I envisioned this method of maintaining breathable oxygen. So each ship would have these gardens generating oxygen like the plants on Earth do. How plausible is this and how would it likely work?

up vote 17 down vote accepted

The Atomic Rockets Web Site has all you could want and more about Closed Ecological Systems.

enter image description here

Selected quotes:

Remember the fundamental rule of rocket design: Every Gram Counts.

and

The main functions of a CELSS are:

1 Turn astronaut's exhaled carbon dioxide into oxygen
2 Turn astronaut poop and table scraps into food
3 Turn astronaut pee and washing wastewater into drinkable water

The current lines of research focus on doing this the same way Terra's ecosystem does: by using plants. In order to make the CELSS hyper-efficient they have to use hyper-efficient plants. Which explains the focus on algae.

and

The advantage of algae is that it can theoretically form a closed ecological cycle. This means that 6 liters of algae water, one human, some equipment, and sunlight can keep the human supplied with food and oxygen forever.

But there is waaayyyy more to this subject than the snippets I included. Please read that site for the details.

Which is better: Closed or Open Loop System?
Later on the page this resource indicates that closed-loop systems only make sense when you need environmental systems to last for a year or more. For times less than this, it takes less mass to just carry the inputs with you and dump the waste overboard.

A garden in space?
You could replace some or all of the algae with other plants, but your closed cycle ecological system will take more space, require more resources, and possess a very much higher mass.

Similarly you could increase the mass of algae in the system and raise animals which ate it for nice variety in the diet (likely sea creatures able to share the algae tanks - crab, shrimp, fish, etc.). But as with replacing algae with trees, adding animal life decreases the mass efficiency of the system.

For near-term (the next couple of hundred years) or possibly forever, space travel will require something less mass intensive to make space flight practical.

The Biosphere 2 experiment was designed, in part, to answer this question (the term "Biosphere 1" being reserved for the planet Earth). It consisted of 3.14 acres of completely-enclosed, air-tight, land (including rainforest, wetlands, savannah, and desert), as well 8 individuals and a number of animals (including goats, chickens, and wild boar). Ultimately, the land was not able to sustain the necessary oxygen levels, which steadily dropped and eventually had to be artificially boosted. However, at least part of this oxygen loss was later found to be due to exposed cement portions of the structure absorbing oxygen from to air.

Another atmospheric issue that was encountered was fluctuating CO2. It would drop during the daylight hours due to photosynthesis, and then rise again during the night. Occasionally, a CO2-scrubber had to be used.

A later, second experiment (that included sealing the cement) was terminated prematurely due to some combination of personal, managerial, and financial issues.

  • 6
    3.14 acres - was that an intentional nod to Pi, or just a coincidence? – Mikey May 31 '15 at 18:16
  • I recall that the loss was due to the fresh cement absorbing CO2 from the atmosphere, causing the crop productivity to drop, and that this was ameliorated by adding more CO2. – EvilSnack Sep 3 at 13:51

"A single mature tree can absorb carbon dioxide at a rate of 48 lbs./year and release enough oxygen back into the atmosphere to support 2 human beings"

  • Arguments for Land Conservation: Documentation and Information Sources for Land Resources Protection

That being said, it becomes a matter of elementary math. Number of people / 2 = number of trees.

However, the $O_2:CO_2$ ratio will be in constant flux, unless you keep lights on continuously. Also, you will have to water the trees. I would probably be easier to use a machine to produce oxygen.

  • You found good source about trees in particular. What about algae mentioned in Jim2B answer? How come machine could "produce" oxygen? – terafforem34 May 31 '15 at 23:23
  • @KacperWikieł Basically, since you need water and electricity for this to work, you can skip the middle man (a lot of these resources are wasted in growing the plant). To skip the middle man, we can use a process called electrolysis. We run electricity through water, breaking up into oxygen and hydrogen gas. – Jimmy360 May 31 '15 at 23:35
  • I feel trees are a bad idea for oxygen generation. In order to convert CO2 --> O2 the tree must grow; this locks up precious carbon. If you keep letting it grow it'll eventually use up all the carbon you brought with you and you'll have loads of wood and no food. You need a closed system CO2 + H2O + waste <---> food + O2. Any loss in that equation (to for example vast amounts of useless wood) and you're in big trouble – Richard Tingle Jun 1 '15 at 19:04
  • @RichardTingle That is another reason to use machines instead of trees. – Jimmy360 Jun 1 '15 at 19:12
  • @Jimmy360 Depends on what the machines outputs the carbon as. If it's anything but food (or something that can be turned into food) you've got the same problem – Richard Tingle Jun 1 '15 at 19:37

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