Houston (AP): Two astronauts floated outside the international space station on Sunday to tackle a variety of maintenance tasks during the final spacewalk of the shuttle Discovery's visit to the orbiting outpost.

The first and primary task for shuttle astronauts Michael Fossum and Ronald Garan Jr. was to replace an empty nitrogen tank. Nitrogen gas is used to push ammonia through the station's main cooling system.

They also planned to do some final outfitting of the space station's new Japanese lab and, if they have time, retrieve debris from a solar wing's rotating joint.

The spacewalk, the third of the mission, began about 30 minutes ahead of schedule.

During the nitrogen tank replacement, Garan was set to go on the ride of his life. While holding the old tank, he would be attached to the end of the space station's robotic arm and be swung from one side of the complex to the other. He will be about 80 feet (24 meters) above the station during the maneuver. After getting the new tank, he would be swung back so he can install it.

``It's going to be an amazing view when he's doing that swing through. That's quite a trip,'' said Emily Nelson, a space station flight director.

They were also set to remove insulation from wrist and elbow cameras of the robotic arm of the space station's newest room: a $1 billion Japanese lab named Kibo.

Fossum was given an extra task for the spacewalk, if time permits: collecting samples of small amounts of dust-like debris detected on a solar wing rotating joint on the space station's left side.

Engineers will analyze the debris to see if the material can help them figure out why a similar joint on the right side of the station was clogged with metal shavings.

That clogged joint on the right side of the station has been used only sparingly since last fall, hampering generation of electricity. The joints enable the space station's solar power arrays to rotate and track the sun.

The left side joint is working normally but engineers are trying to prevent it from experiencing problems as well. Fossum said he was not worried about the joint.

It ``looks to be in pretty darn good shape,'' he said.

The spacewalkers were also going to reinstall a television camera that had a faulty power unit replaced.

On Saturday, Discovery crew members Akihiko Hoshide and Karen Nyberg moved Kibo's robotic arm for the first time, maneuvering very slightly two of its six joints.

Full deployment of the 33-foot (10-meter) arm will be done after Discovery leaves the station next week. However, it won't be used for any actual work until after the launch into orbit next year of the lab's third and final section _ a ``porch'' for exterior experiments _ and a second, smaller robotic arm.

Japanese Prime Minister Yasuo Fukuda spoke to Hoshide and Discovery commander Mark Kelly and congratulated them on Kibo's successful installation.

Kibo, Japanese for hope, was delivered by the shuttle and installed on the space station last week.

Also Saturday, several thermal protective panels on Discovery's right wing that the astronauts had photographed because of some slight pulses in their embedded sensors were given the ``all clear,'' said flight director Annette Hasbrook.

The wing sensors are one of NASA's many safety measures put in place after Columbia was destroyed during re-entry in 2003 as a result of a gashed wing.