We are familiar with the first groups of NASA astronauts, like the Mercury Seven and the New Nine. Something about that struck me as (literary) odd; All the first twelve astronaut groups had an odd number of astronauts, except for Astronaut Group 3 and 4. The probability for such a coincidence is quite low, so there was perhaps a reason for this policy.

Why did NASA hire an odd number of astronauts in almost all the first astronaut groups?

7 , 9 , 14 , 6 , 19 , 11 , 7 , 35 , 19 , 17 , 13 , 15


For comparison, the same pattern does not exist in the Soviet space program. A list of the cosmonaut groups can be found here.

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    $\begingroup$ It's not the odd number of astronauts but the number of odd astronauts that made our jobs interesting. $\endgroup$ Feb 17, 2016 at 0:53
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    $\begingroup$ Does the Russian / Soviet space program do the same thing? $\endgroup$
    – Joe L.
    Feb 19, 2016 at 19:45
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    $\begingroup$ As an extra data-point, the pre-NASA Man in Space Soonist program also had an odd number (9) of test-pilots. $\endgroup$
    – simplicio
    Feb 21, 2016 at 0:37

4 Answers 4


Can't answer the overall question, but for Group 1 specifically, there's a discussion of the recruitment program here. The original plan was to recruit twelve, to allow for some dropping out during the program. When it became clear there were unlikely to be many dropouts, they revised the target to six. However, when making the final selection, they got stuck on bringing the shortlist down to 6:

Sitting in judgment over 18 finalists, Donlan, White, and North pared down the final pool of selectees, choosing each to complement the rest of the group. The going was so difficult that they could not reach the magic number six, so Gilruth decided to recommend seven. Donlan then telephoned each of the seven individually to ask whether he was still willing to accept a position as a Mercury astronaut. Each one gladly volunteered again.

So that one's as much an accident as anything...

(edit:) Group 4 was originally six strong, but one (Graveline) resigned before being assigned to a mission.

(edit:) Group 7 was essentially arbitrary - it was "all the Manned Orbiting Laboratory astronauts under 35". It so happened there were an odd number of them, but there was no set number initially aimed for.

After accounting for these, we end up with:

  1. Planned even, actually odd (7)
  2. Odd (9)
  3. Even (14)
  4. Even (6)
  5. Odd (19)
  6. Odd (11)
  7. No plan for numbers, actually odd (7)
  8. Odd (35)
  9. Odd (19)
  10. Odd (17)
  11. Odd (13)
  12. Odd (15)
  13. Odd (23)
  14. Even (24)
  15. Odd (23)
  16. Even (44)
  17. Even (32)
  18. Odd (17)
  19. Odd (11)
  20. Even (14)
  21. Even (8)

So across all 21 groups, and taking account of the fact we know two were intended to be different sizes, this is eight even, twelve odd, one "whoever happens to qualify" (but happened to be odd). This certainly feels like there's no intentional focus on odd numbers.

  • $\begingroup$ Of the original 7, Deke Slayton was grounded because of a cardiac issue, and in the end, 6 astronauts flew 6 manned missions. Was the program always supposed to be six flights, and would one of the 7 have had to sit out regardless of health? $\endgroup$ Feb 29, 2016 at 0:02
  • $\begingroup$ @RussellBorogove This was mostly coincidence. Mercury was originally intended to have more than six manned flights - there were at least three Redstone flights cancelled after MR-4, for example, per history.nasa.gov/SP-4201/ch11-9.htm $\endgroup$ Feb 29, 2016 at 10:27
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    $\begingroup$ Ok, so the first group seems to be mostly a coincidence . Then there are only 11 more groups... $\endgroup$ Feb 29, 2016 at 10:28
  • $\begingroup$ @Hohmannfan Ten, I think - turns out Group 4 was even as well. Agree it's improbable as a coincidence! $\endgroup$ Feb 29, 2016 at 20:37
  • $\begingroup$ @Andrew I misread the information. You are correct. $\endgroup$ Feb 29, 2016 at 20:39

It's not that much of a coincidence -- about 1 in 52 for 10 or more odd numbers out of 12. You would have been just as surprised by ten or more even numbers, so you can bring that down to 1 in 26. And you would have been just as surprised to see such a discrepancy in the Soviet space program, instead of the US program, so you can bring it down to 1 in 13.

To sum up: your claim that there "must have been a reason for this policy" is unfounded.

  • $\begingroup$ True, I am only observing something that looks like a pattern in random noise. However, that is enough of a reason to ask if the pattern indeed has an explanation. I can reword my question slightly if that detail is nagging you. For balance, here is an unmatched bracket ) $\endgroup$ Oct 12, 2016 at 20:11
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    $\begingroup$ @Hohmannfan: Well, I appreciate your edit, but I still disagree with your "quite likely". There's really nothing to it. $\endgroup$
    – TonyK
    Oct 12, 2016 at 21:37
  • $\begingroup$ Quite likely >> perhaps. $\endgroup$ Oct 13, 2016 at 7:50

Richard Feynman:

You know, the most amazing thing happened to me tonight. I was coming here, on the way to the lecture, and I came in through the parking lot. And you won’t believe what happened. I saw a car with the license plate ARW 357. Can you imagine? Of all the millions of license plates in the state, what was the chance that I would see that particular one tonight?


Based on the excellent answer by @TokyK let's look at the numbers with python:

even_odd = [[(i/2**n)%2 for n in range(12)] for i in range(2**12)]

sums = [sum(x) for x in even_odd]

A, B = np.histogram(sums, bins = range(14))


[1, 12, 66, 220, 495, 792, 924, 792, 495, 220, 66, 12, 1]

4096. / (1 + 12 + 66 + 66 + 12 + 1)


So chances are 1 in 26 that there are two or less groups that are even or odd.

What are the chances that - considering all of the things that happened in the Apollo space program - one of them had a chance of 1 in 26 or less of happening?

Essentially 100%.

Per day?

Essentially 100%.

Per person?

Essentially 100%.

Stuff happens.


I can't remember who told me this. But I've heard that the reason astronauts go up in odd numbers is so they don't form "teams" against each other. For example, if six astronauts spend a year in space together, three astronauts might disagree with the three other astronauts on some problem. This would create a lot of tension with no easy way to solve the problem. But if seven astronauts were to go up, opinions would never be split evenly. A decision would always have a deciding vote, and everyone could move on.

I can't give a source right now. But that's what I've heard.

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    $\begingroup$ This answer doesn't really make any sense. It might possibly make sense if your exposure to astronauts in space has been only something like the movie Apollo 13, but that's not how it happens in real life. Astronauts are very carefully screened and selected, and receive tons of very realistic training on ground, both of which serve to reduce the risk of or bring out any such tendencies in a non-critical environment where they can be dealt with trivially using standard management techniques. $\endgroup$
    – user
    Feb 29, 2016 at 15:28
  • $\begingroup$ Yeah, I agree with you. I believe a professor told me that. I just did a bit of searching and couldn't anything to support that reasoning. Having teams form against each other might be a concern for long distance exploration missions. But when astronauts are in constant contact with Earth, I don't see teams forming being an issue. $\endgroup$
    – JoshK
    Feb 29, 2016 at 15:50
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    $\begingroup$ This is a reasonable argument for flying odd-numbered crews (particularly on long missions), but it doesn't mean you need to hire them in odd-numbered batches. $\endgroup$ Feb 29, 2016 at 20:19

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