2007-10-29

Spacewalking astronauts found evidence of damage to a key part of the International Space Station’s power system today.

It was the second of five scheduled spacewalks during the shuttle mission. More than six hours of outdoor activities were originally to be devoted to unbuckling an solar array atop the International Space Station so it could be moved to the side of the station, and also doing some work on the new “Harmony” module that astronauts had installed earlier in the week and first entered on Saturday.

Those tasks proceeded relatively well, as has virtually everything else in this otherwise exceptionally smooth mission. But those successes could well be overshadowed by the discovery of shavings in one of the shuttle’s enormous rotating joint assemblies, that appeared to be metallic.

The part, known as the Solar Array Rotating Joint, or SARJ, is 10 feet across and one sits toward each end of the station’s long truss. The motorized joint allows solar panels to rotate and constantly face the sun during the sunny part of each orbit.

“It’s quite clear,” said Daniel Tani, one of the two spacewalkers, describing what he saw after removing a protective cover over an assembly of gears and bearings. “There’s metal-to-metal scraping, or something, and it’s widespread.”

A sharp-eyed space station flight controller had noticed several weeks ago that the joint on the right side of the station was experiencing unusual vibrations as it rotated. Further examination revealed that the motor on that joint was using greater-than-expected amounts of current, which suggested that it was having to work harder than it should to turn the paddlewheel-like array. Mission managers added the inspection to the spacewalk schedule on Friday.

Mission managers said in an afternoon briefing that the origin of the shavings is still unclear. The leading theory, they said, was that foil backing on a protective cover could be rubbing against the mechanism and shredding into it. Mission managers had hoped the problem with the rotary joint would be easy to spot and easy to fix — something like a bolt out of place or an insulating blanket that was dragging and increasing friction, or even a leftover shop rag that was carried up to space and became lodged in the wrong place but could be removed. Before taking the cover off, Mr. Tani conducted a visual inspection of every bolt and blanket on the exterior of the device, which was made by Lockheed Martin at its Space Systems facility in Sunnyvale, and found no problems.

The problem could have ripple effects that go beyond this mission. If NASA wants a second look at the joint, a second spacewalk will have to be added to the schedule. With five spacewalks already on the shuttle mission’s calendar, it would be difficult to squeeze in another. At the same time, the days between the end of this shuttle mission and the arrival of the next shuttle in December is packed with activities for the three-person space station crew, and so even a single additional spacewalk could mean delaying the December mission.

Kirk Shireman, the deputy space station program manager at the Johnson Space Center in Houston, noted said in a media briefing on Friday that there are backup motors and controllers for each rotary joint, and so the system might still be able to work after a switchover.

During the same briefing, Derek Hassman, the lead space station flight director, said that the troubled joint could be “parked” in a position that allows it to pick up a fair amount of sunlight throughout the orbit while NASA continues to investigate the problem. “As long as we can get it into an attitude that’s reasonably good for power generation, combined with what the other SARJ can produce, we wouldn’t have any significant power impacts that we couldn’t deal with,” he said.

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