How NASA astronauts repaired a critical backup computer during a spacewalk

Space · Published 11 October 2026 Swipe more like this →

In short: When a vital backup computer failed during a routine health check, NASA engineers had to decide: proceed with a commercial cargo launch or risk a mission-critical repair. The solution required a precise extravehicular activity to restore redundancy to the station's backbone.

The failure occurred on April 11, when a Multiplexer/Demultiplexer (MDM) stopped responding to commands during a routine health check. This specific unit, known as EXT-2, is one of the two external MDMs responsible for routing commands to essential station systems. Components such as the solar alpha rotary joints, the external cooling system, and the Mobile Transporter rail car—which enables the robotic arm to traverse the Integrated Truss Structure—are managed by this unit.

While the primary computer continued to operate flawlessly, the loss of the backup presented a challenge to the station's redundancy. NASA engineers had to evaluate whether the outage posed a risk to the upcoming SpaceX-3 Dragon cargo mission, which was scheduled to arrive via a Falcon 9 rocket. Because the station's robotic arm is vital for capturing the Dragon spacecraft, the loss of a backup controller for the Mobile Transporter was a significant concern.

To resolve the issue, astronauts Steve Swanson and Rick Mastracchio performed a spacewalk to the S0 truss, the central part of the station's backbone. They removed the failed MDM, which had been in place since the truss was delivered in April 2002, and replaced it with a spare unit. This spare had been stored inside the Destiny laboratory module since it was delivered aboard the space shuttle Endeavour in April 2001. The station maintains a total of 45 MDMs, with 24 located internally and 21 positioned externally.

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