Closed Loop Life Support System

Closed Loop Life Support Systems are environmental control systems which maintain a livable environment in a sealed enclosure, such as a spacecraft, and fully recycle all supplies used in that environment. All materials used by the life support system must be contained within the environment and mechanism. These systems are referred to as 'closed loop' in reference to flow charts detailing the theoretical flow of materials through such a system. While modern spacecraft and submarine life support systems have become quite efficient at maintaining livable environments and recycling certain materials (such as water), to date only one type of closed loop life support system has matched the performance of man-rated systems already in use: Controlled Ecological Life Support Systems, or CELSS.

CELSS's employ living organisms, such as plants and bacteria, as part of their recycling process. These systems attempt to maintain an ecology in miniature, taking advantage of natural cycles to provide food, water, and other supplies to the crew using their processed waste as a material source. CELSS's typically use a greenhouse, compost pile (see soil for a brief discussion of compost piles), and other biologically based processing systems instead of disposal or chemical alternatives. Because it can theoretically be started from seed stock which can be imported at low mass, some type of ISRU augmented CELSS is generally favored for long term life support of a Martian Colony. However, fully operational CELSS's tend to be more massive than other life support systems.

Open Loop Life Support Systems remain a viable alternative to closed loop systems. However, these cannot function indefinitely without a source of supplies, such as would be available from in situ resource utilization (ISRU). Closed loop life support systems theoretically only lose material to leakage, and can therefore function far longer on the same amount of supplies than an open system.