Astronaut Peggy Whitson making repairs to the EXT-1.
It doesn't happen often on the ISS, but when a critical electrical command component breaks down, it's great to know there's a backup system in place. In this case, on May 20 the EXT-1 MDM electrical command controller went down. It controlled external US segment systems, which includes things like the Mobile Transporter (MT), Secondary Electrical Power System (SEPS), Passive Thermal Control System (PTCS), and a couple of Truss rotary joints. When the system failed, the EXT-2 took over right away so there was no degradation of systems. However, if THAT item were to fail, NASA would have lost control over the facing direction of the solar radiators and several other critical station systems along the Truss.
Astronaut Jack Fischer moves along the outside of a module.
Immediately the decision was made to go outside and replace the broken equipment. Using components stored aboard the station for such a situation, a new EXT-1 was assembled and tested. Then on May 23, astronauts Peggy Whitson (Commander of Expedition 51) and Jack Fischer made a short spacewalk of over two hours. The mission event was a success, and systems are back to normal.
Record holders for EVAs.
This EVA brings Peggy Whitson into the top three record holders for time spent on spacewalks. Currently, the Russians hold the lead.
You can read more details of the operation at NASA SpaceFlight.com: https://www.nasaspaceflight.com/2017/05/spacewalkers-unplanned-eva-replace-failed-ext-1-mdm/