Spacewalking astronauts Dan Tani and Peggy Whitson are at the starboard end of the International Space Station's central truss, where they aim to inspect two faulty components designed to keep a massive solar wing optimally pointed at the sun.
The first stop: one of two Beta Gimbal Assemblies on the S3-S4 truss segment. The BGAs are about three feet by three feet by three feet and are used to tilt, or pivot, the wing. The component was shut down Dec. 8 when two associated circuit breakers tripped. NASA engineers think a micrometeorite might have struck the components, causing an electrical surge that tripped the breakers.
Tani and Whitson are looking at the three major components of the assembly: the Bearing, Motor and Roll Ring Module, or BMRRM, the Electronic Control Unit, or ECU, and the Beta Gimbal Transition Structure. No obvious problems have been spotted yet.
Later today, they'll inspect the Solar Array Rotary Joint, or SARJ -- a 10-foot diameter mechanism that is designed to allow the wing to turn like a Ferris wheel so that it can remain face-on to the sun as the station circles Earth.
The joint was shut down in October after engineers noted high voltage readings. Two subsequent inspections have undercovered a fine dust and metal shavings within the joint as well as damage to its circumferential race ring.
Engineers hope the inspection today will help them pinpoint the source of the contamination so they can determine what action should be taken to fix the problem.
Here's a graphic that shows the location of both the BGA and the SARJ:
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