The spacewalk’s start was slowed by an hour when airlock pressure checks indicated a small leak in the transfer compartment separating the Poisk and Pirs modules. Padalka and Malenchenko were using Pirs as their airlock. The leak, eventually traced to an internal valve, was stopped after a round of troubleshooting by Russia’s Mission Control.

The Aug. 30 U.S. spacewalk will pair NASA’s Sunita Williams and Japanese astronaut Akihiko Hoshide for the replacement of the Main Bus Switching Unit (MBSU) No. 1., one of four U.S. segment external circuit breaker boxes. The MBSU experienced a partial failure in October 2011.

The relay functions, but the telemetry that provides ground control teams with insight into the MBSU itself has ceased.