NASA astronauts on the International Space Station will have two malfunctions associated with the outpost's solar arrays to inspect when they venture outside Dec. 18 in a spacewalk that can't wait until the space shuttle is flying again.

In addition to the starboard solar alpha rotary joint (SARJ), a beta gimbal assembly (BGA) that rolls half of the array wing around its own axis is out of commission, possibly from a micrometeroid or debris hit detected when circuit-breaker trips stalled it Dec. 8.

Without the electricity that could be generated if the "1-alpha" BGA were able to keep its twin array blankets at the best available solar angle, NASA isn't sure the station can support any assembly missions after the STS-122/1E flight scheduled for Jan. 10.

Expedition 16 commander Peggy Whitson and flight engineer Dan Tani will check out both rotating mechanisms in a six-and-a-half-hour extravehicular activity (EVA). First they will look for evidence of damage to the BGA's bearing rotor roll-ring module, which allows signals and power to pass through the gimbal as it rotates. They also will inspect cables that might have been shorted out by space debris or loose hardware.

After that they will visit the SARJ, pulling off as many of its 22 thermal covers as they can while taking photos and using Kapton tape to collect samples of the ferrous debris Tani first discovered in October (DAILY, Oct. 29).

Whitson and Tani also will inspect both of the redundant drive lock assemblies (DLAs) that turn the SARJ with electric motors, and bring back one of the 12 trundle bearing assemblies (TBAs) that hold the truss together as its outer section rotates. Plans call for removing TBA No. 5, which is a prime suspect based on accelerometer data. However, if the spacewalkers spot another TBA that is obviously the source of the trouble they will take it inside instead.

Kirk Shireman, deputy ISS program manager, says analysis of the SARJ-debris samples returned on Discovery hasn't helped pinpoint the source of the problem, which appears to be a misalignment that has abraded at least one of the three roller surfaces on the rotating race ring in the SARJ. The BGA issue now takes priority as station managers plan recovery of full power capability, in large part because it probably will be easier to fix than the SARJ. Shireman says engineers are leaning toward shifting the SARJ to its backup race ring rather than trying to fix the problem on the primary ring.

But that will reduce redundancy on the starboard SARJ, and require "multiple" EVAs that will consume valuable time in the tight ISS assembly schedule.