Spacewalkers successfully replace failed unit on ISS

In a a short spacewalk just under three hours two astronauts today successfully replaced the failed MDM data relay unit on the outside of ISS.

Some additional details about this unit:

The MDM that failed and an identical unit are part of the tier 2 command architecture and relay commands to a variety of critical station systems, including the station’s guidance, navigation and control system, the lab’s stabilizing gyros, the environmental control system, the station’s cooling system and others.

Both tier 2 computers were installed during spacewalks March 30 and March 24 respectively, replacing two older units with models featuring upgraded data processing cards. It is not yet known whether the problem with MDM-1 involved the upgraded components or some other circuitry or software

I suspect this upgraded but failed unit is going to be looked at very carefully.

Emergency spacewalk on ISS to replace electronics relay box

Astronauts will perform an emergency spacewalk on May 23 to replace the failed MDM electronics relay box that failed on Saturday.

So, they have a spare on board, which is good. The article does note, however, that the unit that failed had only been installed less than two months ago, on March 30. This is not good. These units should last longer than two months, especially considering how critical they appear to be.

Primary data relay electronics unit on ISS truss fails

ISS managers are meeting to make a plan to deal with the failure on Saturday of the #1 data relay electronics box on the S0 truss of the station.

The unit, called an MDM, has an identical backup in place so no station operations have so far been effected. However, that backup had failed back in 2014 and required a spacewalk to replace it, so it is not clear to me as yet whether the station has any additional spares available to replace the newly failed unit. If so, it will still require a spacewalk to make the switch.

If there is no spare, they will likely have to ship one up on the next cargo flight, as the MDM is essential for operating the solar panels and radiators as well as the robot arms.