A multiplexer-demultiplexer (MDM) data relay box aboard the International Space Station failed Saturday, and will be replaced Tuesday during a spacewalk by NASA astronauts Peggy Whitson and Jack Fischer, the space agency announced Sunday. The two-hour “contingency spacewalk” has been given the go-ahead by program managers of the ISS.

The MDM data relay box is housed in the S0 truss, and is one of the two such systems, both of which are redundant — which is to say, both boxes are independent and one can take over in the event the other fails. MDM-1, the failed data relay box, apparently malfunctioned shortly after noon Saturday. Crewmembers tried to restore power to the component multiple times, but were not successful.

According to the NASA statement, the boxes “control the functionality of radiators, solar arrays, cooling loops and other station hardware. The other MDM in the truss is functioning perfectly, providing uninterrupted telemetry routing to the station’s systems. The crew has never been in any danger, and the MDM failure, believed to be internal to the box itself, has had no impact on station activities.”

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Why the MDM failed is not known, but it had been installed March 30 after a software upgrade, during a spacewalk conducted by Whitson — commander of the ongoing Expedition 51 — and her predecessor Robert Shane Kimbrough. Another MDM replacement had been carried out during a spacewalk in April 2014, when crewmembers of expedition 39, Steve Swanson and Rick Mastracchio of NASA, carried out the task.

Whitson prepared a spare data relay box Sunday morning, and tested its components. Spacewalk preparations and crew readiness was reviewed through the day, and Whitson reported everything to be in order, following which the 201st spacewalk was given a green light. The Quest airlock on ISS will be used for the task, the sixth time already in 2017 it is being used for the purpose.

In the two hours or so that the spacewalk will last, Whitson — designated as extravehicular crewmember 1 and wearing a suit with red stripes — will replace the data relay box while Fischer — designated as extravehicular crewmember 2 and wearing a suit with no stripes — will perform the additional task of installing a pair of wireless communication antennas on the Destiny Lab. The third astronaut aboard ISS, Thomas Pesquet of the European Space Agency, will remain inside the station and maneuver the robotic arm that positions the astronauts outside.

The installation of the antennas was originally scheduled for May 12, when Whitson and Fischer conducted the space station’s 200th spacewalk. That was Fischer’s first spacewalk and the ninth for Whitson.

The upcoming spacewalk is scheduled to start around 8 a.m. EDT Tuesday, but could start earlier if the crew is running ahead of time. NASA will start its live coverage of the event at 6:30 a.m. EDT.