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. These open-loop life support systems are very useful and efficient for short-duration space missions. As space missions get longer, however, the supply load gets heavier and becomes prohibitive. Therefore it will be essential to recycle consumables and, consequently, to introduce closed-loop life support technologies on future long-duration space missions. The selection of a suitable life support system in space depends mainly on the destination and duration of a mission and the available technologies.
Closed-loop technologies that provide regenerative functions can use physicochemical and/or biological processes. Systems that include both physicochemical and biological processes are called hybrid life support systems. Physicochemical processes include the use of fans, filters, physical or chemical separation, and concentration. Biological or bioregenerative processes employ living organisms such as plants or microbes to produce or break down organic molecules. Physicochemical processes are typically well understood, relatively compact, and low maintenance and have quick response times. However, these processes cannot replenish food stocks, which still must be resupplied. Biological processes are less well understood. They tend to be large volume and power-and maintenance-intensive, with slow response times, but they have the potential to provide food.
Whereas the water and oxygen loops of a life support system can be closed through the use of both physicochemical and bioregenerative processes, the carbon loop (most human food is based on carbon compounds) can only be closed by using biological means. If all three loops are closed using bioregenerative means, a closed ecosystem is obtained. In this type of closed life support system, all metabolic human waste products are regenerated and fresh oxygen, water, and food are produced.
Designing a Closed Ecosystem for Space Missions
Engineering a scaled-down version of the complex terrestrial biosphere into a spacecraft or planetary colony is a difficult task. An efficient biological system requires the careful selection of organisms that can perform life support functions while being ecologically compatible with other organisms in the system and with the human crew. In the absence of natural terrestrial forces, maintaining the health and productivity of the system requires stringent control of system processes and interfaces.
Although closed ecosystems in space are theoretically feasible, they will not become a reality in the very near future. This is due to the fact that there are extreme environmental conditions in space, such as microgravity and ionizing radiation. In addition, there is a high degree of complexity, and therefore manifold feedback processes, in such a system.
The main challenges in the design of closed ecosystems are:
- harmonization of mass and energy fluxes;
- miniaturization; and
All of these problems are due mainly to the comparatively small size of space ecosystems. Whereas on Earth almost unlimited buffering capacities exist, these capacities are not available in small artificial ecosystems. For example, a problem area in attempts at miniaturization involves mass flow cycles: the mass turnover of the subsystems (producers, consumers, detruents ) that have to be adjusted because large reservoirs cannot be installed. Thus, turnover processes are accelerated and volume is decreased. Because of the lowered buffering capacity, the whole system loses stability and the capacity for self-regulation, and the danger of contracting an infection increases. Finally, in small ecosystems the equilibrium has to be maintained by monitoring and control systems that detect and correct deviations.
To enable the development of a closed ecosystem in space within the next few decades, a comprehensive research program on bioregenerative life support systems has been established by the National Aeronautics and Space Administration (NASA). The site of this research program is the BioPlex facility at the NASA Johnson Space Center in Houston, Texas. Bioplex is a ground-based life support test facility that contains both physicochemical and bioregenerative systems. Long-duration tests with humans are to be conducted in the future.
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