Astronaut Peggy Whitson on a March 30 spacewalk outside the ISS. She and Jack Fischer will perform a "contingency" spacewalk May 23 to replace a faulty electronics unit. Credit: NASA

WASHINGTON — NASA astronauts on the International Space Station will carry out an unplanned spacewalk May 23 to replace an electronics box that failed over the weekend, the agency announced May 21.

Astronauts Jack Fischer and Peggy Whitson will perform the spacewalk to remove a data relay box known as a multiplexer-demultiplexer (MDM) on the station’s truss, which controls the operation of solar arrays and other hardware on the station’s exterior. The failed MDM, designed MDM-1, is one of two such fully-redundant units.

MDM-1 malfunctioned on the afternoon of May 20, NASA said, and ground controllers were unable to restore the unit. Although the other MDM is working normally, allowing the ISS to continue regular operations, NASA station managers approved plans for the “contingency” spacewalk May 21 to replace MDM-1 with a spare unit already on the station.

During the two-hour spacewalk, Whitson will replace the failed MDM unit while Fischer carries out an unrelated task to install wireless communications antennas on the Destiny module. That task was planned for a May 12 spacewalk but deferred when that EVA, which started late because of a problem with a water umbilical inside the station’s airlock, was cut short.

The MDM-1 unit that failed had been installed less than two months earlier during a March 30 spacewalk by Whitson and Shane Kimbrough. The cause of the failure is not known, NASA said.

This is not the first time astronauts have had to perform an unplanned spacewalk to replace a faulty MDM. In April 2014, astronauts Rick Mastracchio and Steve Swanson performed a spacewalk to replace a similar unit that failed earlier that month. The two were able to successfully replace the unit in a spacewalk lasting a little more than 90 minutes.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...