Less than a week before he is due to return to Earth—wrapping up a U.S. national-record-breaking third long-duration stay aboard the International Space Station (ISS)—Expedition 48 Commander Jeff Williams will venture outside the orbital outpost Thursday, 1 September, to handle an unfinished piece of business from last fall. Designated “U.S. EVA-37,” it will represent the 37th “Stage” spacewalk, performed from the U.S. Orbital Segment (USOS), in U.S.-built Extravehicular Mobility Units (EMUs), and without the presence of the space shuttle, since February 2002. Williams will be joined for the 6.5-hour Extravehicular Activity (EVA) by crewmate Kate Rubins, and the duo will retract the accordion-like Trailing Thermal Control Radiator (TTCR) on the station’s port-side P-6 truss and cinch it securely into place.
Nicknamed “the ticker,” this backup radiator was meant to be retracted last November by Expedition 46 astronauts Kjell Lindgren and Scott Kelly, but other tasks and an approaching “saturation” of the station’s Control Moment Gyroscopes (CMGs) caused them to run out of time and the TTCR was left in a fully deployed configuration. This is an undesirable situation, as having the backup radiator deployed unnecessarily elevates the risk of sustaining Micrometeoroid Orbital Debris (MMOD) damage.
On Wednesday, 24 August, EVA-37 Flight Director Zeb Scoville and Lead Spacewalk Officer John Mularski held a press conference at the Johnson Space Center (JSC) in Houston, Texas, to lay out the planning for the excursion. As with last week’s EVA-36, Williams will be designated “EV1,” with red stripes on the legs of his EMU for identification, and Rubins will fill the “EV2” role, clad in a pure-white space suit. Aged 58 years and 217 days on 1 September, Williams will further cement his current record as the United States’ oldest spacewalker, whilst Rubins becomes only the seventh woman in history to have completed at least two EVAs.
Despite the fact that only 12 women have ever gone EVA, they have secured a number of enviable records. Ever since the Soviet Union’s Svetlana Savitskaya and America’s Kathy Sullivan performed their trailblazing spacewalks within months of one another in 1984, others have followed in their footsteps. Kathy Thornton became the first woman to perform more than one EVA, including two to service the Hubble Space Telescope (HST) in December 1993, whilst Linda Godwin became the only woman to spacewalk outside both Russia’s Mir space station and the ISS. Added to that list, Tammy Jernigan, Heidemarie Stefanyshyn-Piper, Nicole Stott, and Tracy Caldwell-Dyson have spacewalked to assemble, maintain, and repair the ISS, whilst Susan Helms helped secure the empirical record for the longest single EVA in history. Rounding out the list, Peggy Whitson and Sunita Williams are currently the most experienced female spacewalkers. Williams sits at No. 7 on the world list of 216 spacewalkers, having spent over 50 hours in vacuum during seven EVAs.
The core goal of EVA-37 on Thursday extends back to the very dawn of ISS assembly and the arrival of the P-6 truss segment—laden with twin solar array wings, photovoltaic radiators, and batteries—during Shuttle Endeavour’s STS-97 mission in late 2000. Initially, the P-6 hardware was installed atop the Z-1 truss, where it provided early power generation and cooling capabilities for the steadily growing space station. As part of the Early External Active Thermal Control System (EEATCS), the accordion-like Trailing Thermal Control Radiator (TTCR) and Starboard Thermal Control Radiator (STCR) provided ammonia cooling during P-6’s first few years in orbit. As the station’s main cooling system came online, the two radiators were retracted and stowed by Expedition 14 spacewalkers Mike Lopez-Alegria and Sunita Williams in early 2007, and the entire P-6 segment was moved to its final location at the furthest-port side of the Integrated Truss Structure (ITS) during the STS-120 shuttle mission, later that same year.
However, from the end of 2006 Power Channel 2B—one of P-6’s two power channels—had exhibited a very slow ammonia coolant leak, which reached about 1.5 pounds (0.7 kg) per year. It received a 5-pound (2.3-kg) top-up by the STS-134 shuttle crew in May 2011, with projections indicating that it probably would not need further attention until 2015. But from June 2012 onward, the leak accelerated to a rate of 5.2 pounds (2.4 kg) per year. This alarming trend threatened to force Channel 2B, which carried significant electrical loads across the space station, into a shutdown scenario within months. Suspicion as to the cause of the leak centered on P-6’s main Photovoltaic Radiator (PVR), which it was theorized might have sustained a Micrometeoroid Orbital Debris (MMOD) strike or suffered age-induced cracking. To better investigate this possibility, managers opted to isolate the Channel 2B coolant loop and redeploy the TTCR for interim cooling.
In November 2012, Expedition 33 spacewalkers Sunita Williams and Akihiko Hoshide executed a 6.5-hour EVA to redeploy the TTCR. Over the next few months, engineers focused on a faulty Pump Flow Control Subassembly (PFCS) as the source of the leak, but in May 2013 a dramatic rise in the leakage rate occurred and ammonia “snow” was seen emanating from Power Channel 2B. This demanded a contingency EVA a few days later by Expedition 35 spacewalkers Chris Cassidy and Tom Marshburn. Although the EVA was unable to locate the exact source of the leak, the pair removed, replaced, and tested the PFCS. This appeared to resolve the problem.
Planning got underway to restore P-6 to its original configuration, reconnecting Power Channel 2B to the main PVR and retracting and stowing the TTCR. Last November, Expedition 45 spacewalkers Kjell Lindgren and Scott Kelly set to work on this task. Although Lindgren successfully retracted the TTCR, the timeline got away from them before they could cinch it down and cover it up with its protective MMOD shroud. An additional problem centered upon the station’s Control Moment Gyroscopes (CMGs), which were approaching saturation level. Normally, these could be “desaturated” by means of Russian Orbital Segment (ROS) thruster firings, but mission rules opted against this course of action during an EVA or whilst the TTCR was not fully secured. As a result, Lindgren was directed to redeploy the TTCR and, in the words of Mr. Scoville at last week’s briefing, the team stepped back “to fight another day.”
Ten months later, Thursday’s task for Williams and Rubins will restore the P-6 cooling architecture to its pre-2012 configuration. According to Mr. Scoville, the TTCR remains one of the station’s “high-priority spares” and its retraction will allow it to be used again in the future, should the need arise. According to current plans, Williams will emerge from the Quest airlock at 8:05 a.m. EDT Thursday, carrying tethers for himself and Rubins. As his crewmate waits at the airlock, Williams will firstly secure the tethers at the P-1 truss, before translating along the port-side truss out to the worksite.
He will halt at the Solar Alpha Rotary Joint (SARJ). This is one of two massive joints, each weighing around 2,500 pounds (1,130 kg), which are tasked with rotating the station’s port-side and starboard-side solar array groups to continuously track the Sun. Williams will be joined by Rubins and they will take up positions for the TTCR retraction. Williams will be located at the Earth-facing (or “nadir”) side of the truss, and Rubins at the space-facing (or “zenith”), and the task will begin with visual inspections to ensure nothing could block the retraction. Armed with a Pistol Grip Tool (PGT), Williams will retract the TTCR into its fully stowed configuration. As well as having Rubins on hand to observe the process, Williams will benefit from ample external camera coverage. When the radiator is folded up, the spacewalkers will install four cinches to secure it in place, before deploying the MMOD shroud to afford thermal and debris protection.
With the primary objective of their EVA behind them, the astronauts will press ahead with their secondary tasks. Williams will collect an Articulating Portable Foot Restraint (APFR) and in short order will hop onto the 57.7-foot-long (17.6-meter) Canadarm2 robotic arm, deftly manipulated by his Expedition 48 crewmate Takuya Onishi of the Japan Aerospace Exploration Agency (JAXA). Rubins will collect a set of high-definition cameras from the airlock and transfer them to Williams for installation, along with a replacement light for Camera Group (CG)-9 on the nadir face of the P-1 truss. Meanwhile, Rubins will return to the TTCR worksite to perform a full inspection of the SARJ, as well as a number of other “clean-up” tasks on the outboard port truss.
Assuming that EVA-37 runs to its expected duration of 6.5 hours, Williams will return inside the space station after pushing himself up the league table from the 62nd most experienced spacewalker in the world to position himself within the Top 50. He will round out a four-mission, five-EVA career with a little over 31 hours spent outside his spacecraft. Meanwhile, Rubins will elevate herself from 179th to around 130th place on the world list of 216 spacewalkers. Chalking up around 12.5 hours of total EVA time, she will also become only the eighth woman to have made two spacewalks and the sixth most experienced female spacewalker of all time.
And for Williams, EVA-37 should close out a spectacular astronaut career. As the first U.S. Mission Specialist member of his astronaut class—the “Sardines,” selected way back in May 1996—to draw a flight assignment, he was also the first of his group to perform an EVA. Williams subsequently became the first non-Russian spacefarer to undertake as many as three discrete space station increments and last week passed Scott Kelly to become the United States’ most flight-experienced astronaut. Joined by his Russian crewmates Alexei Ovchinin and Oleg Skripochka, he is scheduled to return to Earth aboard the Soyuz TMA-20M spacecraft late on 6 September.