Becoming an astronaut is notoriously difficult, with something on the order of 0.01% of all applicants getting picked for the job. However there are two possible explanations for why this is the case:

  1. Because the job of an astronaut is extremely difficult and only 0.01% of all humans could do well doing it
  2. Being an astronaut isn't that difficult, but space agencies get tens of thousands of applicants so any selection process will inevitably weed out 99.99% of all applicants.

Did NASA or other space agencies ever publish estimates of what % of humans could realistically do an astronauts job if given the opportunity? I.e. if NASA was forced to pick the 5000th best candidate instead of the 1st best candidate, would they significantly lower the chance of a successful mission? What about the 50,000th best candidate?

  • $\begingroup$ You are asking more than one thing, you ask why the process is so rigorous and you ask whether space agencies estimate how many humans are suitable. You need to focus onto one of them, if you have two questions ask them in separate questions. $\endgroup$
    – GdD
    Commented Jul 6, 2022 at 8:51
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    $\begingroup$ What definition of an astronaut are you using: a outer space professional who needs to perform a task in outer space, getting there or getting back or are you also including space tourists or passengers who travel to outer space simply for an experience? $\endgroup$
    – Fred
    Commented Jul 6, 2022 at 9:46
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    $\begingroup$ At least partly because far more people want to become astronauts than can become astronauts, so you ramp up the testing until you only have as many left as you need. $\endgroup$
    – Jon Custer
    Commented Jul 6, 2022 at 13:41
  • $\begingroup$ @GdD added a follow up question: space.stackexchange.com/questions/59656/… $\endgroup$ Commented Jul 6, 2022 at 17:10

1 Answer 1


Why is the selection process for astronauts so rigorous?

Frame challenge: it isn't. Chris "Hanks" Sembroski was selected at random from a lottery. (More precisely, his friend was selected at random but couldn't make the flight because he exceeded the weight limit of Crew Dragon. He then gifted the flight to Chris Sembroski.)

Selecting an ordinary person at random is almost the least rigorous process you can have.

Note that while Chris Sembroski is technically an Air Force Veteran, he was an engineer in the Air Force, not a pilot.

On the same mission, Hayley "Nova" Arceneaux was selected but not for any physical or mental abilities. She was selected by her colleagues as an ambassador for St. Jude's Children's Research Hospital. In fact, as far as I know, she became the first astronaut with an artificial body part (unless you count glasses).

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    $\begingroup$ Chris Sembroski was a passenger. While he technically did win his astronaut wings, he is not a NASA astronaut. The same applies to Hayley Arceneaux. The question obviously is about NASA astronauts and the rigorous selection process NASA uses to select its astronauts. $\endgroup$ Commented Jul 6, 2022 at 12:57
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    $\begingroup$ Both Chris Sembroski and Hayley Arceneaux spent about 50 times more time in space than Yuri Gagarin and performed scientific and medical experiments. Hayley Arceneaux also served as the Chief Medical Officer of the mission. I think you would be hard pressed to find a definition of "astronaut" that includes the likes of Gagarin or Shepard (before his Apollo mission) but excludes Arceneaux and Sembroski. The only difference between the former and the latter is that the former were "professionals" (in the technical sense of the word, i.e. they received a salary for doing astronaut stuff). $\endgroup$ Commented Jul 6, 2022 at 15:54
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    $\begingroup$ @JörgWMittag there is an enormous and obvious difference: the NASA and Soviet cosmonauts passed a governmental selection process and were accepted into their respective government human spaceflight programs. $\endgroup$ Commented Jul 6, 2022 at 16:56
  • $\begingroup$ @OrganicMarble so the difference is... in the paperwork? Yes, technically that's the case, but that's a very boring 'gotcha' answer. $\endgroup$ Commented Jul 20, 2022 at 21:23
  • $\begingroup$ @JonathanReez pick one or more 1) it's a comment, not an answer 2) sometimes reality is boring 3) there was more to astronaut selection than paperwork, obviously 4) boring questions get boring answers $\endgroup$ Commented Jul 20, 2022 at 21:24

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