Spacewalkers begin Space Station Cooling System Repair

Spacewalkers begin Space Station Cooling System Repair:

Sunita Williams, center, emerges from U. S. airlock for International Space
Station spacewalk.  Photo Credit/NASA TV

Astronauts Sunita Williams and Akihiko Hoshide emerged from the U.S. airlock of the International Space Station early Thursday in a bid to stem an elusive and growing ammonia 

coolant leak on the far port side of the orbiting lab’s vaulting solar power truss.



The six to seven hour excursion got under way at 8:29 a.m., EDT.




If not isolated, the leak threatens to force the shutdown of 2B -- one of the eight power channels distributed across the six person space station -- by early 2013.
The spacewalk follows a 10 minute maneuver of the station late Wednesday to avoid a succession of close encounters with a small but erratic fragment of the U.S. commercial Iridium 33 satellite. The debris was generated by a February 2009 collision with Russia’s Kosmos 2251 satellite. Thrusters on the Progress 48 supply craft, berthed to the Russian segment Pirs docking compartment, ignited at 7:08 p.m., EDT, to kick off what turned out to be an abbreviated but effective debris avoidance maneuver.
Thursday’s spacewalk takes Williams and Hoshide to P-6, the oldest of the station’s solar power modules. A suspected leak in the P-6 photovoltaic radiator (PVR) system, first noted in 2007, increased substantially in June. Possible sources of the leak include damage from an orbital debris strike or a crack.
The repair strategy requires the two astronauts to reconfigure a pair of coolant jumper cables to circumvent the P-6 PVR panel. The spacewalkers plan to re-route the flow of ammonia coolant for the 2B channel to the trailing thermal control radiator (TTCR).
The TTCR was launched as part of the P-6 solar power module in late 2000, but deactivated in late 2006. P-6 was transferred from its initial central location on the station, where it served as the U.S. segment’s first source of solar power, to its current site by a space shuttle crew in late 2007.




The 45-foot-long TTCR panel was subsequently folded up and covered by Williams and another spacewalking astronaut during her previous space station assignment.
The redeployment of the TTRC will open another reservoir of ammonia available to the station cooling system in case the leak is not isolated during Thursday’s spacewalk.



Further efforts to pinpoint the leak source could lead to additional spacewalk repairs.



Until the leak rate increased in June, the seepage was so small that NASA managers planned



to just recharge the system every few years. The last refill was carried out in May 2011 by visiting shuttle astronauts.



Williams and Hoshide teamed for spacewalks on Aug. 30 and Sept. 5 to replace a failing main bus switching unit on the station’s solar power truss, a troublesome repair.



They are scheduled to return to Earth with cosmonaut Yuri Malenchenko aboard their Soyuz 31 mission capsule on Nov. 19.