In Andy Weir's Project Hail Mary, nitrogen in the atmosphere is lethal for the astrophage predator taumoeba. In part of the story they are trying to breed taumeba that can live in a few percent nitrogen. This taumoeba escapes and kills all the astrophage, but why didn't the 70% nitrogen in the normal human atmosphere on the Hail Mary kill it?

  • 2
    Are you sure the atmosphere on the Hail Mary actually contained nitrogen? The nitrogen in Earth's air is rather useless for humans and I think the Hail Mary's atmosphere just contained lower pressured oxygen. I don't quite remember, though.
    – TARS
    Jun 28, 2021 at 11:19
  • 2
    @TARS Hasn't NASA been mixing in nitrogen since the Apollo 1 incident?
    – wogsland
    Jun 28, 2021 at 12:10
  • @TARS There's plenty on earth though.
    – Jontia
    Aug 23, 2021 at 8:34
  • @jontia Well...yeah, noone denies that.
    – TARS
    Aug 24, 2021 at 12:26
  • @Tars my bad. I'm mixing up my Taumeba with my astrophage.
    – Jontia
    Aug 24, 2021 at 15:22

5 Answers 5


There is a reference that the atmosphere aboard the Hail Mary is 40% sea-level which would suggest that the nitrogen level is certainly reduced from normal (so that the partial pressure of oxygen remains high enough) but I'm not sure if 40% means that it must be entirely oxygen.

Apparently, the entire Hail Mary is at that 40 percent pressure

Hmm. Although later on there is this part:

"Just a centimeter of transparent material separates my one-fifth atmosphere of oxygen pressure from Rocky's 29 atmospheres of ammonia."

Which suggests pure oxygen at 20% normal pressure.

And, finally, there is this reference to

one of the crews selected method of suicide (by nitrogen asphyxiation) which means that they have to provide a tank of it specifically: "'We'll make sure there's a tank with plenty of nitrogen, and a backup tank as well in case the first one leaks'"

So, even though there seems to be an inconsistency about the exact pressure, it does seem to be very strongly implied that the Hail Mary atmosphere is pure Oxygen.

  • 7
    The atmosphere issue is the one thing that bothered me about an otherwise fantastic book. I spotted the same problems. 29 atmospheres but really (based on Rocky analyzing) it would have been 29 x 0.4 = 11.6 atmospheres (though irrelevant - any high pressure ammonia is an issue for humans) and 40% -> 1/5 (=20%). There were enough references to fire to make it clear that the spaceship atmosphere could not have been pure oxygen - and even at 40% = 20% oxygen + 20% inert, nitrogen would have been the natural choice. Jun 29, 2021 at 3:03
  • 1
    Interesting. Isn't pure oxygen a bit dangerous from a human biology point of view and accidentally causing fires point of view? Jul 9, 2021 at 23:19
  • @neverendingqs As I understand it, for human biology it is basically fine. But fire is a different story - see Apollo 1. Jul 14, 2021 at 21:48
  • 3
    My recollection for Apollo is that they were going to use 1/3 pressure oxygen (to make the capsule structure easier) and that was fine for the Apollo mission length (but probably not for longer missions). The (tragic) mistake they made was to run a test at 1+1/3 pressure (still pure oxygen) so that the capsule (which would remain on the ground) would experience the "correct" 1/3 positive pressure. I think there would be health issues but the test was supposed to be short, but the main issues was that substances in pure oxygen at that pressure behave very differently when it comes to burning.
    – AdamT
    Jul 21, 2021 at 17:05
  • 1
    Health issues based on partial pressure of oxygen - so 100% at low pressure is fine.
    – Michael
    Jan 25, 2023 at 19:59

It bothered me too, so I emailed the author. Here's his response: "The Hail Mary's atmosphere is pure oxygen at 30% pressure. Same as they used to do in Apollo spacecraft. So there's no nitrogen at all"

  • I wonder if he is going to correct his book? As that does not agree with either of the direct quotes I took from the book! :-)
    – AdamT
    Feb 8, 2022 at 9:58
  • Guess you can email hm too :) he was quite responsive, to be honest
    – shakedzy
    Feb 10, 2022 at 10:01
  • 1
    That seems incredibly dangerous and not like something you would want in a laboratory with spark generating tools and burners.
    – user
    Feb 21, 2022 at 11:08

Here is a link to a study on the flammability of substances based on both concentration, partial pressures and total pressure.

Flammability characteristics (per NASA-STD-6001 Test 1 MOC self-extinguishment thresholds, material ignition, and burn rates) show a strong dependence on oxygen concentration with little relation to total pressures above 41 kPa (6 psia). Below 41 kPa (6 psia), MOCs and required oxygen partial pressures show increased dependence on total pressure. Power equation models fit trends very precisely across pressure ranges spanning 2.8–119.3 kPa (0.4–17.3 psia) for both MOC and partial pressure plots against total pressures. A notable finding was that required partial pressure of oxygen necessary to sustain propagation decreases with decreased total pressures. This directly implies an increased flammability risk at lower total pressure conditions.

Although the total pressure was bellow 40 kPa the 100% oxygen content still seems worthy of discussion in the book, given that last sentence. Especially given Strats obsession with “tried and tested technology” and it’s possible effect on high temperature equipment.

  • 2
    Welcome Cassiopia. Please take our wonderful tour and refer to the help center as and when for any guidance. Enjoy the site :-) Jun 2, 2023 at 17:20
  • 1
    I’m a little late to this question, but I just finished the book and has the same question
    – Cassiopeia
    Jun 2, 2023 at 17:21
  • I'm not sure how this answers the question; it appears to deal purely with oxygen partial pressures, and no mention is made of nitrogen at all.
    – DavidW
    Jun 2, 2023 at 18:09

When the phage was initially studied on earth the lab was filled with Argon. When astrophage was found to be non-lethal the lab(s) studied and grew it in the composition of Earth's atmosphere which should have killed the phage.

  • 1
    Not quite sure if/how this answers the question of why the atmosphere in the Hail Mary wasn't a problem for the taumoeba. Might help if you edit this answer to expand on your explanation. Aug 22, 2021 at 23:12
  • 2
    You're making the same mistake I did nd mixing up your Taumeba and astrophage. Astrophage is unaffected by nitrogen and is fine in earth conditions labs.
    – Jontia
    Aug 24, 2021 at 15:23

It is possible that the Taumoeba could already survive the amount of nitrogen on board the ship, and just the amount that he added to it would make it so that they could not survive it. I mean if Rocky accidentally screwed up when making the breeder tanks and made a leak then the nitrogen in the Hail Mary would have gotten in.

  • Hi, welcome to SF&F. Do you have any evidence for your theory that the taumoeba could survive any nitrogen concentration at all, and that the ship's atmosphere contained that proportion of nitrogen?
    – DavidW
    Feb 15, 2022 at 17:05

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.