HOUSTON — Spacewalking astronauts isolated an International Space Station thermal control system radiator with a suspected leak during a 6 hr., 30-min. Nov. 1 spacewalk in which they also rerouted the flow of ammonia coolant to a backup circulation loop.

The elusive leak on the photovoltaic radiator (PVR) of the P-6 solar power module, first noted in 2007, increased substantially in June. The loss threatens to prompt a shutdown of the station’s 2B power channel, one of eight channels that distribute electricity through the orbiting laboratory’s solar power grid, by early 2013.

NASA astronaut Sunita Williams, the station’s commander, and Japanese astronaut Akihiko Hoshide re-entered the station’s U.S. airlock at 3:07 p.m. EDT, with the backup P-6 trailing thermal control radiator (TTCR) circulating ammonia through the 2B power channel in place of the PVR. The PVR remained active for the circulation of ammonia for the station’s 4B power channel.

Station engineers plan further troubleshooting to pinpoint the source of the seepage over the coming weeks. The findings will likely drive future repair efforts. In the meantime, the reactivation of the TTCR introduces an additional supply of coolant that can feed the leak, if the loss is not coming from the PVR.

The station, which marks its 12th year of continuous staffing this month, houses six astronauts and supports more than 160 science experiments and technology demonstrations.

Possible sources of the ammonia leak include orbital debris strikes or a small crack.

The spacewalk got under way just hours after a 10-min. maneuver of the station to avoid a succession of predicted 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 on Oct. 31 at 7:08 p.m. EDT, kicking off what turned out to be an abbreviated but nonetheless effective debris avoidance maneuver, NASA reported.

The redeployed, 45-ft.-long 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 position by a space shuttle crew in late 2007.

The TTCR panel was subsequently folded up, covered and secured by Williams and another spacewalking astronaut during her previous space station assignment.

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. The experience seemed to pay off as Williams and Hoshide fell behind schedule while working with the intricate plumbing connections of the cooling system.

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