How many plants does it take to breathe?

Question

I'm working on a space story in which an astronaut is travelling to Neptune to validate our CurrentEarth™ corporation's findings regarding our Solar System. The setting is a moving ship similar to the Mars home base in The Martian, but I came across a bit of a boondoggle, he needs to breathe! Being the crafty astronaut he is, he's using hydroponics not only for food on the long journey, but also for his oxygen supply. Ingenious!

A few details:

He's an average height (5' 11", 180 cm), weight (195 lbs., 88 kg), and lung capacity (Normal) human male
The size of the ship is not a problem (although weight still matters to an extent), he has plenty of funding from CurrentEarth™
He needs to be able to constantly do strenuous activity without suffocation
He has emergency oxygen supplies that will last for 1 year
His ship has been preoxygenated for his trip, so he doesn't have a ramp up time while the plants grow
He's really fond of his ficus Wilson

How many plants would it take for my astronaut to breathe while doing strenuous tasks in space?

@NexTerren I didn't have a hard reason for naming his ficus, but now I do. Gratz on naming the ficus in my story! — Anoplexian, Aug 31 '16 at 14:56
I don't have any sources, so I won't put this as an aswer. I remember learning that it takes a hill's worth of trees to produce enough oxygen for 1 person. Furthermore, algae produce far more breathable air than trees. Add to this, it's far more efficient (given spacial confines) to use either mechanical or chemical air scrubbers (which Nasa already has quite some studies and papers on). — Fayth85, Aug 31 '16 at 15:02
What plants, exactly? Or you want most effective plants for the job, plus one Wilson? — Mołot, Aug 31 '16 at 15:05
Probably, my relevant question from Space Exploration SE can be helpful. — Z.., Aug 31 '16 at 18:00
You might find information related to the Biosphere 2 project. — Basil Bourque, Aug 31 '16 at 18:17
I keep reading my own question in the HNQ as "How many planets does it take to breathe"..... — Anoplexian, Aug 31 '16 at 19:29
Don't forget about the gas balance from the plant's perspective as well, when you're writing your story. If you have too many plants, the environment could get too oxygen-rich and the plants could become CO2 starved. Check this post: http://biology.stackexchange.com/questions/3043/what-is-the-effect-of-a-pure-oxygen-environment-on-a-plant — Tophandour, Aug 31 '16 at 20:46
"using hydroponics not only for food on the long journey, but also for his oxygen supply" is redundant. The amount of CO2 one exhales from consuming food is the exact net amount of CO2 removed from the air to grow that food. (Note that if you're eating meat, the animal already put back some of the CO2 removed by the plants it ate.) — Monty Harder, Aug 31 '16 at 21:12
This question doesn't make sense. How is your sneaky astronaut going to get all the sunlight and warmth to let the plants grow and do photosynthesis, at Neptune, if he hasn't got the power to do a bit of electrolysis to be able to breathe? — Karl, Aug 31 '16 at 22:47
A minor nitpick: 5'11" is actually tall. 5'6" is the worldwide average adult male height (though this seems relevant — Brian, Sep 01 '16 at 14:29
Another minor nitpick: athletes don't usually have normal lung capacity and astronaut probably shouldn' either. — Daerdemandt, Sep 01 '16 at 17:30
Geez, you all are just full of nitpicks! In all seriousness, thanks for the feedback! — Anoplexian, Sep 01 '16 at 19:47

score 47 · Accepted Answer · edited Jun 16 '20 at 11:03

13 square meters

BIOS-3, a sealed, underground compound designed to mimic a spacecraft, managed to generate oxygen for humans using algae. Its specifics are outlined in an article by Salisbury et al.:

A crew of up to three people
315 cubic meters of living space (14 m by 9 m by 2.5 m)
3 phytotrons, or algal cultivators, covering 63 square meters
Originally, 20 6kW lamps in each phytotron, though this was later increased
One catalytic converter, to do partial air purification; the plants did all the rest

Some of these figures are for the original setup; modifications were made to increase efficiency. Later experiments showed that "higher plants" could supply the crew with enough food to survive; they included beets, cucumbers and potatoes. In theory, to meet the full dietary and oxygen needs of one person, you would need 56 square meters of plants; to only meet the full oxygen needs and 35% of the dietary needs, you need only 13 square meters.

Gitelson & Lisovsky (2008) presented a slightly different overview, though with the same numbers. Here's a diagram of BIOS-3 from their paper:

Keep in mind that technology has improved since the BIOS experiments; newer experiments, such as MELiSSA, may show greater improvements. See Nelson et al. (2009) for more information on other tests, and Johansson (2006) for an excellent comparison. There's also a great article on Science2.0.com.

@Anoplexian That's gas, not gasoline - as in, air. As for the charcoal filter, even if it did need replacements, it's not a big deal to bring enough charcoal with you. The report goes into more detail - the "growing wheat" part was optional, for a scenario where you removed the algal compartment. This also sufficed to recycle enough oxygen, though it required vastly more area (though also providing almost enough food). — Luaan, Aug 31 '16 at 19:31
The algae farm is an interesting idea. In fact some people have already implemented it in a way. Switzerland has started using small algae farms to scrub pollutants from the air above highways. And the charcoal filter is no big deal to replace, a single one lasts a long time. In The Martian (the book anyways, haven't seen the movie) they do a piece of the triviality of that particular component. — thanby, Sep 02 '16 at 12:40

Pere · Answer 2 · 2017-07-08T11:22:50.783

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Humans and animals use oxygen to metabolize our food, while plants produce oxygen while producing food. For example, the amount oxygen produced by a potato plant to produce a single potato equals the amount of oxygen the astronaut will need to metabolize that potato. Therefore, if the astronaut has plants to produce food, and they produce all the food he eats, those plants will produce enough oxygen for him to breathe.

Of course, we must assume a closed stationary system but we need to take in account some deviations from a closed stationary system. Some deviations could cause an excess or a lack of oxygen:

Lack of oxygen: Oxygen being consumed for things different than oxidize organic matter produced in the ship. For example, if something in the ship is made of iron and it gets rusty, it consumes oxygen, or if the astronaut eats food from Earth, he will need extra oxygen to metabolize it.
Excess of oxygen: If some organic mater produced by plants in the ship is stored, the equivalent amount of oxygen will build up - in fact, that's how Earth's atmosphere got the sizeable amount of oxygen it has now. For example, all plants produce non edible parts (wood, leafs, roots...). If those parts are stored instead of being recycled as compost or burned, oxygen will accumulate. Planting trees is unnecessary.

In summary, the astronaut just need the plants to produce his food. They will produce his oxygen, too.

edited Jul 08 '17 at 11:22

answered Aug 31 '16 at 22:09

Pere

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+1, this is the correct (and probably the most useful) answer. Of course, some of the oxygen will not be consumed by the astronauts, but by microorganisms that decompose any uneaten plant material and the astronauts' own waste products. But in the end, it must balance out regardless -- in a closed system, the oxygen produced by photosynthesis will always be exactly enough to oxidize all the carbohydrates produced by the same process (and anything else derived from them) back into water and carbon dioxide. – Ilmari Karonen Sep 01 '16 at 01:58
Didn't know about the equivalent food eat/oxygen need - food produced/oxygen produced thing but; does your answer also takes in consideration that plants also breath during "night" ? I mean yes, you could enlighten them all the time but in this case we have to take in consideration the fact that since plants are using the day/night cycle to position themselves during the year, a constant light would be bad at long term (do we really want this for Wilson?). Don't know if I'm fully correct, just questionning around your answer. – comicurus Sep 01 '16 at 08:33
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I'm not sure to understand, you focus on the amount of oxygen the astronaut will need to metabolize that potato, but that ignores the body needs, for example how does this relate to the fact that the whole body will consume oxygen to feed the brain, the muscles, etc. with oxygen? – A.L Sep 01 '16 at 11:45
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@A.L Body never consumes just oxygen. It consumes oxygen, carbon and hydrogen. Carbon and hydrogen come from the metaphorical potato. Energy goes to different purposes including varied amount of waste heat, but ratio of oxygen and potato (carbon and hydrogen) atoms is fixed. – hyde Sep 01 '16 at 14:05
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@comicurus Respiration is the exact reverse of photosynthesis. It's the net production of oxygen that is reflected by the carbon and hydrogen added to the plant's mass, which the astronaut consumes along with exactly the net amount of oxygen (after subtracting the oxygen consumed in respiration) that was released by the plant in the first place. – Monty Harder Sep 01 '16 at 16:00
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@comicurus. As Monty Harder said, is the net production that matters. When plants "breath" (day or night) they just combine part of the oxigen they have produced with part of the "food" they have produce. That is, part of the carbohydrates and oxigen made by the plant are being consumed by the same plant. If the astronaut takes some food from the plant (e.g. he picks a cherry) the oxigen produced by the plant when producing that food (the cherry) won't be consumed by the plant: it will be consumed by the astronaut (as he metabolizes the cherry). – Pere Sep 01 '16 at 20:34
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@A.L As hide said, when the body feeds the brain, the muscles, etc., it feeds them with oxigen and food. The body doesn't need oxigen to anything different than oxidizing food, and oxidizing food produces energy to fuell every process in the body. – Pere Sep 01 '16 at 20:37

score 10 · Answer 3 · edited Sep 07 '16 at 22:10

10

according to:

This site

a mature leafy tree produces as much oxygen in a season as 10 people inhale in a year.

So a single tree could produce enough oxygen to let breath 40 people (if you have 4 seasons like in European countries) or to let breath 20 people (if you have 2 seasons only like in tropical countries).

However a single tree is not practical because if it dies, you are f****.

The best thing is to have 3 sections in the ship that simulates day/night alternatively (because vegetation during night burn oxygen instead of producing). What you want to achieve is that at any given time you consume as much oxygen as you produce. Have many different plants of different species is another must, you never want all your plants dying because of some disease or some climatic effect of the ship.

To keep the oxygen balance to let plants still grow and your human to not die because of excess CO2 you could just adjust the night cycle by few minutes every day in order to let plants burn/produce more/less oxygen

You'll need roughly 10 square meters per section. (surface taken by a single tree, slightly rounded off) You also basically have one redundant section which can be handy if clever exploited in case of various emergencies (Sure putting more mass in orbit has high cost, but if this travel is really important you want to be sure the pilot don't die).

edited Sep 07 '16 at 22:10

Vincent

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answered Aug 31 '16 at 16:33

CoffeDeveloper

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I think a tree would be a bad idea to grow for oxygen because you can't eat a tree. – Pieter B Sep 01 '16 at 08:43
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@PieterB but you can eat fruit from a tree, depending on the fruit and if the tree produces fruit – gabe3886 Sep 01 '16 at 11:21
I infact told to use many different plants not just 1 tree, The tree is my "measure sample" to get idea of order of magnitude of vegetation required. Also growing roots may be damaging the spaceship at some degree, better smaller plants with shorter roots. – CoffeDeveloper Sep 01 '16 at 12:19
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@gabe3886 sorry I should have been more precise, if you take a plant where you can use the whole plant for food you keep the carbon cycle intact. With a tree you end up with a lot of the carbon stored in the wood (which you can't eat) and to keep the carbon cycle intact you need to import food and water. If you then choose to like use the wood and burn it, you change your atmosphere. I just meant, a tree would be a particularly bad plant. Other plants would be better. – Pieter B Sep 01 '16 at 16:17
cool, this is interesting stuff going to read more about that. – CoffeDeveloper Sep 02 '16 at 09:41

How many plants does it take to breathe?

3 Answers3

13 square meters