U.S. spacewalkers Chris Cassidy and Tom Marshburn searched hard Saturday but failed to find the “smoking gun" source of an ammonia coolant leak on the International Space Station's vaulting solar power system.
However, they replaced a bulky suspect pump and flow control system (PFCS) electronics box nested in an Integrated Electronics Assembly (IEA) bed on the station's 13-year-old P-6 solar power module in a bid to stem the leak of hazardous coolant. The surprise seepage of frozen ammonia from the region around the old PFCS that was first noted by ISS commander Chris Hadfield on Thursday did not resume after the replacement.
As Saturday’s hastily planned 5.5 hour spacewalk drew to a close, all troubleshooting pointed to an elusive leak source within the old PFCS box itself.
NASA astronauts Tom Marshburn, left, and Chris Cassidy address thermal control system leak outside the International Space Station on Saturday. Photo Credit: NASA TV
The replacement PFCS was activated with Cassidy and Marshburn positioned close by the assembly to watch for telltale signs of frozen ammonia floating away.
"We are not seeing anything," Cassidy reported.
"So far, so good," NASA Mission Control spacecraft communicator Mike Fincke, a former ISS commander, told the spacewalkers. "You guys have done fantastic work.
However, a final determination will likely follow a substantial period of leak free thermal control system operations and follow up analysis by ground experts, with the new pump circulating coolant.
"We are happy, very happy we did not see any obvious signs of a leak. But it is going to take some time, weeks and perhaps months, to evaluate the system to make sure we did indeed stop the leak," Joel Mantalbano, NASA's deputy ISS program manager, told a post spacewalk news briefing.
The spacewalk ended at 2:14 p.m., EST
Prior to Saturday's excursion, the leak prompted the shutdown of one of eight power channels on the six person orbiting science laboratory, which over time could impact research activities. If not solved with the installation of the new PFCS, the seepage will require more focus from the ISS ground personnel to monitor power demands against cooling system limitations.
Radiators panels jut from the starboard and port sides of the station's vaulting power system truss to dissipate heat from companion solar arrays, power storage batteries, life support systems, research gear and other avionics.
The deactivated power channel will remain off at least through Monday, when three of the station's crew, including Hadfield and Marshburn are scheduled to return to Earth.
"If we change out the pump and the pump is not the cause of the problem, then it's going to take us quite a bit of time, I imagine, to figure out where this leak could be, identify it, isolate it, overcome it, fix it and then recharge the system," Mike Suffredini, NASA's ISS program manager, cautioned on the eve of the spacewalk.
The lengthy appraisal will include assessments of day night cycles and changing solar beta angles on the station's orbit.
A very small leak of ammonia in the P-6 module was first noted in 2007. The system was topped off to manage the seepage until last Nov. 1, when former station astronaut Sunita Williams and Akihiko Hoshide installed a jumper cable during a spacewalk and reactived an older radiator panel to circumvent the suspected leak source, the primary radiator panel.
Cassidy and Marshburn looked everywhere Saturday for an obvious source of the ammonia loss.
"I don't see any smoking guns," reported Cassidy as the two men reached the work site after translating hand over hand 150 feet from their airlock. "It looks like a nominal IEA."
The leaking power module's 2B radiator cooling loop was deactivated ahead of the excursion and thermal control transferred to the companion 2A circulation system. But ISS mission managers and NASA flight controllers were hopeful there would be enough residual pressure in the 2B loop to push free distinctive white flakes of frozen ammonia that would point the way to the leak source.
"I don't see any flakes either," Marshburn reported.
That assessment was repeated over and over throughout the outing.
As the spacewalk got under way at 8:44 a.m., EST, NASA's flight control team planned to have the spacewalkers inspect the IEA for evidence of a leak source in the PFCS box and associated coolant plumbing before committing to the installation of a replacement. Cassidy and Marshburn were familiar with the work site from previous excursions to the region during a July 2009 shuttle space station assembly mission they shared.
But Mission Control directed the spacewalkers to proceed with further photo documentation after they removed the suspect 260-pound electronics box, then to install one of two spares stowed previously on the power system truss in case a rapid repair was necessary.
"It looks really, really clean, surprisingly so," reported Cassidy as the old PFCS was removed. "All the pipes look clean, no crud."
Though initially disappointed at the elusiveness of the leak source, Mission Control urged the two men on.
"Everyone is really happy with the data so far," controllers assured Cassidy and Marshburn.
Near the four hour mark of the spacewalk, coolant flowed through the newly installed PFCS pump. Still, no signs of leaking ammonia were evident and internal accumulator pressures within the assembly held steady.
As much as Saturday's troubleshooting pointed to the old PFCS as the leak source, it may be impossible to know for certain.
An actual failure analysis of the old hardware is unlikely. The ammonia exposure makes the assembly too hazardous to astronauts to haul into the station so that it could be loaded aboard a re-supply craft destined for a return to Earth. And at the moment, there is no supply craft capable of descending with unpressurized equipment.
Hadfield and Marshburn will join cosmonaut Roman Romanenko aboard the Soyuz TMA-07M spacecraft as it departs the station on Monday just after 7 p.m., EST, for a scheduled landing under parachute in Kazakhstan at 10:30 p.m., EST, ending a 146 day mission.
Command of the ISS will transfer to cosmonaut Pavel Vinogradov, who remains with Cassidy and cosmonaut Alexander Misurkin. They are scheduled to be joined by a new U. S., Russian, European crew, Karen Nyberg, Fyodor Yurchikhin and Luca Parmitano, following a Soyuz launch on May 28.
Parmitano will arrive prepared to join Cassidy for further troubleshooting if another spacewalk is required.