Past, Present, and Future of Closed Human Life Support Ecosystems - A Review

During the development of human space exploration, the idea of simulating the Earth’s biosphere to provide human life support led to the convergence of space biology and the field of ecology to develop closed manmade ecological systems. A space habitat can be thought of as an ecological system of human beings exchanging energy and material within a spacecraft. In order to understand and control material and energy exchange processes, one must combine basic ecological principles with the knowledge gained through Bioregenerative Life Support Systems (BLSS) research to date. Experimentation in closed manmade ecosystems for spacecraft life support began in the 1960’s and over the years has shown biological life support systems to be realizable. By building on lessons learned, we can identify and prioritize future research objectives. Future development should focus on improved reliability of mechanical components, autonomous ecosystem control, closure of the carbon cycle (food generation and waste recycling), and the maintenance of long-term stability and robustness. A common need identified throughout all closed ecological life support systems (CELSS) research is mass and energy exchange models that enable intelligent autonomous control and design optimization. To validate mass balance models, integrated system level experiments are needed, but full-scale tests are time consuming and expensive. Small closed experimental ecosystems (microcosms) could allow observation of stability limits and response to perturbation with repeated short duration experiments. However, they must have proper scaling to represent larger system dynamics. Thus, the definition of non-dimensional ecological parameters (or invariant system descriptions) that define similarity between biological life support systems of different temporal and spatial scales is a potentially critical yet little studied research area that will enable prediction of mass and energy exchange for various system configurations and operating scenarios.