Assuming an on-time liftoff, the four-hour excursion is scheduled to begin at 2:46 p.m. on Dec. 4. The goal of the outing is to install thermal blankets around the motors and bearings used to rotate the P6 solar arrays to track the sun.

Over the past several months, engineers noticed higher-than-expected currents in the motor assemblies, indicating something was causing internal bearings to bind, making them harder to turn. Engineers grew concerned that one of the motor assemblies could lock up with an array in a less-than-optimal orientation, reducing the amount of power the panel could produce.

"The concern ... was if we get a solar array that we can't position optimally toward the sun we'd have to power down parts of the station and some critical equipment may have to be powered down," said station flight director Sally Davis.

"That reduces our fault tolerance and it gives us less flexibility in our operations. So we've been very careful with those arrays while we were trying to figure out what was causing the motors to increase their current and in some cases, stall."

Studying telemetry, engineers were able to rule out a problem with the electronic control systems used to operate the motor assemblies. Without knowing what was causing the binding, NASA planners had Godwin and Tani start training for a high-stakes spacewalk to replace the port-side bearing motor roll ring module assembly, or BMRRM.

"That was high risk in that we had to power down almost everything on that power channel except that which we could run on batteries for several hours," Davis said. "Then we would have to power everything back up. Also, it was highly choreographed with power and data connectors into that data canister."

This image illustrates the items of note on the space station's solar arrays. Photo: NASA

But after additional analysis, the engineering community began to believe the temperature extremes of space "was causing a distortion in the roundness of the beta gimbal assembly BMRRM motor bearings, which would cause the bearings to bunch up," Davis said.

And that led NASA managers to forego the high-risk replacement of the port-side motor bearing assembly and instead to ask Godwin and Tani to perform a much simpler task: Installation of insulation blankets around the port and starboard drive motors to keep the temperatures they experience more constant and to minimize, if not eliminate, any distortion in their housings.

Engineers will monitor the performance of the motors through December to determine if the blankets fix the problem. If not, a bearing replacement spacewalk likely will be added to a May shuttle mission.

"It's kind of a big barrel-looking thing, about the size of a barrel," Godwin said of the BMRRM. "It gets full sun on one side sometimes while it has full shadow on the other. And those temperature extremes basically get it out of round so it becomes a slight oval rather than a cylinder.

"So we're carrying up two thermal blankets to go and wrap these barrels, which are the beta gimbal housings, in hopes that will keep the temperature more constant all the way around its perimeter. They think that will take away some of the problems they've seen with the current spiking a bit high when they're moving the arrays."

After installing the blankets, Godwin and Tani will perform a few so-called "get-ahead" tasks for spacewalkers on the next station assembly flight, moving a set of tools inside, removing a thermal blanket on an S-band transponder and securing a solar array truss that failed to fully lock in place when the panel was deployed.

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