After a spectacular run of two EVAs, lasting a cumulative 13 hours and 24 minutes, over the last week, Expedition 42 astronauts Barry “Butch” Wilmore and Terry Virts will venture outside the International Space Station (ISS) for the third and final time tomorrow (Sunday). Scheduled to run for six hours and 45 minutes, EVA-31 will install the expansive Common Communications for Visiting Vehicles (C2V2) infrastructure, providing standardized communications between the ISS and future visiting vehicles—including Boeing’s CST-100 and SpaceX’s Dragon V-2 piloted craft—with a common data link for exchanging audio, video, and telemetry at varying rates and communications ranges. For unpiloted visiting vehicles, this will include trajectory and spacecraft health data, whilst for the piloted craft it will also feature a two-way audio capability. When the C2V2 hardware is fully operational, it will operate for up to 13 years, throughout the expected remainder of the station’s lifetime, and consolidate ISS-based communications, save weight and volume, and greatly simplify logistics for co-ordinating the arrival and departure of all visiting vehicles.
EVA-31 comes hard on the heels of last Saturday’s highly successful EVA-29, during which Wilmore and Virts began the laborious task of laying and configuring 340 feet (103 meters) of cables and utilities on the forward end of the Harmony node and Pressurized Mating Adapter (PMA)-2 to prepare it for the delivery of two International Docking Adapters (IDAs), later this year, and Wednesday’s super-fast EVA-30, in which the spacewalking duo completed the cable work, lubricated the “sticky” Latching End Effector (LEE) of the 57.7-foot-long (17.6-meter) Canadarm2 robotic arm, and readied the forward and aft Common Berthing Mechanisms (CBMs) of the Tranquility node for the relocation of the Leonardo Permanent Multipurpose Module (PMM) and the arrival of the Bigelow Expandable Activity Module (BEAM). However, EVA-30 ended on a cautious note, when Virts detected a small quantity of water in his helmet, during the repressurization of the Quest airlock.
The incident garnered unpleasant reminders of the water intrusion incident which engulfed Italian astronaut Luca Parmitano during the curtailed EVA-23, back in July 2013, although it proved to be of far less severity. Virts’ first reference to water came when the repressurization of the airlock reached 5 psi. He noted that its taste ruled out a leakage from his drinking bag and the crew estimated a quantity of about 15 ml. When fellow Expedition 42 crewmate Samantha Cristoforetti removed Virts’ helmet, she became aware that the lower part of his Helmet Absorption Pad (HAP)—one of the corrective actions implemented in the aftermath of the EVA-23 incident and capable of holding up to 800 ml of water—exhibited slight dampness, but at no point were any of the astronauts in any danger.
Following an investigation of the issue, ISS Program managers convened yesterday (Friday) and gave approval for EVA-31 on Sunday. It was pointed out that Virts’ suit (No. 3005) has a history of “sublimator water carryover.” This is a small quantity of residual water in the sublimator cooling component which could condense when the environment around the suit is repressurized, following exposure to vacuum during an EVA, and results in a tiny amount of fluid pushing into the helmet. The Mission Management Team (MMT) expressed “a high degree of confidence” that the suit’s systems are in good shape and ready to support EVA-31 and its multitude of objectives.
On this occasion, the spacewalkers will swap designations, with Virts now taking the lead (or “EV1”) role, with red stripes on the legs of his suit, and Wilmore (“EV2”) clad in a pure white ensemble. Early tomorrow morning, the pair will undertake 60 minutes of “pre-breathing” on masks, during which time the inner “equipment lock” of the Quest airlock will be depressed from its ambient 14.7 psi to 10.2 psi. Upon completion of this well-trodden protocol—and assisted by Cristoforetti and Russian cosmonaut Anton Shkaplerov—the spacewalkers will don and purge their suits and the airlock’s atmosphere will be repressurized back up to 14.7 psi.
This will allow Virts and Wilmore to enter a nominal pre-breathing regime, lasting 50 minutes, followed by a further 50 minutes of In-Suit Light Exercise (ISLE). The latter was first trialed during the STS-134 shuttle mission in May 2011 and will require the two men to flex their knees for about four minutes, rest for 60 seconds, then repeat until the 50 minutes are up. This technique serves to rapidly remove nitrogen from the bloodstream, thereby avoiding a potentially fatal attack of the “bends.” At length, Cristoforetti will transfer the fully-suited pair and their equipment—including the Simplified Aid for EVA Rescue (SAFER) units, affixed to the lower section of their life-sustaining backpacks—from the equipment lock into Quest’s outer “crew lock.” Hatches between the two locks will be closed and depressurization will get underway. When it reaches 5 psi, it will briefly halt for pressure and leak checks, then resume until it achieves a condition of near-vacuum.
EVA-31 will officially begin when Virts and Wilmore transfer their suits’ life-support utilities from ISS power to internal batteries, with an estimated start time of 7:10 a.m. EST Sunday. This will mark Virts’ third career spacewalk and Wilmore’s fourth. After departing the airlock, Virts will receive a Large Orbital Replacement Unit (ORU) Bag from Wilmore, which carries four cable reels, as well as the larger of the two antenna booms, known as the “P-3 Boom.” Wilmore will then emerge from the airlock with the shorter of the two booms, the “S-3 Boom,” and the two astronauts will press into critical “buddy checks” to ensure that their suits, tethers, and equipment are in proper order.
Overseeing tomorrow’s spacewalk will be Flight Director Tony Ceccacci, who fulfilled the same role on last Saturday’s EVA-29, and Art Thomason will serve as the Lead EVA Officer. In a briefing at the Johnson Space Center (JSC) in Houston, Texas, earlier this month, the outline for this ambitious spacewalk was set out. Having already laid 340 feet (103 meters) of IDA Prep cables during EVA-29 and 30, the spacewalkers will establish a further 400 feet (122 meters) of cables on EVA-31 for the C2V2 architecture, which, when complete, will see four antennas and three laser reflectors erected on four booms on the port and starboard trusses of the space station. The cables will be routed in four 100-foot (30-meter) “legs,” running from the central S-0 truss and radiating outward to the C2V2 antenna locations on the P-3 and S-3 trusses.
The spacewalkers will take different routes to reach their initial worksites, with Virts making his way to the port-side truss and Wilmore heading starboard. To reach his destination, Virts will navigate through the “Rat’s Nest”—an area of the central Z-1 truss, which provides a hub for electrical and thermal control utilities and is so named because of its tricky maze of cables—to reach the P-3 truss. Once there, he will install his boom onto a Worksite Interface (WIF) and stow his bags, before removing both antennas. He will attach the antennas onto the boom, remove their protective covers, and place them into his Crew Lock Bag. The system also includes laser reflectors, but the delicate nature of their glass means that they will not be installed until later in the EVA.
As Virts works on his tasks, Wilmore will head out to the S-3 truss to install the shorter of the booms onto a WIF. He will essentially duplicate Virts’ actions, leaving his Crew Lock Bag at the worksite, in order for it to be used to hold protective caps at the end of the spacewalk. Both men will then head back to the airlock. Wilmore will collect a Large ORU Bag with the four cables and translate to the U.S. Destiny laboratory module to establish it in close proximity to the debris shield which the C2V2 system will be laid under. Working together, the astronauts will remove the Micrometeoroid Orbital Debris (MMOD) shield, temporarily demate a Global Positioning System (GPS) and Ku-band connector, install the new C2V2 connector, and reintegrate them. They will then replace the MMOD shield.
Next, Wilmore will head to the Large ORU Bag and retrieve two cable reels, passing off one to Virts and retaining the other for himself. They will route the port-side “leg” of the cables in the first instance, followed by the starboard leg. For the port-side work, Virts will follow the aft translational path, along the port side of the truss, securing the cable by means of copper wire-ties as he goes, before mating them to the inboard antenna at P-3. Meanwhile, Wilmore will follow about 20 feet (6 meters) behind Virts, routing cables along the forward translation path, along the Earth-facing (or “nadir”) side of the truss. This will separate both cables by about 18 inches (45 cm), thus protecting the system from the risk of being taken out by a future MMOD impact. Wilmore will hand his cable to Virts and it will be mated to the outboard antenna. Lastly, Virts will install the two laser reflectors onto the boom, which will be used as ranging aids for future visiting vehicles.
After cleaning up the P-3 worksite, the astronauts will return their cable and antenna bags back to the airlock and collect more cable for the S-3 task. Wilmore will take the lead for this installation effort, heading along the aft translational path of the truss, as Virts follows the forward route. Wilmore will remove two protective caps from the antenna, stow them, and attach his cable to the inboard antenna. Virts will then drop off his reflector bag and Wilmore will complete the final C2V2 connections on the outboard antenna. The spacewalkers are expected to clean up the S-3 worksite and return to Quest to close out the third and final U.S. spacewalk of Expedition 42 after six hours and 45 minutes.
Assuming EVA-31 runs to schedule, this will leave Wilmore with around 26 hours and 43 minutes of spacewalking time in four EVAs and Virts with about 20 hours and 9 minutes in three EVAs. This will elevate Wilmore from his current position as the 82nd most experienced spacewalker in the world—out of 211 individuals who have departed their spacecraft in pressurized suits since the pioneering excursion of Soviet cosmonaut Alexei Leonov, almost 50 years ago, in March 1965—to 55th place, slightly ahead of Hubble Space Telescope (HST) servicing veteran Story Musgrave and behind Ron Garan, who participated in the final EVA within the payload bay of a shuttle. Virts, meanwhile, will move from his present standing at No. 115 on the list to No. 78, just behind Moonwalker and former shuttle commander John Young and ahead of Steve Robinson, who dramatically spacewalked “underneath” the shuttle to remove gap fillers in July 2005.
The work of Wilmore and Virts to install and activate the C2V2 architecture comes at a pivotal time, as Boeing and SpaceX prepare for the first piloted missions by their CST-100 and Dragon V-2 vehicles in about two years’ time. Developed under contract between NASA and Mason, Ohio-based L-3 Cincinnati Electronics, C2V2 “provides standardized communications between the International Space Station (ISS) and docked vehicles by using a standard data link for exchanging audio, video and telemetry data at varying data rates and communications ranges.” For unpiloted vehicles, this includes trajectory and overall spacecraft health data, whilst for the crewed vehicles it will also feature a two-way audio capability. When its hardware is fully installed and activated, C2V2 will operate for up to 13 years—throughout the remaining lifetime of the ISS—and consolidate station-based communications, save weight and volume, and greatly simplify logistics for co-ordinating the arrival and departure of all Visiting Vehicles (VV).
In September 2012, L-3 was awarded a $24 million contract by NASA to deliver four S-band flight units and associated equipment for C2V2. This hardware will “provide coverage to all ISS docking ports” and will occur “throughout the VV trajectory,” with locations for the antennas and their laser retroreflectors ranging across the forward and aft faces of the starboard-side S-3 and port-side P-3 trusses, together with three of the Express Logistics Carriers (ELCs), the space-facing (or “zenith”) face of the central S-0 truss, and the nadir sides of the Quest airlock and the Tranquility node. Under the terms of the contract, the integrated C2V2 hardware should be ready “to support on-orbit operation by March 2015.” Documentation pertaining to the technical requirements and constraints of the system notes that the number of C2V2-related EVAs should be minimized and restricted only to the physical installation of the hardware.