The Biosphere 2 experiment was an experiment on constructing a self-sufficient closed habitat. Even though it is only about the size of two and a half football fields, it is still the biggest closed system ever created.
Biosphere 2 contained several biomes: A tropical rainforest, small ocean with a coral reef, a mangrove wetlands, a savannah grasslands, and a fog desert. There was also an agricultural area, a human habitat, and below ground-level technical infrastructure. The entire facility is airtight, and is powered by an external natural gas energy center. Four men, four women, and plants and small to mid-sized animals belonging to their respective ecosystems were sealed inside. The experiment was aborted after two missions because the "crew" became bad-tempered, there were financial disputes, and various other difficulties.
Even though for many years, the mainstream interpretation was that all closed habitats were psychologically doomed to fail, probably one of the biggest reasons that Biosphere 2 failed is because oxygen seemed to keep "disappearing" from the habitat. Many suspected the severe drops in oxygen levels to be due to microbes in the soil. If respiration were the sole cause of the drop in oxygen levels, then a corresponding increase in carbon dioxide should have occurred. This was not the case. Later, an investigation proved that concrete inside the structure was reacting with carbon dioxide to form calcium carbonate. As the oxygen was part of the CO2, the oxygen seemed to disappear.
When the oxygen level dropped to the same level as 5500 meters above sea level, the "crew" developed hypoxia, a condition in which the body is deprived of adequate oxygen supply. This caused the crew to become bad tempered, and when the situation became very unstable, the experiment was abandoned.
Why is this important to space colonization?
Even though the experiment was a failure, it was an important milestone in creating a closed habitat. If we ever want to put a base on the Moon (or Mars if we don't terraform the planet first), we will need a closed habitat like Biosphere 2, but without the problems.
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