STS-124 Mission Specialist Ron Garan exits the Quest airlock at the start of the mission's third spacewalk

Two U.S. astronauts from the space shuttle Discovery have completed a third spacewalk to make external repairs on the International Space Station.

Michael Fossum and Ronald Garan spent more than six hours Sunday outside the orbiting laboratory, where they replaced a nitrogen tank used to cool the station.

They also inspected a problematic joint attached to the station's solar power panels. The astronauts collected samples of debris from the joint for analysis on Earth. This was the astronauts' final spacewalk of the 14-day shuttle mission.

The Discovery arrived at the station last week to install a new Japanese laboratory named Kibo.

On Saturday, Japanese Prime Minister Yasuo Fukuda spoke to Japanese astronaut Akihiko Hoshide through a video link to congratulate the team on its work.

Hoshide and the other astronauts tested the Kibo's robotic arm for the first time on Saturday.

Kibo is Japanese for "hope."