Here's the picture. Earth is beyond saving and is too polluted to be habitable anymore. My plan is to have this ship built on the moon, using materials from our asteroid belt. A bit of a problem is, I'm not exactly sure how big the ship would have to be, and how the size of it affects its functions and speed, but the latter can be a question for another time. If the average density of population ranged anywhere from places such as Manhattan to Manila, and the interior of the ship was laid flat like a boat (layers possible if necessary), along with considering room for necessary crops and creatures needed for survival, about how big would this ship need to be to fit roughly 7 billion people? For around let's say, a century or two?

edit : I appreciate the different input, while I understand it's unrealistic for the damaged to Earth to be irreversible, I would still like something immanent requiring the need of worldwide evacuation, which may need to stem into another question. Flaws in my question have been pointed out, so I'll be sure to work that through. Thank you for all your help :)

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    $\begingroup$ You are pretty much standing on it. $\endgroup$
    – Pelinore
    Oct 2, 2022 at 11:43
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    $\begingroup$ If you’re able to build a massive self-sustaining biosphere tough enough to survive in space and fit 7 billion people… why not just build it on earth? And leave it there? $\endgroup$
    – Daniel B
    Oct 2, 2022 at 14:05
  • $\begingroup$ I think the problem needs clarification. For how long do you need to keep these wonderful but messy folks on the ship? And do you plan to feed them and keep them entertained, or put them to cryosleep? $\endgroup$ Oct 2, 2022 at 17:32
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    $\begingroup$ @DanielB is right. So, just to make a point: if humanity could build a self-sustaining ship to carry the whole population, there wouldn't be any pollution forcing the migration. Some years ago I coined a phrase to describe this condition: technology dichotomy. You can't have problem-solving technology operating in the midst of the problem it's capable of solving and still have the problem. And if the problem is somehow irreversible (no such thing, but I'll go with it) and they can build such a ship, why can't they simply build domes here on Earth? The sun is still useful and hard to replace. $\endgroup$
    – JBH
    Oct 2, 2022 at 19:56
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    $\begingroup$ @JBH That's a very good phrase, and it works not only for tech but for magic, too. I've written an essay on the past (My Kings aren't Dumb) where I discussed how magically-enabled settings are most often governed by incredibly dumb kings that can't figure out that they can invest in magical learning to solve most of their kingdom's problems. This seems a similar effect. $\endgroup$
    – Mermaker
    Oct 3, 2022 at 13:59

2 Answers 2


Pack the people in densely.

Manhattan has 70000 ish people per square mile, so dividing 7.75 billion by that we get 110000 square miles of land. If you want enough land to feed everyone with crops and such, you need a planet, but if you just want enough to feed people, you'll want stacked farms.

They can be included in the space for humans, and can grow a huge amount of plant life. In addition, you can easily grow insects that grow off plants and fish for some protein, since both are pretty efficient.

You can make your spaceship a cuboid, with a surface area of 50 by 50 for 2500 square miles. You just need fifty layers to contain everyone.

Notably, this is a massive expenditure that would likely require orders of magnitude more energy than cleaning the planet, but maybe they are fine with that.

  • $\begingroup$ Split the structure in two, 25 floors each structure. Connect the corners with very strong cables (like a box kite), spin it around a central axis, and you have gravity. If this ship isn't going anywhere (not a generation ship, but just in a solar orbit, no need for massive propulsiuon either. Just wait it out somewhere in Earth's current orbit. $\endgroup$ Oct 4, 2022 at 1:20
  • $\begingroup$ Rode Island is almost 1100 Square miles, so each layer is twice the size of Rode Island. So fifty Rode Islands stacked on top of each other. That, really, is NOT a lot of resources. $\endgroup$ Oct 4, 2022 at 1:30

Since I assume you want the ship to be self-sustaining, I think a good approximation can be in using the Earth overshoot day

Earth Overshoot Day (EOD) is the calculated illustrative calendar date on which humanity's resource consumption for the year exceeds Earth’s capacity to regenerate those resources that year.

Earth overshoot days in 2022 was around the end of July, which means that, in a first approximation, you would need 12/7 of the Earth surface to sustain those 7 billion people.

True, most of the surface of Earth is not used for food production, but it is still needed to keep all the cycles which support life running, so I would not throw that surface out of window.

And I am also approximating the life bearing layer to be 2 dimensional rather than a volume.


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