For Mike Hopkins it will be a Christmas gift like no other. It has been granted to a little over 200 individuals to walk in space, clad only in a pressurized suit, and to fewer still—just three, in fact—to have walked in space on the traditional date of Christ’s birth. On Christmas Day, Hopkins and fellow Expedition 38 crewmate Rick Mastracchio may complete the third in a critical series of EVAs outside the International Space Station (ISS), whose primary objective is to replace the malfunctioning starboard pump module and hopefully resolve a problem with one of two external ammonia coolant loops. Today (Wednesday), NASA unveiled its plans for up to three 6.5-hour spacewalks, currently targeted to begin at about 7:10 a.m. EST on 21, 23, and maybe 25 December.
It was way back in 1973 that U.S. astronauts Gerry Carr and Bill Pogue floated outside Skylab on Christmas Day and performed a lengthy EVA to photograph Comet Kohoutek and replace film aboard the space station’s Apollo Telescope Mount. For Pogue, the experience reminded him of his childhood, “doing a mud-crawl in a four-foot-deep stock tank used for watering cows and horses.” By the time the two men returned inside Skylab and rejoined crewmate Ed Gibson, they were advised that they had established a new world record for the longest spacewalk to date, at 6 hours and 54 minutes. This duration record has since been broken several times, and others have spacewalked close to the big day—STS-103 astronauts Steve Smith and John Grunsfeld made their final EVA to service the Hubble Space Telescope on Christmas Eve in 1999—but Carr and Pogue’s accomplishment at celebrating the traditional date of Christ’s birth, unhindered by spacecraft walls, high above Earth, has never been duplicated.
Until now, it seems. “NASA managers have planned for the first spacewalk to begin Saturday, the second on Monday and if necessary a third spacewalk on Christmas Day,” the agency revealed today (Wednesday). The situation has drawn some parallels with the August 2010 failure in the starboard-side coolant loop, which left the ISS with barely half of its normal cooling capability and severely restricted the redundancy of several systems. In particular, two of the four Control Moment Gyroscopes were shut down and three EVAs were executed by U.S. astronauts Doug Wheelock and Tracy Caldwell-Dyson to remove the failed pump module and restore the starboard loop to its proper functionality.
For Saturday’s opening EVA—the first spacewalk to be conducted from the station’s U.S. segment in U.S.-built suits since the terminated EVA-23 by Chris Cassidy and Luca Parmitano on 16 July—the two main participants have spent the past several days readying their equipment, tools, and procedures checklists. Rick Mastracchio is no stranger to spacewalking. Expedition 38 is his fourth space mission, and he has chalked up a cumulative six EVAs, lasting a total of 38 hours and 30 minutes, which currently establishes him as the 27th most experienced spacewalker in history. Mike Hopkins, on the other hand, is making his first space mission on Expedition 38, and although both he and Mastracchio had trained extensively for contingency EVAs before launch, neither of them could have anticipated the suddenness with which the events of the last week have unfolded.
On Wednesday, 11 December, the pump module on one of the space station’s two coolant loops automatically shut down when it reached pre-set temperature limits. Suspicion quickly centered on the improper functionality of a regulating flow control valve inside the pump module, and NASA engineers have been hard at work to develop a clearer understanding of the issue through testing over the last week. By regulating the temperature of ammonia in the coolant loop, the valve ensures that when it is re-introduced into the heat exchanger of the station’s Harmony node, it does not freeze the water also passing through the exchanger. Although NASA stressed that the Expedition 38 crew was placed in no danger, engineers worked to move certain critical station systems over to the second coolant loop. Some non-critical elements were also powered down inside Harmony, as well as Japan’s Kibo and Europe’s Columbus laboratory modules.
Replacing the valve itself is not an option, since its location within the pump module is inaccessible to spacewalkers. Only the removal and replacement of the entire pump module—of which several “spares” were delivered to the ISS by the shuttle and are situated aboard the station’s External Stowage Platforms (ESP)—is a realistic option. Yesterday (Tuesday), NASA announced its decision to postpone the scheduled 19 December launch of Orbital Sciences’ first dedicated Cygnus cargo mission (ORB-1) until no earlier than 13 January, in order to focus on the repair effort.
Early Saturday, 21 December, after post-sleep and personal hygiene activities, Mastracchio and Hopkins will jump right 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), after which the atmosphere will be repressurized to 14.7 psi. The astronauts will then enter a nominal pre-breathing period, lasting about 50 minutes, followed by a further 50 minutes of In-Suit Light Exercise (ISLE). This protocol was first debuted on the STS-134 mission in May 2011 and will involve Mastracchio and Hopkins flexing their knees for four minutes, resting for one minute, and repeating over and over until the 50 minutes are up. ISLE serves to remove nitrogen from the spacewalkers’ blood in a much shorter time period.
An hour before the EVA’s scheduled 7:10 a.m. EST start time, the fully-suited pair and their equipment will be transferred into Quest’s outer “crew lock,” and Expedition 38 crewmates Koichi Wakata and Commander Oleg Kotov 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 near-vacuum conditions. EVA-24 will officially commence when Mastracchio and Hopkins transfer their suits’ critical life-support utilities onto internal battery power.
Principal tasks for Saturday’s EVA-24 will be the preparations to remove the failed starboard pump module, which includes the demating of four ammonia fluid quick disconnects, the installation of jumpers, and the removal of five electrical connectors. Mastracchio (designated “EV1,” with red stripes on the legs of his space suit) and Hopkins (“EV2,” clad in a pure-white suit) will then turn their attention to preparing the replacement pump module—currently located on External Stowage Platform (ESP)-3 and delivered to orbit by the STS-127 shuttle crew, back in July 2009—for movement to its new home. The two men will open multi-layer insulation on the new pump module to ready it for installation during EVA-25 on Monday, 23 December.
The second spacewalk will see the astronauts remove the failed pump module and stow it on the Payload Orbital Replacement Unit (ORU) Accommodation (POA) of the station’s Mobile Base System (MBS). They will then install the bolts and electrical connectors for the replacement pump module. Assuming that the historic Christmas Day EVA-26 goes ahead, NASA graphics indicate a change in the roles of the two spacewalkers, with Hopkins now taking on the EV1 duties and Mastracchio serving as EV2. They will finish up the installation of the replacement pump module, securing its ammonia fluid quick disconnects, and will permanently stow its failed counterpart before cleaning up their worksite. With all three EVAs expected to last in the region of 6.5 hours, the New Year should see Hopkins accrue a total of 19.5 hours of spacewalking time, whilst Mastracchio’s record will jump to about 58 hours in nine spacewalks. As a result, this will push Mastracchio from 27th place to 5th place (behind Anatoli Solovyov, Mike Lopez-Alegria, Jerry Ross, and John Grunsfeld) in the list of the most experienced spacewalkers of all time.