In act two of Seveneves it is suggested that dead bodies are jettisoned into space. I don’t understand how you can expect to have a sustainable society in space if you throw away anything.

Thinking about this, I was reminded of Dune. In this scifi novel, some city-dwellers seem to suggest that Fremen, living in the desert, drink the blood of their dead. They are corrected that the Fremen drink the ‘water’ of the dead. If water is by far your most precious resource, you are not going to bury or evaporate it when the current owners die. Water is both more and less than blood, because more than just blood contains water, so the Fremen have developed technology to extract all water from the human body.

I am not saying the spacers in Seveneves should eat their dead directly, but they should preserve the nutrition. Burn or freeze-dry the bodies, bury them or sprinkle their remains over the soil they use for farming, or drown them in the water used to grow algae, but never throw anything away that you can still use. Not if you’re isolated in space.

So how does the population of the cloud ark Endurance expect to live on for 5000 years without recycling human remains?


The ark was designed to be moderately self-sustaining but it's not a totally closed system. Over time supplies of vital resources will run low. They, for example, use lithium filters to remove excess CO2 from the air, which can't be replenished. They simply bring lots of them with them, more than they will ever conceivably need.

It went without saying that, in the long run, the Cloud Ark as a whole was going to have to be self-sustaining in terms of food production. Water would have to be recycled. Carbon dioxide exhaled by humans would have to be used to sustain plants, which would produce oxygen for the humans to breathe and food for them to eat.

Preserving mass is practically a religion for some of the inhabitants, but it's made clear that they do have plenty of leftover mass (presumably including all of the ingredients found in a human) to spare and aren't averse to using it when they absolutely need to.

“We’re dumping mass,” Sean said, nodding. “As soon as the Cloud Ark runs out of propellant, it loses the ability to do all of the things that make it a viable architecture for long-term survival. It becomes a big sitting duck.”

He let them roll that around in their heads for a bit, then went on: “Mind you, almost everything else that we do up here can be done with minimal effect on mass balance. We can recycle our urine to make drinking water and our poo to make fertilizer. Very few of our activities involve just releasing mass into space in a way that we can’t get it back. This is the exception. I have been ranting and raving about this ever since the idea of the Cloud Ark was announced. So far all I get in return, from the powers that be, are vague answers and hand-wavy happy talk.”

Bodies are pretty much the only exception to their rule. 5000 years worth of bodies (assuming a static population of 2000 and an average life expectancy of 50 years) would contain about the same amount of water as a few Olympic-sized swimming pools, a not insignificant amount, but hardly a big deal when you're using that much or more as reaction mass each time you do a minor course correction. Water and the majority elements in a human body are relatively abundant on the Ark in any case.

Ivy made a unilateral decision that they would carve out an exception to that new policy. The deceased were moved into an empty orbital module docked at the truss. This was left open to space, so that freeze-drying of the bodies could happen out of sight and out of mind. When it filled up with dead people, they would have some kind of ceremony, the thing would be deorbited, and they would watch in silence as it drew a white-hot streak across the atmosphere below.

  • Can you share your calcs for the water? If a fiftieth of the population die each year = 40 per year x 5000 years = 200,000 bodies. At 40 litres per body = 8,000,000 litres = 8,000 tonnes = 3 Olympic sized swimming pools....I think?
    – user83948
    Mar 24 at 0:21
  • 1
    @Martin - I'm not going to redo the calcs. I'm happy enough to say that it's not a large amount and possibly very little when they're talking about what to do with gigatonnes of water they've captured from small comets.
    – Valorum
    Mar 24 at 0:38
  • I think your answer is the best in-world I will get, though I will wait some more time before I pick this as the one. It sounds to me like plot device: If the protagonists would dissolve human remains and reuse them as nutrition to their food, it would be a step towards cannibalism. One that could be completely justified of course, we do it as well in a way, but it would make the actions of the antagonists more acceptable and thus them more likable.
    – Johan
    Mar 24 at 8:57
  • Valorum's Isn't that ignoring how much they'd really have to lug along for 5,000 years of more than they will ever conceivably need? Recycling poo for fertilizer slows, not stop, exhaustion of the soil - or whatever growth medium… Unless there's a special reason, 50 years seems very little life expectancy - though with a static population, that matters less. Still, isn't 5,000 years of replacing 2,000 people every 50 years a good 200,000 bodies-worth of lost mass? Mar 24 at 15:18
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    Dumping phosphorus embodied in a population of human corpses would seem to be a serious issue, since all cellular metabolism (human and non human) depends on ATP. So spacing human corpses would mean draining this vital element out of the food web.
    – Lexible
    Mar 24 at 20:50

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