Astronauts begin spacewalk to replace faulty antenna

Date:
Thu, 02 Dec 2021 11:24:48 +0000

Description:
Two astronauts aboard the International Space Station (ISS) have begun a planned 6.5-hour spacewalk (or The post Astronauts begin spacewalk to replace faulty antenna appeared first on NASASpaceFlight.com .

FULL STORY ======================================================================

Two astronauts aboard the International Space Station (ISS) have begun a planned 6.5-hour spacewalk (or Extravehicular Activity EVA) to replace a failing component of the stations 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 a US EVA.



Overview

The objective of US EVA-78 is 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 stations 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 would like to remove and replace the degraded SASA in order to restore redundancy prior to the high-beta period.

A spare SASA resides 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. The spacesuits for todays spacewalk await their users in teh Quest Airlock. (Credit: NASA)

A SASA replacement has been performed before; the String-1 SASA was replaced during the STS-115 mission in 2006.

Procedures

EV-1 for EVA-78 will be NASA astronaut Thomas Marshburn, wearing the suit with the red stripes, and making the fifth EVA of his career. EV-2 will be 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 should aid them in familiarity with the hardware and worksites.

The first order of business after exiting the Quest airlock will be for Marshburn to translate to the Space Station Remote Manipulator System (SSRMS, or Canadarm2), positioned near the P1 Truss, where hell begin to set up a
foot restraint which will allow him to ride the arm.

Barron meanwhile will translate out to ELC-3 on the P3 Truss and prepare the spare SASA for removal. She will then head to the degraded SASA on the P1 Truss where she will meet up with Marshburn.

The task of removing the degraded SASA will then begin, with Barron disconnecting three connectors from the SASA, which provide power, data, and heater power respectively.



Marshburn meanwhile will install gimbal locks via driving four bolts, in order to prevent the high-gain antenna from swinging around once the unit is removed.

Barron will then drive a single bolt to release the stanchion on which the SASA is mounted from the P1 Truss.

The duo will then translate to ELC-3 with the degraded unit in tow,
temp-stow it, and then remove a thermal blanket covering the replacement
unit.

Each will then release two bolts on the new unit using a special right-angle drive, and then drive a further two bolts to release the new SASA from its Flight Releasable Attachment Mechanism (FRAM) mounting plate connected to ELC-3.

Marshburn will then hold onto the new unit as he is flown back to the P1 Truss worksite on the end of the SSRMS, whereupon he and Barron will begin
the process of installing it in place of the old SASA.

The new SASA will be soft-docked, following which its single stanchion bolt will be driven. Three electrical connectors will then be re-mated, followed
by releasing of the gimbal lock bolts. A checkout of the new unit will 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 Ill 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 will be 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.

The pair will then head back to the airlock to conclude the EVA.

The spacewalk is expected to last approximately 6.5 hours; however, a few get-ahead tasks routine parts of EVAs that permit the crew to perform other tasks if they work quickly enough have been postponed due to the increased debris risk following last weeks 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 astronauts 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 astronauts 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.

(Lead image: A NASA astronaut conducting a spacewalk. Credit: NASA)

The post Astronauts begin spacewalk to replace faulty antenna appeared first on NASASpaceFlight.com .



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Link to news story:
https://www.nasaspaceflight.com/2021/12/us-eva-78/


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