Peggy Whitson, the ISS commander, and Flight Engineer Dan Tani started to replace a motor at the base of one of the International Space Station's solar wings at 4:56 a.m. EST on Wednesday .

The problems were identified back in December, when Whitson and Tani conducted another spacewalk 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.

Today, in the mission lead by Peggy Whitson, the astronauts will replace the Bearing Motor Roll Ring Module (BMRRM). Called the 'broom," the motor drives the starboard solar wings as they tilt along their axis to follow the sun for optimal power generation as the station orbits the Earth.

Major activities of the BMRRM replacement take place during the night part of the orbit, when less voltage is generated by the solar wings than when they are exposed to the sun. The replacement BMRRM was retrieved from its stowage place in Pressurized Mating Adapter-3 early this month in preparation for the spacewalk.

The removal and replacement of the BMRRM is expected to take about four hours, including pauses during the daylight part of the orbit.

A second major task of the spacewalk is inspection of the starboard Solar Alpha Rotary Joint (SARJ). That device causes the starboard solar wings to rotate in a paddlewheel-like motion to keep the arrays pointed toward the sun.

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. Previous inspections revealed contamination and debris inside that joint.

Station Flight Engineer Yuri Malenchenko helped the crew with suiting up and is providing other support. Astronaut Tom Marshburn is in the station flight control room in Houston's Mission Control Center, acting as spacewalk choreographer.