The answer to this question is very difficult, so I can only put forward my superficial views here. When people have left the Earth, they need to have a close, cyclic and repetitive use system. Artificial soil technology possesses that feature. During the astronavigation in the space ship, by using artificial soil technology seeds can grow grain. People and animals can eat grain and drink water as well as breathe oxygen and then they excrete dung and urine as well as carbon dioxide. Microbes can decompose dung and urine into fertilizer and produce carbon dioxide and water. Following, the crops can assimilate nutriments (from fertilizer) and carbon dioxide and water meanwhile they let out oxygen and grow grain again .People and animals can eat grain again and drink water again as well as breathe oxygen again. So the cycle will repeat in that closed system where the community of creature can intergrow, coexist and promote each other until the space ship finishes their missions.
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