Expedition 33 Commander Sunita Williams and Flight Engineer Aki Hoshide concluded a spacewalk earlier today as part of efforts to resolve an ammonia coolant leak from one of the power channels of the International Space Station’s port-side P-6 solar array. Launched in November 2000 with 52 pounds of ammonia, P-6 exhibited a leak of around 1.5 pounds per year since the end of 2006. This prompted a decision to top it up during last year’s STS-134 Shuttle mission and projections at the time suggested that it would not need further attention until 2015. However, since June 2012 the leak has accelerated to around 5.2 pounds per year. This alarming trend meant that P-6’s critical 2B power channel, which carries significant electrical loads across the entire station, could have been forced to shut itself down before the end of December. Since the next EVA crew are not scheduled to arrive until shortly before Christmas, NASA opted to assign the task to Williams and Hoshide, who are well-established in orbit and have recent EVA experience.
Suspicion about the cause of the leak presently centres upon P-6’s Photovoltaic Radiator (PVR), which may have sustained a micrometeoroid strike or simply suffered age-induced cracking. To better investigate this possibility, today’s spacewalk sought to isolate the 2B coolant loop in the PVR and use the Trailing Thermal Control Radiator (TTCR) for future cooling. The other P-6 power channel (‘4B’) is unaffected by the leak and will continue to use the PVR. Although engineers are presently uncertain as to whether or not the leak does originate within the PVR, the actions taken by Williams and Hoshide will at least enable them to determine its exact location. Moreover, the TTCR was ‘backfilled’ with ammonia last year and its connection to 2B will essentially give it a ‘free’ top-up. As a result, even if the leak worsens, the channel should still run without difficulty until the end of 2013.
To protect the two spacewalkers from plasma build-up on the exterior of the station, a different orbital attitude was adopted. Instead of flying its normal +XVV attitude, with the forward end of the Harmony node facing into the direction of travel, the station was placed into a +YVV attitude, with the starboard-side arrays facing into the direction of travel and their port-side counterparts – and Williams and Hoshide’s worksite – at the point furthest aft. Both Solar Alpha Rotary Joints were parked for the duration of the EVA, as were both Thermal Radiator Rotary Joints, and veteran astronaut Mike Fincke, who performed last year’s ammonia top-up on STS-134, served as the Capcom in Mission Control. His experience of the P-6 worksite and the challenges faced by Williams and Hoshide made him an indispensable member of today’s team.
The excursion was designated ‘US EVA-20’, meaning it was the 20th spacewalk conducted from the US Segment of the station by expedition crew members. Monitoring and supporting the proceedings from the Quest airlock were Expedition 33 Flight Engineers Kevin Ford and Yuri Malenchenko. By 4:30 CDT this morning, Williams and Hoshide were fully-suited in Quest’s equipment lock, flailing their arms at one stage in response to Madonna’s song, ‘Vogue’. A Ku-band communications issue prompted a slight delay, but the spacewalkers entered the airlock’s crew lock at 6:54 am and the connecting hatch was closed and sealed by Ford. A halt in the depressurisation was called at 7:10 am, for a standard leak check, after which Williams and Hoshide were given a ‘Go’ to open the outer hatch of Quest into the vacuum of space.The EVA officially began at 7:29 am, when external station cameras revealed the popping open of the hatch’s thermal cover and Williams and Hoshide’s life-support utilities transferred to their suits’ internal power supplies. Emerging into orbital daylight, high above New Zealand, the spacewalkers quickly acclimatised and greeted each other – Williams with a brief “Hello!” for her crewmate – and commenced their translation along the port-side truss to the P-6 worksite. Hoshide paused at the boxy Z-1 truss to collect a 12-inch wobble socket tool and by 8:00 am both astronauts had reached their destination. Their first task at P-6 was the removal of four bolts of a metallic shroud covering the Fluid Quick Disconnect Coupling (FQDC), after which Williams used a pistol grip tool to close the supply and return valves of the 2B channel’s cooling system to its photovoltaic radiator. This was the first time the fluid quick disconnects have been manipulated in space.
After replacing the shroud, the pair moved to their next task: the reconfiguration of two Early Ammonia Servicer (EAS) jumpers to bypass the PVR and connect the 2B cooling loop to the TTCR. The risk of contamination led EVA planners to schedule sufficient time to ‘bake’ any ammonia out of Williams and Hoshide’s suits before they returned to the airlock. As circumstances transpired, very little of the hazardous substance was released and at one stage – seeing a couple of small flakes appear to harmlessly strike Hoshide’s helmet – Mike Fincke quipped: “Good thing none of those hit me!” It mattered little. By the end of the EVA, both astronauts had baked out their suits more than three times longer than required.
The spacewalkers moved crisply through the next stages of the EVA, removing the TTCR beta-cloth shroud and releasing six cinches and two winch pip pins holding the radiator in its stowed position. As the time for deployment of the TTCR approached, Fincke told the spacewalkers that it was “time to get paid”, as their efforts finally bore fruit. At 12:34 pm, the TTCR commenced deployment – with the International Space Station sailing high above the central United States – and within minutes was confirmed to be fully unfurled.Their primary task completed, the spacewalkers returned to the airlock. Hoshide hooked their tethers and re-entered Quest at 1:54 pm CDT, followed by Williams a couple of minutes later. The outer hatch’s thermal cover was closed at 1:57 pm and US EVA-20 officially concluded at 2:07 pm with the start of airlock repressurisation. The excursion represented the 166th spacewalk conducted on the International Space Station since December 1998 and the 20th from the US Segment by members of an expedition crew and lasted six hours and 38 minutes. Williams became the world’s most experienced female spacewalker, with more than 50 hours in seven career EVAs, and Hoshide became the most experienced Japanese spacewalker, with over 21 hours across three EVAs – eclipsing Soichi Noguchi, the previous record-holder for his homeland, by a little over an hour. From Mission Control, Mike Fincke and the entire team relayed their congratulations to the spacewalkers on a job well done, although it will take several weeks for NASA to determine whether the ammonia leak has been rectified.Missions » ISS » Missions » ISS » Soyuz TMA-M »