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Supposing a long-term manned flight is planned would a biological resource-system be considered as an option?

Would a biological resource-system work or is a technical solution superior?

Currently technical solutions turn urine into drinkable water again and turn CO2 (carbon dioxide) to O2 (oxygen) [and C (carbon)]. Would the same be feasible with plants or other biological solutions?

This isn't limited to oxygen and water; food could be grown as well. Would it be considered or dismissed, because testing such a system could be difficult.

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    $\begingroup$ Is your question whether it would work or whether they would consider doing it? $\endgroup$
    – Purag
    Jul 19, 2013 at 10:15
  • $\begingroup$ @Purmou Probably both. Would it work in the first place AND would it be considered. Seeing the answer below it turns out to work, though not in a way that appears to be feasible AND it would probably be only considered as a part of a hybrid system. That makes obviously sense. $\endgroup$
    – bastik
    Jul 19, 2013 at 10:56
  • $\begingroup$ See also space.stackexchange.com/q/26933/10831 $\endgroup$
    – Suma
    Nov 16, 2021 at 11:50

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If I understand what you are asking correctly, this is essentially the same idea as the Biosphere 2, where your spaceship air and water is managed by plants, to keep it livable. The Biosphere experiment provided a baseline for what would be required to pull that off, and actually tested such a system. The design called for 8 people to stay inside of the 204,000 square meter facility for 2 years, with the only outside environmental change to come from the form of managing the temperature and rainfall, all from within the contained system. A photo of the system can be seen below, sourced from Wikipedia.

The Biosphere

In order to pull this off, you need to have a huge facility. Just to put this in perspective, the entire volume of the International Space Station is 837 meters squared. The biosphere is thus about 240 times larger than the ISS. Granted, there are aspects of the Biosphere which would not need to be reproduced to completely maintain systems on a space station (Variable air chambers, etc), but that would only reduce the size maybe by a factor of two. And even at that, the system wasn't able to maintain a truly independent environment, during the first test, they had to bring in oxygen and resorted to using emergency food stores.

The bottom line is, to use this would require an extensive system. A hybrid system could be put in place, where machines do some of the conversion from Carbon Dioxide to Oxygen, and food is grown, with a significant reduction in the required space, and that's the most likely solution that we could see anytime in the near future.

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    $\begingroup$ There have been several other projects similar to Biosphere 2, it's just that Biosphere 2 was the largest of them and therefore gets the most publicity. You don't need a system that large if you optimize (use algae, etc instead of inefficiently large plants). $\endgroup$
    – Gwen
    Jul 19, 2013 at 16:34
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On one hand, a biological resource system should be avoided because they are large and energy intensive. On the other hand, it is likely necessary due to the complex biochemical requirements of humans.

Recycling O2 and H2O are technically easy. But we do not live on O2 and H2O alone. There are many essential micro- and macro- nutrients which must be provided. But “essential” is species-specific and there is no assurance we know them all. There may be, for instance, a coenzyme we have yet to identify. Sending humans on a prolonged space voyage with a synthetic diet would be a perfect way to discover “space scurvy”.

Algae is a promising “Space Food” since it can be grown from inputs of inorganic salts, CO2 and light. Since algae are metabolically vastly more sophisticated than humans, there is a reasonable chance they manufacture the mystery micronutrients we have yet to discover.

Besides palatability problems, there are significant resource problems growing algae in space.

They are energy inefficient. Only about 12% of incident light is captured. They require a “Goldilocks” range of light intensity. Too much is toxic. Too little and they don’t photosynthesize. Solar radiation is very difficult to utilize for a light source, so the bioreactor becomes a significant drain on spacecraft electrical supply.

Due to self-shading, algae require a narrow range of suspended concentration in the growth media. This means a significant mass of media is required.

I worked on a project to build an Earth-based algae system to generate medical oxygen for isolated clinics in developing countries. The system required a cubic meter (one metric ton) of media to provide O2 for a single person. This is similar growth media requirements as the algae demonstration in the ISS. https://www.nasa.gov/mission_pages/station/research/news/photobioreactor-better-life-support

Another problem is growing algae in zero-g. On earth, bubbles are a convenient way to segregate gasses from growth media and agitate the media. Bubbles don’t serve the same uses in space.

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