Thermodynamically speaking, humans as living creatures are open systems. To maintain their physical structure, people exchange matter and energy with their environment. Humans live in a closed terrestrial life support system known as the biosphere. The biosphere is a basically closed system in terms of matter but an open system in terms of energy. For spaceflight purposes, the goal is to develop techniques to ensure the biological autonomy of humans when isolated from the terrestrial biosphere.

Life Support Systems

Onboard any spacecraft, space station, or planetary base, a controlled and physiologically acceptable environment for the crew is provided by a life support system. The traditional components of life support are air, water, and food. Since the beginning of human space missions, these supplies have been launched from Earth along with the crew or sent as dedicated supply missions. Waste was typically stored and returned to Earth...