Two astronauts have stepped outside the International Space Station (ISS) on Thursday on what is the first US Segment-based spacewalk of the post-Space Shuttle era. American astronaut Suni Williams and Japanese astronaut Aki Hoshide egressed the Quest airlock at 8:16 AM EDT/12:16 PM GMT, ahead of a challenging spacewalk that resulted in the failure to install a new MBSU.
Thursday’s spacewalk, known as US EVA-18, will mainly focus on replacements of failed hardware and routing of cabling for future ISS elements.
While such tasks were previously handled by Space Shuttle crews during Shuttle-mission related EVAs on the ISS, last year’s retirement of the Space Shuttles means that duty now falls onto the long duration ISS Expedition crews.
Due to the fact that the ISS has now entered the utilisation phase, meaning as much crew time as possible is devoted to science, rather than maintenance, US Segment-based EVAs by ISS crews will be much less frequent than Shuttle mission EVAs.
The target is to only perform one US-based EVA per year, with external tasks that require human intervention in the intervening period being “saved up” for such annual EVAs.
While the annual US Segment-based EVAs are conducted out of the US Segment’s Quest airlock using American Extra-vehicular Mobility Unit (EMU) spacesuits, Russian Segment (RS)-based EVAs will continue at their usual rate of two-to-three per year, performed out of the Russian Docking Compartment-1 (DC-1) “Pirs” airlock using Russian Orlan spacesuits.
The last US-based EVA on the ISS was performed just over one year ago, during the final Space Shuttle mission, STS-135, in July 2011.
Since then, a number of external maintenance tasks have built up, which will be the focus of Thursday’s EVA.
US astronaut Suni Williams will serve as lead spacewalker, designated EV-1, for this EVA, and will be wearing the EMU marked with a red stripe. Japanese astronaut Aki Hoshide, who will serve as EV-2, will be wearing the all-white EMU.
Williams has previously performed four EVAs during her past ISS mission, however this will be Hoshide’s first ever spacewalk, during which he will become only the third ever Japanese astronaut to perform an EVA.
Each spacewalker will carry a digital camera with them, with one camera being equipped with a standard 28mm lens, and the other being equipped with a 10mm wide-angle lens that was responsible for the amazing wide-angle photos of the docked ISS/Shuttle complex during the STS-134 and STS-135 EVAs in 2011.
While no Shuttle orbiter will be present at the ISS for today’s EVA, Europe’s ATV-3 and Japan’s HTV-3 will both be attached to the station.
EVA procedures (More images will be added, and the article will be updated, during the EVA):
Following suit-up, the first order of business for Williams and Hoshide will be to perform the In Suit Light Exercise (ISLE) protocol, demonstrated during the final EVAs of the Space Shuttle Program in 2011.
ISLE, as its name suggests, involves the EVA crew donning their spacesuits and then performing some light exercise inside the ISS in order to purge their bloodstreams of nitrogen.
The purge is necessary since nitrogen in the bloodstream could cause “the bends” at the conclusion of the spacewalk when astronauts exit their EMUs, which are pressurised at around 5psi of pure oxygen in order to facilitate better suit mobility, and return to the 14.7psi oxygen/nitrogen atmosphere of the ISS.
ISLE eliminates the need for an overnight “campout” inside the Quest airlock at a reduced pressure by the spacewalkers, and thus allows nitrogen purges to take significantly less time than in the past, and allows spacewalks to be performed at shorter notice, if required.
After the two spacewalkers had egressed the Quest airlock, the first task for Suni Williams will be to route and connect two new power cables between the US Segment and the Russian Segment.
Once fully connected via future Russian EVAs, will be used to provide power to the future Multipurpose Laboratory Module (MLM) and to provide power to the Russian Service Module (SM) from via all four US Segment Main Bus Switching Units (MBSUs), instead of the current two.
In order to route the cables, Williams had to enter the Starboard Zero (S0) Truss through an access hatch.
Meanwhile, Aki Hoshide prepared to replace the failing MBSU-1 atop the S0 Truss. MBSUs are electrical boxes that distribute generated electrical power around the station’s power channels, to which electrical loads are attached.
MBSU-1 is still working, however ground controllers lost contact with it last year after it experienced a series of bit flips – or anomalous logic upsets. MBSU-1 is no longer sending data on its health to the ground, leading ISS managers to deicide to replace it now before it could hard fail unexpectedly sometime in the future.
The first task for Hoshide was to prepare the new MBSU – which resided in a Flight Releasable Attachment Mechanism (FRAM) Flight Support Equipment (FSE) enclosure on External Stowage Platform-2 (ESP-2) just outside the Quest airlock. Preparations included removing a thermal cover, and setting up the EVA worksite.
Next, Hoshide translated to atop the S0 Truss, and removed the failing MBSU-1 by turning two bolts, following which he ingressed an Articulating Portable Foot Restraint (APFR) on the end of the Space Station Remote Manipulator System (SSRMS), prior to himself, holding the then removed failing MBSU, being “flown” by the SSRMS back to the new MBSU on ESP-2. The bolts were notably troublesome during the work on the failed unit.
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The failing MBSU was then temp stowed on ESP-2, following which Hoshide removed the new MBSU from its FRAM FSE, and again “flown” by the SSRMS back to the MBSU-1 install site atop the S0 Truss, where he was joined by Suni Williams who helped Hoshide install the new MBSU-1 in place of the removed failing unit.
However, the bolts were again troublesome, requiring a large amount of evaluation from the ground, including the Mission Evaluation Room (MER). Debris – in the form of shavings – seemed to be causing problems with the installation of the unit and the tightening of the bolts. Cleaning out the holes with a nitrogen canister was called for, with controllers joking it should be renamed “Puff the Magic Dragon”.
All efforts to install it failed, likely to result in a contingency EVA at a later date, while the unit was stowed with a tie down.
The failing MBSU was installed into the then empty FRAM FSE, and the thermal cover replaced.
It is possible that the failed MBSU could return to Earth inside a SpaceX Dragon capsule at some point in the future, once the JEM ORU Transfer Interface (JOTI) hardware arrives at the ISS early next year, which will allow MBSU-sized payloads to be transferred to/from the ISS interior and exterior via the JEM Airlock.
In preparation for the MBSU-1 replacement, an internal Lab Truss Contingency Jumper (LTCJ) and Secondary Power Distribution Assembly (SPDA) jumper were both installed in the US Lab in order to temporarily transfer electrical loads from MBSU-1 to MBSU-4, prior to MBSU-1 being removed and replaced.
The next task would have been to replace the end B Camera, Light, Pan/tilt Assembly (CLPA) on the SSRMS by removing the old unit and replacing it with a spare. It was previously planned to replace a mast CLPA on the Mobile Base System (MBS) during this time period, however that task was deferred in lieu of the SSRMS end B CLPA replacement by a recent failure of the vital focus mechanism in said camera.
However, due to the challenges with the MBSU, this task was cancelled from the timeline.
Several get-ahead tasks were also available, such as the installation of a Micro Meteoroid Orbital Debris (MMOD) and thermal cover over the end of Pressurised Mating Adapter-2 (PMA-2), which, following the retirement of the Space Shuttle, is now unneeded for dockings until the installation of the International Docking Adapter (IDA) in 2014, prior to dockings by commercial crew vehicles in 2017.
The PMA-2 MMOD/thermal cover one is the same type that was installed on PMA-3 during the STS-135 EVA in July 2011.
Other get-ahead tasks included the replacement of the MBS mast CLPA which was deferred in favour of the SSRMS end B CLPA replacement, replacement of the Japanese Exposed Facility (JEF) Vision Equipment (VE) forward camera and light, troubleshooting exposed grounding wires on the Functional Cargo Block (FGB) Power Data Grapple Fixture (PDGF), and reconfiguring some EVA tools.
However, due to the MBSU issues, none of the get-ahead tasks could be completed.
Ingress back into the Quest airlock was planned for 2:45 PM EDT/6:45 PM GMT. However, due to the issues, the EVA was extended to what became the third longest spacewalk in history, ending at eight hours and 17 minutes.
(Images: Via NASA and L2. Content aided by L2’s extensive ISS Update Section.)
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