Two astronauts on Sunday completed a spacewalk outside the International Space Station (ISS) as part of a complex procedure to move a solar power module. Scott Parazynski and Dan Tani spent six and a half hours outside the station uncoupling the 17,500-kilo module with the aid of the station's robotic arms, controlled by colleagues inside.

However, they have found a significant problem. A joint that turns a pair of the station's solar wing panels so they can track the sun for energy is malfunctioning. The motors operating the joint were previously determined to draw more power than expected, and produced suspicious vibrations.

Dan Tani did not find a protruding bolt, as some NASA engineers had expected, but found instead metal shavings inside the joint. He collected a sample to be analyzed back on earth. "It's quite clear," Tani told Mission Control, "there is metal-to-metal scraping and it's widespread. Wow."

The device will be moved to its new position on Tuesday. Parazynski also worked outside Harmony, the new module installed by the astronauts during the mission's first spacewalk, on Friday. The European space laboratory Columbus is set to be attached to this new ISS "wing" in December.

A NASA spokeswoman said following completion of the spacewalk that it had proceeded "as well as we could have wished."

For the Discovery this is the twenty-third mission to the International Space Station. Discovery delivered its precious cargo, the U.S. Node 2 Harmony module, which expands the space station's capability for future international laboratories. Built in Italy for the United States, Harmony is a high-tech hallway and Tinker toy-like hub. It is a 23- by 14-foot passageway that will connect the U.S. segment of the station to the European and Japanese modules, to be installed later this year and early next year, respectively.

Harmony, the ISS' seventh "room", will be the first new U.S. pressurized component to be added to the station since the Quest Airlock was attached to one of Unity's six berthing ports in 2001. This means that the crew faces one of its most complicated construction missions yet.

The trickiest part is that Harmony fits where the Discovery is actually docked. This means the module will have to be assembled in space and attached after Discovery leaves the ISS. Before that, astronauts will be restricted on how long they can spend inside the new compartment because of the makeshift ventilation system currently in place.

"STS-120 is such a cool mission," said Commander Pam Melroy in a statement before the mission. "Node 2 is the expansion of the space station’s capability to bring international laboratories up. It’s the expansion of our capability to carry additional people. "It has additional life support equipment that will allow us to expand out beyond a three-person crew. It’s this big boost in the capability which is really exciting," she added.

Harmony will provide air, electricity and water for the space station, as well as additional lodging for crew members.

Mission Specialist Daniel Tani will switch places with Clayton Anderson, who will be wrapping up a four-month tour of duty as an Expedition 16 crew member. Tani will stay on the station until he returns to Earth with STS-122 later this year.