A life-support system provides a comfortable environment for terrestrial life forms, including
human beings, in surroundings hostile to life. Such systems include units for a single organism and large spaces for many organisms. They must be able to function for periods ranging from a few minutes to indefinitely long stretches. The hostile surroundings can range from underwater (see bathyscaphe, submarine, oceanography, and scuba diving) to high mountains, from very hot to very cold conditions (as well as poisonous or infectious environments), all the way to outer space and the surfaces of other worlds. This article discusses life support in space. BASIC REQUIREMENTS At a minimum a life-support system supplies breathable
air, potable water, and nutritious
food, along with conditions that avoid extremes in
pressure, temperature, and radiation. Besides providing all necessary inputs, it processes outputs such as exhaled gases, excreted waste, and heat. Two important characteristics of a system are the duration it must serve and the "consumables"Ñnecessarily expended fresh materialsÑit must provide. For short-term operations, very simple systems can suffice. As missions lengthen, the mass of required consumables grows. It then becomes advantageous to add equipment for recycling some of them. The extent of recycling defines what is called the system's "degree of closure." A simple system has no recycling, whereas the Earth's biosphere is a fully closed system. Space life-support systems fall somewhere between these extremes, depending on the mission and the available technology. Factors influencing the system selected also include weight, power, and cost constraints, reliability requirements, and the degree of acceptable risk. Air The Earth's air pressure at sea level, about 1
kg/cm6 (14
lb/in6), is commonly referred to as 1 bar, or 1,000 millibars, of pressure. The atmosphere consists of a mixture of gases.
Oxygen is a primary requirement for animal metabolism, and the average human consumes about 200 g (0.44 lb) per day. Since oxygen comprises only 20% of the atmosphere, its partial pressure is about 208 millibars at sea level and about 100 millibars at the highest elevations where humans can live comfortably. Providing this pressure range is the main requirement of any spacecraft cabin atmosphere. Some nitrogen is also needed, both to reduce fire hazards and to avoid long-term lung damage from exposure to pure oxygen. Carbon dioxide is the main gaseous waste product of metabolism. The average human exhales about 1,000 g (2.2 lb) per day. The gas must be kept at a partial pressure below 0.01 bar, or it leads to the loss of blood's oxygen-carrying ability. For health reasons air humidity should be maintained in the 40-60% range. Trace gases and aerosols must also be controlled. Water, Food, and Waste Each day the average human consumes about 1.9 kg (4.2 lb) of water as liquid and 0.7 kg (1.6 lb) in food. About 1.5 kg (3.3 lb) of water is excreted as urine, 0.9 kg (2.0 lb) as exhaled moisture, and 0.9 kg as evaporated sweat. These values do not add up precisely, because some of the water changes form during metabolism. Water is also needed for washing. As for solid food, an average human consumes about 0.6 kg (1.4 lb) dry weight per day and excretes about 0.2 kg (0.44 lb) dry weight of solid wastes. Most of the rest of the food mass is converted into carbon dioxide and waste water.
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