Two astronauts aboard the International Space Station (ISS) have completed a planned spacewalk (or Extravehicular Activity — EVA) to replace a failing component of the station’s communication system.
The spacewalk got underway at 11:15 UTC — 55 minutes ahead of the planned timeline — when NASA astronauts Thomas Marshburn and Kayla Barron took their spacesuits to battery power — the event that marks the start of US EVAs. The spacewalk concluded 6 hours 32 minutes later at 17:47 UTC.
The objective of US EVA-78 was to replace a faulty S-Band Antenna Subassembly (SASA).
The ISS uses an S-band communication system to provide command and control capability of all the station’s systems, relay live telemetry, and provide voice communications between the station and ground controllers.
It does not however support live video downlink. That is provided by the Ku-band system, which provides higher bandwidth for transferring larger amounts of data between the ISS and Earth.
The S-band system is comprised of two “strings,” with each string having its own SASA. String-1 is located on the S1 Truss, with String-2 on the P1 Truss.
The SASA itself features a high-gain antenna, with pointing capability provided by a gimbal, and a fixed low-gain antenna, both mounted on a stanchion.
In September, a routine check of the String-2 SASA on the P1 Truss – which has been operating for the past 20 years – revealed that the return link had degraded, meaning the SASA could not provide any downlink to the ground. The uplink, however, continued to operate normally.
Since the String-1 SASA continued to function, and with the Ku-band system providing a backup command and control capability, replacing the unit was not considered a priority.
However, a high beta-angle period coming up in December could cause the fully operable String-1 SASA to reach its temperature limits and cease functioning, which would leave the ISS without any S-band downlink capability due to the degraded String-2 SASA.
For this reason, ISS program managers decided to remove and replace the degraded SASA in order to restore redundancy prior to the high-beta period.
A spare SASA resided on ExPrESS Logistics Carrier-3 (ELC-3) on the P3 Truss, having been launched on STS-134 in May 2011 along with one other spare SASA unit.
A SASA replacement has been performed before; the String-1 SASA was replaced during the STS-115 mission in 2006.
EV-1 for EVA-78 was NASA astronaut Thomas Marshburn, wearing the suit with the red stripes, and making the fifth EVA of his career. EV-2 was Kayla Barron, wearing the all-white suit, and making her first EVA.
The duo had the chance to rehearse the EVA procedures in the Neutral Buoyancy Laboratory (NBL) prior to their launch, which aided them in familiarity with the hardware and worksites.
The first order of business after exiting the Quest airlock was for Marshburn to translate to the Space Station Remote Manipulator System (SSRMS, or Canadarm2), positioned near the P1 Truss, where he began to set up a foot restraint which allowed him to “ride” the arm.
Barron meanwhile translated out to ELC-3 on the P3 Truss and prepared the spare SASA for removal. She then headed to the degraded SASA on the P1 Truss where she met up with Marshburn.
The task of removing the degraded SASA then began, with Barron disconnecting three connectors from the SASA, which provided power, data, and heater power respectively.
Marshburn meanwhile installed gimbal locks by driving four bolts, in order to prevent the high-gain antenna from swinging around once the unit is removed.
Barron then drove a single bolt to release the stanchion on which the SASA was mounted from the P1 Truss.
The duo then translated to ELC-3 with the degraded unit in tow, temp-stowed it, and then removed a thermal blanket covering the replacement unit.
Each then released two bolts on the new unit using a special right-angle drive, and then drove a further two bolts to release the new SASA from its Flight Releasable Attachment Mechanism (FRAM) mounting plate connected to ELC-3.
Marshburn then held onto the new unit as he wass “flown” back to the P1 Truss worksite on the end of the SSRMS, whereupon he and Barron began the process of installing it in place of the old SASA.
The new SASA was soft-docked, following which its single stanchion bolt was driven. Three electrical connectors were then re-mated, followed by releasing of the gimbal lock bolts. A checkout of the new unit was then be performed by ground controllers.
Unpack your EVA suit 👩🚀👨🚀 Tomorrow Kayla Barron and @AstroMarshburn will exit the @Space_Station to replace a faulty antenna system. For this spacewalk I’ll manipulate the @csa_asc Canadarm2 as onboard operator M1, supported by @Astro_Raja as M2 🦾 #CosmicKiss pic.twitter.com/ZPRiYdipZb
— Matthias Maurer (@astro_matthias) November 29, 2021
Following this, the old degraded SASA was attached to the empty FRAM mounting on ELC-3 via two bolts, with the thermal cover being placed over it for protection, as it could still serve as a spare unit if necessary despite having a degraded downlink.
Working far ahead of the timeline, NASA decided to add back all five get ahead tasks to the duo’s work, including cable wire ties to a Solar Array Blanket Box.
The get-ahead tasks — routine parts of EVAs that permit the crew to perform other tasks if they work quickly enough — had been postponed due to the increased debris risk following last week’s Russian Antisatellite (ASAT) Test.
Those get-ahead tasks were removed from the EVA plan immediately following the ASAT test when it became clear that a full picture of the debris environment would not be known for several months.
According to NASA during a briefing on Monday, November 29, the ASAT test has presently created a known 7% increase in the debris strike potential to an astronaut’s spacesuit (Extravehicular Mobility Unit) during a spacewalk.
While NASA noted that a debris strike to a suit would not necessarily result in a “catastrophic penetration” — elements such as impact location, relative impact velocity, and relative impact trajectory would all play a role in determining total severity.
However, any penetration of a pressure suit in the vacuum of space is a serious condition that can quickly lead to much more serious issues for the wearer of the breached suit.
Moreover, a debris strike could very easily not just penetrate the suit but also an astronaut’s body.
Nevertheless, NASA flight controllers, ISS and Human spaceflight program managers, and health officers have collectively determined that the current 7% known debris strike increase is acceptable for the nature of the EVA and its importance to continued station operations.
As if on cue, a late-breaking debris conjunction on November 30 caused a last-minute postponement to the EVA’s original date when NASA was unable to verify the nature and threat of the debris. The EVA was rescheduled for Thrusday, December 2.
(Lead image: A NASA astronaut conducting a spacewalk. Credit: NASA)