Spacewalk 98: Menon and Adenot Swap a Failed ISS Antenna

On the morning of the spacewalk, NASA astronaut Anil Menon and ESA astronaut Sophie Adenot set out from the Quest airlock of the International Space Station to finish replacing a failed high-speed communications antenna. US Spacewalk 98 began at 7:27 a.m. Central time (8:27 a.m. Eastern), when their suits were switched to internal battery power. It was the 283rd spacewalk in support of station assembly, maintenance and upgrades, the third of the month and the sixth outside the station that year, and the third for Expedition 75.

The antenna and why it was being replaced

The space-to-ground antenna, called the SGANT or Escant in the broadcast, is a dish-shaped unit. Two of them sit side by side on the station's Z1 truss. Last November, antenna number two began having periodic pointing problems during autotrack operations, meaning it was not properly gimbaling, or orienting itself, to communicate with NASA's Tracking and Data Relay Satellite System (TDRS). Flight controllers switched operations to antenna number one, which continues to carry high-rate video, voice and telemetry for control centers around the world. Installing the replacement would restore redundancy to the station's space-to-ground communications.

The work began on August 18, when Menon and Adenot spent 6 hours and 23 minutes outside removing the failed antenna and securing it on the truss structure. They needed more time than expected to disconnect electrical cables and loosen bolts, which delayed installation of the replacement. The spare, the only one left aboard the station, was waiting on External Stowage Platform 3 (ESP3) on the port 3 truss.

The antennas have a spaceflight lineage. The first one on the Z1 truss was carried up on STS-92 in October 2000. The spare being installed was delivered on STS-127 in July 2009 and installed by Dave Wolf and Tom Marshburn. The failed unit had first been installed on STS-132 in May 2010 by Garrett Reisman and Steve Bowen.

People and roles

Menon was EV1, in the suit with red stripes, on his third spacewalk after two earlier in August, on the 6th and 18th, which had logged 12 hours and 50 minutes. Adenot was EV2 in the unmarked suit, on her second spacewalk. Inside the station, NASA astronauts Jack Hathaway and Jessica Meir supported the walkers; Hathaway was prime on the Canadarm2 robotic arm, with Meir backing him up. Other Expedition 75 crew aboard were cosmonauts Andrey Fedyaev and Pyotr Dubrov, and Anna Kikina.

In Mission Control Houston, the Orbit 2 team under flight director Chris Dobbins ran the spacewalk, taking over from the Orbit 1 team led by Audi Bulos. Sandy Fletcher was lead spacewalk officer, Steven Zenter was the Capcom for the crew inside, and NASA astronaut Ann McClain was the ground IV, guiding the walkers through procedures.

Preparing to go outside

Before the walk the crew did a two-phase pre-breathe: first a mask pre-breathe on 100% oxygen, then in-suit light exercise, moving arms and legs to speed metabolism and purge nitrogen. This prevents decompression sickness, because the suit runs at about 4.3 psi while the station is at 14.7 psi. In-suit light exercise was completed around 6:50 a.m. Eastern.

The suit is the Extravehicular Mobility Unit (EMU), with a suit assembly and a life support system that supplies oxygen, cooling water, carbon dioxide scrubbing, battery power and radio. Each suit is tailored from about 80 measurements and roughly 16 major elements. Medical oxygen (Medox) canisters were changed out, and the crew tracked their serial numbers. A pressure reading of 3.4 on Adenot's suit was lower than the roughly 4.2 expected; Hathaway noted it made sense as the suit slowly breathes down, and the ground called it a good value that would rise as part of the Medox change-out.

Both walkers wore the SAFER (Simplified Aid for EVA Rescue), a jetpack-like backup should they become detached, although each always has two tethers to the station. When Hathaway was asked whether the SAFER battery had been reseated on Menon's unit, he answered that only the multilayer insulation had been adjusted for comfort and everything was nominal.

The crew lock was then depressurized from 14.7 psi, with a pause at 5 psi for suit leak checks. Both leak checks were good. The process takes about 30 minutes. At 2 psi, Menon and Adenot reported their initial tether configurations, and the ground asked for part of Adenot's report again because of a handover. With the crew lock down to vacuum, the hatch was opened, the suits were switched to battery power, and both suits read 4.3 and 4.4 psi. The ground then told them it would be nighttime as they went.

Out the hatch and setting up the arm

Menon exited first, followed by Adenot, and they carried out buddy checks of each other's suits and tethers. During them, Adenot's left SAFER handle appeared slightly up, and the pair rechecked until the handles were confirmed down. The station sees 16 sunrises and sunsets a day, so they alternated between light and darkness.

Adenot led the translation to the port crew and equipment translation aid (CETA) cart for the adjustable portable foot restraint (APFR), while Menon headed for the arm. The ground cautioned them to avoid the deployed test cable. The pair installed the APFR on the arm roughly 36 minutes into the walk, and Menon ingressed. The ground remarked that they were working incredibly efficiently, at just over half the time allotted for arm setup. Adenot then translated to ESP3, her first translation along the mobile transporter track, and chose a diagonal path with fewer cables over the briefed route.

Releasing the spare antenna

At ESP3, about 1 hour and 2 minutes in, Menon used a ratchet wrench to break torque on the two flight support equipment (FSE) bolts, a half turn on bolt two and then one turn on bolt one. He reported a significant decrease in running torque after the half turn. With the pistol grip tool he then released the bolts, with 27 to 35 turns expected for each, while Adenot stabilized the antenna by its T-handle. About an hour and 17 minutes in, the antenna slid off the passive FSE pins, with the rear plate clear. Menon's body position had to stay clear of a grapple fixture, with about 20 cm above his helmet, and of the dish.

The arm then carried Menon and the spare toward Z1. The remaining plan was to seat the antenna on the boom, remove two gimbal locks and fit a thermal cover, then retrieve and permanently stow the failed antenna. If time allowed, they would replace a retroreflector on the forward port of the Harmony module to improve navigation data for visiting spacecraft. The spacewalk was expected to last about 6.5 hours.