Astronauts Barry “Butch” Wilmore and Terry Virts floated outside the International Space Station Saturday, kicking off a planned six-and-a-half-hour spacewalk to install wiring needed for two new docking mechanisms that will be installed later this year.
The Boeing-built International Docking Adapters, or IDAs, will allow commercially developed crew capsules built by Boeing and SpaceX to dock at the front and top of the forward Harmony module starting in 2017. Two other ports on the Earth-facing side of the station will be available for use by U.S. cargo ships.
Floating in the station’s Quest airlock module, Wilmore and Virts switched their spacesuits to battery power at 7:45 a.m. EST (GMT-5) to officially begin EVA-29, the 185th spacewalk devoted to station assembly and maintenance since construction began in 1998.
For identification, Wilmore, call sign EV-1, is wearing a suit with red stripes and will be using helmet camera No. 18. Virts, EV-2, is wearing an unmarked suit and will use helmetcam 20. Astronaut Doug Wheelock in mission control at the Johnson Space Center handled space-to-ground communications.
This is the first of three spacewalks by Wilmore and Virts to install more than 760 feet of cabling and to attach new antennas to prepare the space station for eventual dockings by commercial crew capsules. The two additional spacewalks are planned for Wednesday and March 1.
During Saturday’s excursion, Wilmore and Virts spent most of their time working at the front of the station at the forward end of the Harmony module where a docking port extension known as Pressurized Mating Adapter No. 2 is attached. PMA-2 is where visiting space shuttles once docked and where one of the new IDAs will be attached later this year.
After removing micrometeoroid shields from the forward end cone of Harmony, Wilmore and Virts planned to unplug no-longer-needed cables and to install new power and data lines to support IDA-1. A half-dozen cables were needed, each one with multiple connectors.
While the work was not considered especially challenging, the number of cables and connectors made for a complicated timeline, with color-coded lines running to and from multiple sites on the front end of the station.
IDA-1 will be launched this summer aboard a SpaceX Dragon cargo ship and robotically attached to PMA-2. IDA-2 will be launched in December. But before it an be attached to the station, the lab crew must robotically move PMA-3 from an unused port on the Tranquility module to Harmony’s upper port.
As part of the station’s reconfiguration, a cargo storage module currently attached to the Earth-facing port of the central Unity module will be moved to a port on the forward side of Tranquility, freeing up the Unity port for use by visiting cargo ships. A second cargo port remains available on the Earth-facing side of Harmony.
All told, seven U.S. spacewalks will be needed to complete the station reconfiguration, requiring nearly 650 hours of crew time. Another 234 hours will be required for internal wiring changes and robot arm operations.
Saturday’s spacewalk was originally planned for Friday, but mission managers delayed it one day in the wake of extensive troubleshooting to verify the health of critical spacesuit components known as fan/pump/separators.
The modules in question use a common motor and drive shaft to power three different components: a fan to circulate oxygen, a pump to circulate cooling water and a system to remove, or separate, condensation from the air supply and return it to the cooling water loop.
Failure of a fan/pump/separator during a spacewalk would stop the flow of oxygen to the astronaut, but an emergency backup system would give the crew member time to return to the station’s airlock.
During testing in orbit, two fan/pump/separators failed to spin up and both were shipped back to Earth aboard a SpaceX cargo ship earlier this month for detailed analysis. An inspection revealed corrosion around drive shaft bearings. Engineers concluded the corrosion likely was the result of additional testing implemented after water backed up into the helmet of spacewalker Luca Parmitano in 2013.
Based on telemetry, a small amount of bearing corrosion is believed to be present in the fan/pump/separator in Virts’ spacesuit. But analysis indicated it was well below the threshold that could cause the unit to fail during use.
As a result, mission managers cleared Virts and Wilmore to press ahead with the first of their three planned spacewalks. Both suits powered up normally Saturday, with no signs of trouble with either fan/pump/separator.
This is the second spacewalk for Wilmore and the first for Virts. Going into Saturday’s EVA, 119 astronauts and cosmonauts representing nine nations had logged 1,152 hours and 27 minutes of station EVA time, or 48 days.