Expedition 41 astronauts Reid Wiseman of NASA and Alexander Gerst of the European Space Agency (ESA) will step outside the International Space Station (ISS) tomorrow (Tuesday, 7 October) for 6.5 hours to move a failed ammonia cooling pump module from temporary to long-term storage and install a new Relay Assembly to provide power redundancy for the railroad-like Mobile Transporter (MT), along which the 57.7-foot-long (17.4-meter) Canadarm2 robotic arm traverses between its various work sites. Tuesday’s EVA-27 will be followed on Wednesday, 15 October, by EVA-28—featuring Wiseman and Expedition 41 crewmate Barry “Butch” Wilmore—to replace a Sequential Shunt Unit (SSU) on the starboard truss, following its failure back in May, as well as relocating external cameras in support of next year’s arrival of the first International Docking Adapters (IDAs) for the long-awaited Commercial Crew vehicles.
The requirement for EVA-27 came about following last December’s automatic shutdown of a pump module on one of the station’s two ammonia coolant loops. Suspicion centered upon the improper functionality of a regulating flow control valve inside the pump module, and NASA engineers worked quickly to transfer certain systems over to the second loop and power down a number of non-critical elements of the Harmony node and Europe’s Columbus and Japan’s Kibo laboratory modules. Since the valve itself was inaccessible to spacewalkers, the sole option was to remove the entire pump module from its location on the S-1 truss and replace it, which Expedition 38 astronauts Rick Mastracchio and Mike Hopkins did during the 5.5-hour EVA-24 on 21 December and the 7.5-hour EVA-25 last Christmas Eve.
As intended, the failed pump module was moved from the S-1 truss and attached to a robotically-controlled stowage location, known as the Payload Orbital Replacement Unit (ORU) Accommodation (POA) on the station’s Mobile Base System (MBS). This was always expected to be a temporary location, with forward plans to stage another EVA—originally scheduled for August 2014, during Expedition 40, but delayed due to problems with EMU Long Life Batteries (LLBs)—to move the failed pump module for permanent storage at External Stowage Platform (ESP)-2, which lies on the starboard side of the U.S. Destiny laboratory module, close to the Quest airlock.
Although they have been in space for over four months, both Wiseman and Gerst are on their first career space missions and neither has performed an EVA before, making them the first all-rookie ISS spacewalking duo since Expedition 17 cosmonauts Sergei Volkov and Oleg Kononenko in July 2008. From the perspective of spacewalkers utilizing U.S.-built Extravehicular Mobility Units (EMUs), it has been even longer. Not since shuttle mission STS-51 in September 1993 and the EVA by Carl Walz and Jim Newman have two “rookie” spacefarers embarked on their first spacewalk together. Tuesday’s EVA-27 will also make Gerst only the third German citizen—after his countrymen Thomas Reiter and Hans Schlegel—to participate in a spacewalk.
However, as described in an EVA-27 press briefing on Friday, 3 October, Space Station Integration Operations Manager Kenny Todd noted that both Wiseman and Gerst have trained extensively for their tasks in the Neutral Buoyancy Laboratory (NBL) at the Johnson Space Center (JSC) in Houston, Texas, in March, only weeks before their 28 May launch to the space station aboard Soyuz TMA-13M.
Final preparations for Tuesday’s spacewalk will begin at 3:15 a.m. EDT, when Wiseman and Gerst will jump onto a well-trodden path of 60 minutes of pre-breathing on masks, during which time the Quest airlock’s inner “equipment lock” will be depressed from its “ambient” 14.7 psi down to 10.2 psi. Their next step will be the process of donning and purging their bulky Extravehicular Mobility Units (EMUs)—assisted by Wilmore, who arrived at the station aboard Soyuz TMA-14M late 25 September and will serve as EVA-27’s “Intravehicular” (IV) crewman—after which the atmosphere will be repressurized to 14.7 psi. Wiseman and Gerst will then enter a nominal pre-breathing period, lasting about 50 minutes, followed by another 50 minutes of In-Suit Light Exercise (ISLE). The latter protocol was first trialed on the STS-134 shuttle mission in May 2011 and will involve the spacewalkers flexing their knees for about four minutes, resting for one minute, then repeating over and over until the 50 minutes are up. ISLE is scheduled to begin at 5:15 a.m. EDT serves to remove nitrogen from their blood streams in a much shorter timeframe.
An hour before EVA-27’s scheduled 8:15 a.m. EDT start time, the fully-suited astronauts and their equipment will be transferred into Quest’s outer “crew lock” and crewmates Wilmore and Expedition 41 Commander Max Surayev will confirm the closure of hatches between the two locks. Depressurization will be briefly halted at 5 psi for standard leak checks, after which the process will resume and continue until the crew lock is at conditions of near-vacuum. EVA-27 will officially get underway when Wiseman and Gerst transfer their space suits’ critical life-support utilities onto internal battery power.
According to EVA-27 Spacewalk Officer Jaclyn Kagey, Wiseman (designated “EV1,” with red stripes on the legs of his suit for identification) will be first to leave the airlock, followed by Gerst (“EV2,” clad in a pure-white suit), who will hand out equipment bags for the Mobile Transporter (MT) Relay Assembly (MTRA) replacement task, later in EVA-27. As is customary in the opening minutes of all spacewalks, the astronauts will pause to check each other’s suits and safety tethers, before parting on their respective tasks. Gerst will head to the Starboard Crew and Equipment Translation Aid (CETA) to temporarily stow the MTRA bags.
The first goal of the spacewalk will be the permanent stowage of the failed pump module, almost 10 months after Mastracchio and Hopkins initially transferred it from the S-1 truss to the temporary POA location. As Gerst wraps up his first task, Wiseman will head to ESP-2 to prepare the installation site, stowing equipment bags and opening the thermal cover to accept the 780-pound (350-kg) pump module. Elsewhere, Gerst will translate to the Port CETA to retrieve the Articulating Portable Foot Restraint (APFR), which he will move to the Canadarm2 installation point at the interface between the station’s central S-0 truss and port-side P-1 truss. He will install the APFR onto the end of Canadarm2—controlled by Butch Wilmore from workstations inside the multi-windowed cupola—and after exchanging safety tethers will ingress the foot restraint.
In the meantime, Wiseman will transfer the two MTRA equipment bags (one of which will hold the MTRA itself, whilst the other will hold four installation cables) out to the work site on the space-facing (or “zenith”) side of the MT to prepare for this task. He will then relocate himself to Gerst’s safety tether, gather it, and move it to ESP-2.
Riding Canadarm2, Gerst will remove the failed pump module from the POA and rotate it through 180 degrees to get it to the correct installation orientation. Wilmore will then “fly” him to join Wiseman at ESP-2, who will provide an extra pair of eyes to monitor the maneuver to insert the pump module into its permanent stowage location. Halfway in, the team will pause to allow Wiseman to remove the Adjustable Grapple Bar (AGB)—which has been used as a grapple fixture to secure the pump module to the POA for the last 10 months—after which they will drive it fully into place. The spacewalkers will then stow the AGB onto a spare ORU on ESP-2 and prepare for their next tasks.
Gerst’s assignment is to replace a failed light at External Camera Port 13, on the starboard side of the U.S. Destiny laboratory module. “That light has one of two bulbs currently failed in it,” explained Scott Stover, who will serve as Lead Flight Director for EVA-27, “and it’s one of our primary lights that we use for the visiting vehicles … and also for external robotics. We want to go out and replace that light before the second bulb fails.” Carrying the spare light in an equipment bag, Gerst will be flown by Wilmore on Canadarm2 to the work site, where he will remove the old light, fit the new one, and secure it with a single bolt and electrical connector.
Elsewhere, Wiseman will clean up the ESP-2 location and close the thermal cover, then move the pump module equipment bags back to the Quest airlock, before translating to the MTRA site on the zenith face of the MT. This location promises to afford him a spectacular perspective “forward,” over much of the U.S. Orbital Segment (USOS), although there will be little time to admire the view. Using a Pistol Grip Tool (PGT), Wiseman will install the MTRA, which provides backup power options for the MT, and route and mate electrical cables to its Earth-facing (or “nadir”) surface.
As EVA-27 enters its homestretch, Gerst will return to ESP-2 to “clean up” Canadarm2, swapping back his safety tethers, removing the APFR, and releasing himself from the robotic arm. He will then translate back to the airlock to stow the failed External Camera Port 13 light, then join Wiseman at the MTRA work site. With Wiseman working the port side and Gerst the starboard side, the two men will route cables to the MT nadir, then return to the zenith location and secure all wire ties to keep them out of the Mobile Transporter’s translation path. With this objective completed, they will clean up their equipment and head back to the airlock after about six hours and 30 minutes.
According to Kenny Todd in his introductory remarks, EVA-27 marks the first in a salvo of U.S. spacewalks which will run into the spring and summer of 2015. Mr. Todd stressed that the two contingency EVAs last December and also a third unplanned spacewalk in April 2014 by Expedition 39’s Rick Mastracchio and Steve Swanson “had left some things out there that we knew we wanted put back in proper order.” He added that there was an urgent need to tend to power-related issues and improve the station’s fault-tolerant capability, ahead of several more complex EVAs next year.
Current plans call for two U.S. spacewalks in January 2015 by Butch Wilmore and Terry Virts, two others in April and June, and a further pair in August to route cables and utilities in support of the relocation of the Leonardo Permanent Multipurpose Module (PMM) and the Pressurized Mating Adapter (PMA)-3. The names of the spacewalkers for the April, June and August EVAs have yet to be announced, but crew members rotating in and out of the USOS during this period include U.S. astronauts Virts, Scott Kelly, and Kjell Lindgren, Italy’s Samantha Cristoforetti, and Japan’s Kimiya Yui. These EVAs will lay the groundwork for the delivery of two International Docking Adapters (IDAs) in support of Boeing’s CST-100 and SpaceX’s Dragon V2 Commercial Crew vehicles. The Leonardo PMM will be robotically transferred in July 2015 from the nadir port of the Unity node to the forward port of the Tranquility node, whilst PMA-3 will be moved in late August from its current berth on Tranquility to the zenith port of the Harmony node for Commercial Crew operations.