Generally, we can divide the things that a worker, in this case an astronaut, does, into two categories:

  • Value adding: Activities which contribute directly towards the employer's goals. In a company, this means profit, but in the context of NASA, this--broadly--means "science output".

  • Non value adding: Activities which may be required in support of value adding activities, but do themselves not produce value. In the context of the ISS, this is tasks like repair, maintenance, or cleaning.

My question is how many on-orbit FTEs (Full-time Equivalent units) does the ISS require to operate?

For example, if it takes 3 FTEs (so three astronauts working full time) to maintain/repair/clean/do chores on the station, this would mean that if there were four astronauts aboard, then there would be 1 collective FTE available for scientific, value-adding pursuits.

My hypothesis is that there is a "baseline" of "chores" that the ISS requires, and this has all sorts of implications: for example, taking the previous example of it requiring 3 FTEs, this would suggest that if we upped the "onboard astronaut count" from four to five, we would effectively be doubling the amount of science that could be done on the station as we now have 2 FTEs spare after subtracting those that accrue from non-value adding activities.

Additionally, if we knew this figure, we you could calculate the costs for a more economical space station: if a modern and brand new station only requires 1 FTE to keep in operation, you can have a station with fewer astronauts aboard producing more science value.

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    $\begingroup$ You are only asking about crew in space, not the ground support? Because that's a lot more than the onboard crew. If you include everything, it's about $1 billion a year. spaceref.com/status-report/… $\endgroup$ Commented Feb 22 at 20:37
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    $\begingroup$ @OrganicMarble Yes, specifically only the crew in space. I've edited for clarity $\endgroup$
    – Dragongeek
    Commented Feb 22 at 20:47
  • $\begingroup$ Does this (space.stackexchange.com/questions/11935/…) or this (space.stackexchange.com/questions/57788/…) answer your question? $\endgroup$
    – Hobbes
    Commented Feb 22 at 20:55
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    $\begingroup$ The 1st answer Hobbes linked to flat out states " NASA estimates that 2.5 persons are needed for maintenance, on average." That sounds like the answer to me. If not, what are you asking? The source is (From that answer) nap.nationalacademies.org/download/10614 and it does indeed say that on page 4. $\endgroup$ Commented Feb 22 at 22:41
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    $\begingroup$ @OrganicMarble Ah, I overlooked that! Still, this is basically exactly the information that I'm looking for, however one that reflects the current state rather than the state of the ISS rather than decade(s) ago $\endgroup$
    – Dragongeek
    Commented Feb 23 at 10:19

1 Answer 1


The NASA ISS payload utilization (science experiments) for FY 2019 was 967 hours, according to the ISS National Laboratory.

EDIT: My previous conclusion seemed low on the payloads utilization numbers. The above number was just for non-NASA payloads only. The number below in the FY 2019 financial report appears to be total USOS crew payload utilization.

The NASA FY 2019 Agency Financial Report (October 1, 2018-September 30, 2019) lists payload utilization crew time at 2944 hours on page 135.

Between Expeditions 56-61 during that time frame, there were 1129 total person-days on-orbit for USOS crew. Provided 5/7ths of those were work days at 6.5 hours each, that gives an estimated 5242 total USOS crew work hours for FY2019.

Those numbers come out to 56% payloads utilization and 44% maintenance/cargo ops.

For the specific question of on-orbit FTE requirements for maintenance, the 2298 hour remainder would equate to 353.5 total days. Adjusted for weekends, it's 1.35 USOS FTE. However, that number does NOT include any Russian segment maintenance.

  • $\begingroup$ Fantastic! Is the Russian segment maintenance completely covered by the Russian cosmonauts? Additionally, if you can guess, is the maintenance associated with the propulsion/life support in Zvezda higher than the baseline? $\endgroup$
    – Dragongeek
    Commented Feb 27 at 10:29
  • $\begingroup$ Yes, Russian segment maintenance was covered completely by cosmonauts. When I worked at POIC, the only times I really paid attention to their timelines was on the rare occasion they performes USOS payload activities or if their activities somehow affected USOS resources. That and whenever HRF needed to use CHIBIS for Fluid Shifts. Otherwise it was almost like there were two different space stations! $\endgroup$
    – Doresoom
    Commented Feb 27 at 12:11

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