There is a list of Critical Contingency items on the US side of the International Space Station called the "Big 13". The failure of any of these items would force an unscheduled spacewalk aka Extravehicular Activity (EVA).

What are the Big 13 Critical Contingency EVAs? Have any ever been done in real life?


1 Answer 1


The Big 131 are

  1. Pump Module (PM) R&R
  2. Flex Hose Rotary Coupler (FHRC) R&R
  3. Interface Heat Exchanger (IFHX) R&R
  4. Solar Array Wing (SAW) Bearing Motor Roll Ring Module (BMRRM) R&R
  5. SAW Electronics Control Unit (ECU) R&R
  6. Ammonia Tank Assembly (ATA) R&R
  7. Nitrogen Tank Assembly (NTA) R&R
  8. Main Bus Switching Unit (MBSU) R&R
  9. External (EXT) Multiplexer/Demultiplexer (MDM) R&R
  10. DC-to-DC Converter Unit (DDCU) R&R
  11. External Remote Power Control Module (RPCM) R&R
  12. Ammonia (NH3) Leak Isolation and Recovery
  13. Micrometeoroid/Orbital Debris (MMOD) penetration pinpoint and repair

They, unsurprisingly, are related to critical power, cooling, computer control, and life support functionality of the ISS and involve Orbital Replacement Units that are not accessible from within the station.

  • Electrical power system: 4, 5, 8, 10, 11
  • Cooling system: 1, 2, 3, 6, 12
  • Computer control: 9
  • Life support: 7, 13

To date, contingency EVAs have been performed for "Big 13" numbers 1, 8, 9, and 12.


  • Experiences with Extra-Vehicular Activities in Response to Critical ISS Contingencies (paper)
  • Experiences with Extra-Vehicular Activities in Response to Critical ISS Contingencies (presentation)

1 The number has varied from “Big 8” to “Big 9” to “Big 14” to “Big 11” to now the “Big 13”

  • 3
    $\begingroup$ I'm going to assume that "R&R" in this context has nothing to do with rest and relaxation. Probably something more like "Repair and Replace"? $\endgroup$ May 5, 2021 at 16:56
  • 3
    $\begingroup$ @FredLarson likely Remove and Replace but I can't rule out what you say. Not defined in either source. $\endgroup$ May 5, 2021 at 17:19
  • 6
    $\begingroup$ In the automotive industry, "R&R" is often "Remove and Reinstall" so that's what I assumed here. $\endgroup$
    – Xcali
    May 6, 2021 at 0:18
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    $\begingroup$ I'm thinkin' that, for the subject at hand, perhaps the most "definitive" resolution of the "R&R" question, in the space business context, would come from someone with formal training in the maintenance of the Space Shuttle Orbiter. Having said that, I think it is reasonable to surmise that, for said subject at hand, "Remove and Replace" is the correct choice. I can't imagine too many situations in which a spacewalker would remove a faulty component, repair same, then reinstall it...unless it's Don Pettit who is on the job... $\endgroup$
    – Digger
    Jun 5, 2021 at 16:37

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