How many plants does it take to supply oxygen to a spacecraft?

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Recently one of our subscribers sent the following question:

The answer to the first question is very easy — no, plants are not used for these purposes and, as far as I know, have never been used. In general, the idea of providing a space station or spacecraft with oxygen through photosynthesis looks logical, but in reality it is difficult to implement.

Calculations show that approximately 10,000 leaves are required to provide oxygen to one person through photosynthesis. The ISS has a crew of 6, i.e. you will need about 60,000 leaves. There is nowhere to grow trees on the ISS, so most likely these leaves should grow on some small plants, such as, for example, tomatoes.

One tomato bush has an average of 30 leaves, i.e. the ISS will need somewhere to put 2,000 pots of tomatoes. Mark Watney from the movie «The Martian» with his potato plantation nervously smokes on the sidelines.

Photosynthesis requires light, but sunlight obviously cannot be used. Firstly, the ISS is only 50% of the time on the sunny side, and secondly, there are very few windows in the ISS and they are almost always battened down. Almost no sunlight enters the ISS.

Artificial lighting can be used, but this will require additional energy, and besides, it will be necessary to somehow dissipate and remove the heat generated by the lighting elements.

Also, water, soil and nutrients (fertilizers) for plants must also come from somewhere, and I’m not even talking about the fact that the crew will have to devote almost all the time, instead of scientific experiments, to caring for plants.

However, there is a good alternative. In the 60s of the last century, a series of experiments BIOS-1, BIOS-2, BIOS-3 was carried out in the USSR to create closed ecosystems. During the BIOS-1 experiment, using 20 kilograms of chlorella (green algae), it was possible to fully provide a person with oxygen, while the aquarium with algae had a volume of only 8 cubic meters.

In theory, a large enough algae farm could meet the oxygen needs of the ISS or other space station crew. Subsequent experiments, both in the USSR and in the USA (Biosphere-1, Biosphere-2, etc.) showed the fundamental possibility of creating a closed ecosystem self-sufficient in oxygen.

It still makes no sense to introduce reservoirs with algae on the ISS to provide people with oxygen. The Earth is close enough and it is not difficult to replenish oxygen reserves from the Earth, and shifting the care of the crew’s breathing to plants that require careful care and may die is a deliberate decrease in reliability. A chlorella tank is less reliable than a compressed oxygen tank. But for long flights to other planets, as well as for potential colonies on Mars, the Moon, etc. algae can become a truly irreplaceable renewable source of oxygen.