Two astronauts onboard the International Space Station will conduct today the 100th spacewalk for the construction and maintenance of the station, during which they will try to assess the nature of previously-observed damages.

The ISS is currently affected by electrical problems stemming from the station’s solar array. Expedition 16 astronauts Dan Tani and Peggy Whitson will try to determine what caused the partial loss of electrical power to one of the station's two Beta Gimbal Assemblies (BGAs) for starboard solar wings, according to NASA. Secondary objectives also include damage-assessment to the starboard Solar Alpha Rotary Joint (SARJ).

The Alpha joint is the main rotary joint allowing the solar arrays to track the sun; in nominal operation the alpha joint rotates by 360° each orbit. One Solar Alpha Rotary Joint (SARJ) is located between the P3 and P4 truss segments and the other is located between the S3 and S4 truss segments. When in operation, these joints continuously rotate to keep the solar array wings on the outboard truss segments oriented towards the Sun.

Astronaut Dan Tani is the leader of the team, and his official status will be visible on his “garments”: a white spacesuit with candy-stripe markings. He is to be accompanied by 47 year old Peggy Whitson, who will be dressed in a simple all-white suit.

The spacewalk is scheduled to last for about 6.5 hours. The two ISS crew members will start their journey from the US-built airlock Quest and their destination is the near the end of the main truss’ starboard side. There the first task is inspection of the BGA 1A's power sources, expected to last for approximately 45 minutes.

Tani and Whitson’s priority is the inspection of cables and other elements feeding power to the BGA, which have probably been damaged by a micrometeoroid or orbital debris strike. "We're still in the throes of understanding what happened and how to move forward," said Kirk Shireman, NASA's deputy ISS program manager, in a mission briefing.

After the BGA inspection, Tani and Whitson will continue their mission with the relocation next to the SARJ, which has been locked after vibrations and increased power consumption were noticed. There, the two spacewalkers will together remove two large drive lock assembly covers. Next in their schedule is to inspect the race rings and bearings beneath them.

The 100th spacewalk for the construction and maintenance of the station will also set a new record, this time for cumulative spacewalk time by a woman (Peggy Whitson). The former record (still) belongs to Sunita Williams, with a total of 29 hours and 17 minutes during four spacewalks.