Physico Chemical Life Support Systems
With the exception of the Space Station MIR that incorporates some recycling, life support systems in spacecraft in all manned missions so far has been consumables based and physico chemical.
Physico chemical life support systems take care of the basic necessities of the astronauts by generating the necessary elements by means of physico chemical processes. Oxygen and hydrogen tanks are fed into fuel cells where they produce electricity and water as a by product. Oxygen tanks are carried on board for breathing and carbon dioxide is absorbed in a chemical process with lithium hydroxide canisters. Waste is dumped or stored for analysis back on Earth. Food is taken on board according to the astronautsí wish list in dehydrated or even in natural form.
All these luxuries can of course be afforded because of the short duration of the missions involved. In an eventual manned expedition to Mars, where times are in the order of three years, it would be absolutely impossible to take all consumables on board and store or dispose of the waste generated. For any interplanetary mission it is imperative to develop a life support system that can more or less take care of itself, that is, regenerate or recycle the waste in order to save consumables.
NASA and other scientific institutions have been conducting extensive research on the topic and developing some models of CELSS (Closed Ecological Life Support Systems) .