WASHINGTON - Astronauts aboard the International Space Station (ISS) are gearing up for an extra spacewalk to tackle solar array joint malfunctions outside their orbital laboratory.

Expedition 16 commander Peggy Whitson and flight engineer Dan Tani will don their NASA spacesuits on Tuesday to inspect the station's starboard solar array joint and potential damage at the base of one of the segment's power-producing wings.

"I think both Dan and I are looking forward to the [spacewalk] and getting going again," Whitson told reporters Thursday via a video link.

Whitson, Tani and crewmate Yuri Malenchenko have enjoyed a relatively light work week after a November packed with orbital construction work and the delay of NASA's next shuttle flight earlier this week.

During Tuesday's spacewalk, they will focus all of their attention on the station's starboard solar array truss segment, where they will conduct a comprehensive inspection of a vital rotary joint contaminated with metallic grit.

Tani first discovered metal shavings inside the 10-foot (3-meter) wide gear, known as a Solar Alpha Rotary Joint (SARJ) in a late October spacewalk. The gear is one of two designed to rotate the station's outboard solar arrays like paddlewheels to constantly track the sun.

Kirk Shireman, NASA's deputy space station program manager, said Whitson and Tani will peer under as many of the SARJ joint's 22 protective covers during their inspection as time permits, use a mirror to seek debris inside the joint and retrieve one of 12 trundle bearings that may be the source of the damage.

"The big, key question for us is what caused the issue," Shireman said in an afternoon briefing, adding that engineers are hesitant to switch to the starboard segment's sole remaining backup SARJ ring until the source of the initial damage is found. "We need to understand what is the root cause — so we're working very hard to get that answer."

Whitson and Tani will also examine a starboard beta gimbal joint that allows one of the segment's two solar wing panels to pivot from side to side, on a different axis from the SARJ, to continuously face the sun. Last weekend, the joint suffered what may have been a micrometeorite hit that shorted out two power feeds and locked it in place, mission managers said.

Some spare parts for the system are onboard the station, and Tani is trained to repair the vital gimbal joint, Shireman said. The new glitch has forced mission managers to revisit the station's power margins for NASA's planned January launch of the space shuttle Atlantis, he added.

"I hope that we'll find out what it is based on the ground testing and our observations in this [spacewalk], and we'll be able to go fix it without any additional equipment," Shireman said.

Tuesday's spacewalk will mark the 100th dedicated to space station construction and the 23rd of 2007, which will tie the all-time record for a single year set in 2002, NASA officials have said.

"So far they've all gone very, very well and we're pleased to get through 99 of them without any major problems," Tani said.

Tani had hoped to return to Earth before Christmas with the STS-122 crew of Atlantis this month, but NASA postponed the mission to early January after faulty fuel tank sensors thwarted two launch attempts.

"As you know, launches are very unpredictable," said Whitson, adding that she looks forward to Atlantis' delivery of a new European lab in January and a planned February shuttle mission to install part of Japan's Kibo lab at the ISS. "I'm still optimistic [and] looking forward to getting a lot more work done in the future."