For the sake of this question, let's say that there would be a space mission that would send one man on a 40 year journey that will require him to be in a completely self sufficient habitat for the duration of the trip. What would be required for him to be completely self sufficient, and not need any external matter or energy?

From what I can come up with, the man would be in a completely sealed spacecraft with near perfect recycling systems that would gather any moisture in the room, and condense it back to water. Life support systems would possibly include genetically engineered varieties of yeast or bacteria in a special ecosystem akin to a winogradsky column,where heat from a nuclear reactor would power thermophile bacteria that would excrete oxygen from carbon dioxide, etc.

Specialized 3D printers would produce any replacement parts for non-essential mechanisms that would break, while automated repair machines and robotics would perform any work. Food for this astronaut could be possibly some sort of Soylent-like material that can be synthesised from yeast or bacteria as mentioned above.

  • $\begingroup$ Welcome to Worldbuilding! I'm not sure how on-topic this is; Space Exploration could give you come good answers. That said, I think you'll get a lot more answers here, and I actually think it is on-topic here. Anyway, welcome again, and +1 for a good question! $\endgroup$
    – HDE 226868
    Oct 25, 2014 at 18:01
  • $\begingroup$ Are we to assume that the man won't go insane from being isolated for 40 years? You say "not need any external matter or energy"; it is likely only peripherally related to your question, but does that include things like power for the on-board spacecraft systems as well? $\endgroup$
    – user
    Oct 25, 2014 at 20:42
  • $\begingroup$ By not needing any external matter or energy, I mean that solar panels or interstellar hydrogen collectors are not within the scope of this question. In the example I provided above, power is generated using a nuclear reactor. Of course, the sanity of the man or women in question would be important - perhaps this can be addressed in a possible response! Maybe if we drug this astronaut, and leave him in a low metabolic state such as torpor, would help the sanity of the person - assuming he is fed via an IV line. $\endgroup$
    – user2417
    Oct 25, 2014 at 20:49
  • $\begingroup$ Where is the human going, and does he need to bring something to keep him alive after he arrives there? And, do you want responses to consider their psychological needs, too? (40 years in a coffin-like ship alone would take a massive amount of convincing to get me to volunteer. And I suggest you check volunteers for sanity, and have a plan for keeping them sane en route.) $\endgroup$
    – Dronz
    Oct 25, 2014 at 23:38

1 Answer 1


I'm going to make a slightly wacky suggestion. I wouldn't use a fully recycling system for only one person on a 40 year journey. An average US citizen eats about 1 ton of food every year, and this is widely recognised as more than is needed. For an astronaut with reduced physical activity and carefully chosen food types this can very easily be reduced to 60-70% of this (probably further). Freeze-dried and recombined with recycled water the mass that actually needs to be carried is only about 20% of this. In total about 5 tons of dried food would be needed for a 40 year journey. For comparison an Apollo Command/Service Module has a dry mass of about 11 tons and a NERVA rocket engine has a dry mass of about 34 tons.

Now you may say that 5 tons is still 5 tons, and waste should be avoided like the plague. Maybe you can imagine a bacterial faeces-to-food recycling system (yum!) that weighs less. But it is still going to be complex and bulky and weigh some not insignificant fraction of 5 tons, and wasted mass counts against you not only when accelerating your rocket up to cruise speed, but also when decelerating it at the end of your journey.

I'm guessing you aren't planning a 40 year one-way Hohmann transfer to Uranus where we can assume a relatively small capture burn, but either a round trip or some high velocity voyage with a large initial burn, high cruise speed and significant deceleration at the other end. If you take a large, bulky recycling system inside your crew compartment where it can actually be accessed, then you need to decelerate that as well. You also need to carry the extra fuel for this deceleration and accelerate that fuel to your cruise speed when you set off. It's mass relative to its remaining utility only increases during your journey. By contrast the mass relative to remaining utility for stored food remains constant.

Interestingly, your minimally treated, unrecycled waste could actually be helpful during deceleration. If you are planning a nuclear reactor for power generation then your means of propulsion should definitely be a nuclear thermal rocket engine, with the reactor being used for combined power and propulsion. In a liquid fuel rocket the energy is generated by a chemical process in the fuel itself, while in a nuclear thermal rocket the energy is generated by nuclear fission in the reactor and then transferred to an inert reaction mass, which then expands through the rocket nozzle, creating thrust. Why not put that waste to use as reaction mass during your deceleration burn?

Additionally, the stored carbohydrates in your food will liberate water vapour and carbon dioxide once metabolised (containing slightly more oxygen that you inhaled), which could help compensate for any inefficiency in your water and oxygen recycling systems. I'm finding it hard to pin down exact numbers, but it seems the total mass of faeces output by a human being is only around 10% of the food eaten. If we assume somewhere up to 80% of food eaten by mass is water, then perhaps half of the weight of the freeze-dried, stored food will be liberated as carbon dioxide and water.

Finally, the psychological value of freeze dried ice-cream to an astronaut isolated for FORTY years should not be overlooked.

  • $\begingroup$ Such an excellent response! I think the real difference between your scenario and mine is how much value we place on the human astronaut in question. In my mind, I imagine the astronaut to be strapped into a coffin like structure, with electric vibrators to prevent muscle wastage and various tubes for the excavation of waste and insertion of nutrient - effectively a biological component of the spacecraft that must be kept alive. $\endgroup$
    – user2417
    Oct 25, 2014 at 20:53
  • $\begingroup$ Your paragraph about reaction mass also gave me an idea - perhaps stockpiles of each common element (carbon, nitrogen, etc) can be kept in the spacecraft, which would be used as feedstock for bacteria. The effective mass of this stockpile would never decrease as all biological outputs of the astronaut would be carefully recycled. At the end of the journey, the feedstock can be converted into various gases by bacteria, and then used as reaction mass with the nuclear thermal rocket! $\endgroup$
    – user2417
    Oct 25, 2014 at 20:56
  • $\begingroup$ If you recycle then you have less food and/or waste by a factor on the order of your recycling cycle length divided into your journey time, so you will have much less of it to use as reaction mass - maybe 10,000 times less. You will still have a whole recycling system to decelerate. With a nuclear thermal rocket almost anything that can be vaporised and blasted out of a rocket nozzle is "fuel", but a bulky recycling system probably doesn't fit into that category. But, if you have an inner environmentalist that absolutely must recycle then that's cool. (ps. if you like the answer... upvote?) $\endgroup$ Oct 25, 2014 at 23:00
  • $\begingroup$ If you don't need life-support at the destination, you might be able to detach your life-support and/or waste before decelerating. $\endgroup$
    – Dronz
    Oct 25, 2014 at 23:39
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    $\begingroup$ Hello @Black, we were discussing you, not me. Please tell me more about "spill our guts". Do you like them? $\endgroup$ Oct 28, 2014 at 3:57

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