchored outside the Kibo lab.

ISS Flight Director Annette Hasbrook said at a press briefing after the spacewalk that one of the planned activities for Monday was a trial run of Kibo's robotic arm.

"This will be the first large motion of the arm, so we'll just be watching to make sure that it's performing as expected and it's staying within the guidelines and constraints that the Japanese have laid out for the deploy," she said.

Hasbrook added that Fossum during the spacewalk retrieved samples of a powdery substance from the left solar alpha rotary joint, which will be analyzed later by experts on the ground.

"It could be expected debris, it could be something unusual," she said. "Until we get the data home, we really can't characterize what that is."

It was the third and final spacewalk of the planned 14-day mission, begun when Discovery launched from Kennedy Space Center in Cape Canaveral, Florida on May 31.

The principal job of the mission has been to deliver and install the Kibo lab's huge central pressurized module, along with its robotic arm.

Astronauts completed that setup and also attached an earlier-delivered storage module to the new lab during the first two spacewalks.

Kibo -- "Hope" in Japanese -- marks a huge advance in Japan's participation in the ISS, which already has units from NASA and the Russian and European space agencies, and key equipment from Canada as well.

The 11.2-meter (36.7-feet) long pressurized module, with room for four astronauts to do research, is now the ISS's biggest single room.

Installation was aided from inside the ISS by Japan Aerospace Exploration Agency astronaut Akihiko Hoshide, one of the seven astronauts who rode up on the Discovery.

Earlier Sunday Hoshide told Japanese Prime Minister Yasuo Fukuda he was in awe of the view of the Earth from 338 kilometers (210 miles) in space.

"Earth, as I see it from here, is very beautiful," he said, pointing out one of the round windows of the lab.

"I think to myself it is so wonderful and fragile."

Fukuda, amazed at how sharp the image was, said: "I can see you very clearly."

"Maybe you can see my face getting dark, too," Hoshide replied, referring to the stubble on his chin.

After the completion of the spacewalk Sunday, the astronauts have two days of rest and odd jobs before they are scheduled to undock Discovery from the orbiting station on Wednesday at 1142 GMT.

The shuttle is set to return to the Kennedy Space Center on June 14 at 1513 GMT.

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