Why NASA sent astronauts into space to replace a single backup computer

Space · Published 11 October 2026 Swipe more like this →

In short: When a backup computer failed on the International Space Station, the primary system continued to function perfectly. However, NASA engineers didn't take the risk of losing redundancy. On April 23, 2014, two astronauts performed a high-stakes repair to restore the station's critical command systems.

A failed Multiplexer-Demultiplexer (MDM) was identified on April 11 during a standard health assessment by Houston's Mission Control. While the primary computer was operating flawlessly, the loss of the backup posed a theoretical risk to the station's redundancy. To mitigate this, NASA managers ordered a spacewalk to replace the faulty unit located on the S0 truss, the central component of the station's Integrated Truss Structure. This specific unit had been housed inside the Destiny laboratory since its delivery in April 2001. The component serves as a backbone for various essential hardware systems.

The repair was executed by Expedition 39 Flight Engineers Rick Mastracchio and Steve Swanson. The mission was highly efficient; they began the excursion at 9:56 a.m. EDT and completed the installation of the spare MDM in less than an hour. The astronauts finished the task and began re-pressurizing the Quest airlock at 11:32 a.m. EDT, well within the slated 2.5-hour window. The duo successfully installed a spare unit that had been stored within the Destiny module. This swift replacement ensured the mission stayed well within its scheduled timeframe.

Crucial for station functions, the MDM transmits commands and telemetry to the solar alpha rotary joints, truss systems, and the Mobile Transporter rail car. The station relies on a total of 45 MDMs to maintain these functions: 24 are located internally and 21 are positioned externally. This specific repair was deemed non-critical for the upcoming SpaceX-3 mission, as engineering reviews confirmed the station maintained sufficient redundancy for the Dragon cargo craft's arrival. The MDM is vital for managing the solar arrays and the robotic arm's rail car. Engineers determined that the remaining systems were robust enough to support the upcoming SpaceX-3 Dragon cargo arrival.

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