 | By Tariq Malik Staff Writer posted: 18 December 2007 12:51 p.m. ET |
This story was updated at 3:00 p.m. EST.
Two spacewalking astronauts took a close look at a pair of balky solar array joints outside the International Space Station (ISS) Tuesday to help engineers on Earth draw up repair plans.
Expedition 16 commander Peggy Whitson and flight engineer Dan Tani found widespread contamination inside a massive gear that rotates both of the station's starboard solar arrays, but no sign of damage to a joint at the base of one of those solar wings, during nearly seven hours of orbital work.
"Hopefully we got a good amount of data for the folks on the ground," said Tani, who led the spacewalk, after the reentering station's Quest airlock.
Whitson and Tani began Tuesday's spacewalk at 4:50 a.m. EST (0950 GMT), marking the fourth for their Expedition 16 mission. The excursion was initially scheduled as an extra spacewalk during NASA's STS-122 mission aboard the shuttle Atlantis, but the flight's planned December launch was thwarted twice by faulty fuel tank sensors.
As the Expedition 16 crew worked in orbit high above Earth, engineers at NASA's Kennedy Space Center in Florida fueled Atlantis' external tank to test the erratic sensors. Shuttle workers hoped to recreate, and then isolate, persistent glitches with the fuel gauge-like sensors in order to proceed with Atlantis' rescheduled Jan. 10 launch.
Fact-finding inspection
Whitson and Tani focused most of their spacewalk on a massive gear designed to rotate the station's starboard solar arrays like a paddlewheel to keep its power-generating wings continuously facing the sun.
Engineers will use the astronauts' findings as a reference for any plans to repair the joint's bearings, motors and metal race ring. Solving the station's solar array joint woes is vital to allow the outpost to support the addition of a large Japanese-built laboratory and other modules, mission managers have said.
"We didn't find anything that stood out," said NASA's space station program manager Mike Suffredini after the spacewalk. "It would be really nice if something stood out and said, 'Hey, I'm the cause of your problem,' and we didn't get that. But we do know more about the condition of the ring."
Tani first discovered metal grit inside the joint, known as a Solar Alpha Rotary Joint (SARJ), during a late October spacewalk after flight controllers noticed odd vibrations and power spikes in the mechanism's telemetry. A second inspection by spacewalkers last month confirmed the contamination, which Tani and Whitson found to be widely distributed around the 10-foot (3-meter) wide joint during today's excursion.
"You can see the motion of the gear because the debris is kind of walking across the housing," said Tani, adding that magnetized metal shavings appeared to walk end-over-end on one of the SARJ motors. "It's hilarious...it's animated, like they're alive. They're like ants."
The spacewalkers retrieved a suspect set of bearings, one of 12 on the joint, which may be responsible for the contamination. They also used orange Kapton tape to take samples of the metal grit and clean the SARJ gear's damaged metal race ring.
"I am getting almost all of the debris off," Whitson said. "It seems less splotchy."
Using a mirror, Tani and Whitson looked inside the SARJ gear to find that two sides of its three-sided race ring were untouched by damage, boosting hopes that engineers can find a way to continue using the joint in short spurts while continuing to study the glitch.
"If we can figure out to live with this, maybe roll on it for awhile when we have to...it gives us more time to figure out root cause and figure out what the real issue is," Suffredini said. By taking that extra time, engineers could determine whether a major switch to a backup race ring, which could potentially take up to four spacewalks and be performed during a fall 2008 space station mission, will be required, he added.
Tuesday's spacewalk marked the 100th outside the ISS and the 23rd this year alone, tying the all-time record for excursions in a single year. The spacewalk was the fifth career outing for both Tani and Whitson, who set a new world record during the outing for the most spacewalking time for a female astronaut.
"Congratulations, there is no pressure now because you are the queen of EVA," Mission Control told Whitson, using NASA's abbreviation for extravehicular activities.
"It's just being in the right place at the right time," replied Whitson, who is the space station's first female commander and beat NASA astronaut Sunita Williams' benchmark of 29 hours and 17 minutes to nab the title.
Hunting for damage
In addition to their SARJ joint inspection, the spacewalkers also surveyed cables and other hardware for a beta gimbal joint that swivels one of the station's starboard solar wings on a different axis from the SARJ. The joint suffered triple electrical failures on Dec. 8, prompting concerns on Earth of a possible micrometeorite strike.
"Everything I can see is nominal," Tani said. "There is no damage, no frays that I can see."
Despite her bulky NASA spacesuit, Whitson squeezed herself inside the station's backbone-like truss to reach the gimbal joint's power cables. Mission controllers warned her that it would be a tight fit, prompting a laugh from the spacewalker.
"Does this [spacesuit] make me look fat," she joked.
Space station managers said they are now confident that the gimbal joint's glitch lies in its motor box. A spare for the unit is aboard the ISS and Tani is trained to replace the motor.
Whitson ended Tuesday's spacewalk with 32 hours and 36 minutes of spacewalking time under her belt across five career excursions. Tani, meanwhile, ended with 32 hours and one minute as he concluded his fifth career spacewalk.
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