HOUSTON — ’s mission operations team expects to conduct weeks or even months of analysis before declaring the elusive leak of ammonia coolant from the International Space Station’s solar power generation system to have been fully repaired by the crew’s May 11 emergency spacewalk.
As a result, one of eight channels that distributes solar power throughout the six-person orbiting science lab remains offline at least through the early stages of the post-spacewalk troubleshooting, although this is not expected to affect normal operations, according to the operations team.
U.S. astronauts Chris Cassidy and Tom Marshburn, responding rapidly to the leak first spotted May 9, replaced a Pump and Flow Control Systems (PFCS) electronics box on the oldest of the orbiting lab’s four solar power modules, the 13-year-old P-6.
Neither the two men nor the flight control team at’s spotted any further signs of the leak that erupted with a flurry of snowy looking ammonia flakes during the 5 1/2-hr. spacewalk. Ground controllers monitored the work site through helmet cams donned by the spacewalkers and video cameras posted outside the orbiting lab and trained on the leak site.
“I don’t see any smoking gun,” Cassidy reported after reaching the suspected PFCS leak source. It was a refrain he and Marshburn would repeat frequently while they inspected the area before receiving clearance from NASA’s Mission Control to replace the bulky PFCS box with one of two spares pre-positioned on the port side solar power system truss for just such an eventuality.
Then the spacewalkers stood by as the new pump was activated and commanded to flow coolant, with the expectation the recharged thermal control system would reveal a leak somewhere in the associated plumbing. But no seepage was apparent, a strong suggestion the old PFCA was to blame for the ammonia loss.
“We are happy we did not see any obvious signs of a leak. But it will take some time, weeks, maybe months, to evaluate the system and make sure we did indeed stop the leak,” Joel Montalbano, NASA’s deputy ISS program manager, told a post-spacewalk news briefing. “There are no obvious signs of a leak, and we are happy with that.”
The temporary absence of a single power channel can be managed by reassigning current draws and is not expected to effect ISS operations during extended troubleshooting.
“We can do everything we need to do in that configuration for quite some time,” Ed Van Cise, a NASA space station flight director involved in the spacewalk planning, told the briefing. “It’s a capability the ISS was designed for. We have cross connects in place.”
Some of the analysis will track the cooling system response during day/night cycles and over changes in solar beta angles on the station’s orbital course.
Prior to the spacewalk, cooling responsibilities for the leaky 2B cooling loop were reassigned to a companion 2A loop. Radiator panels extend from the solar modules to thermally condition external power storage batteries as well as the many life support, science and avionics components laced throughout the station.
Seven channels were deemed sufficient for the scheduled late May 13 departures of three ISS crew members, including Marshburn. He was to join ISS commander Chris Hadfield and cosmonaut Roman Romanenko aboard the Soyuz TMA-07M spacecraft late in the day as they undocked and descended to a parachute-assisted landing in Kazakhstan, ending a 146-day expedition.
A factor in the spacewalk’s urgency was Marshburn’s familiarity with the leak site. During a July 2009 shuttle space station assembly mission, he and Cassidy teamed for a spacewalk at P-6 to replace power storage batteries.