the artificial gaseous medium in the closed, hermetically sealed space of a spacecraft’s cabin. The cabin atmosphere is optimum for man when it corresponds exactly in physical properties and chemical composition to the earth’s atmosphere.
The cabin atmosphere may consist of only one gas, such as gaseous oxygen at a pressure of 33–56 kilonewtons per sq in (kN/m2; 1 kN/m2 ≈ 7.5 mm Hg), or of several gases (O2, N2, CO2, and so on). The advantage of a single-gas cabin atmosphere is the somewhat lower possibility of decompression disorders and the lesser effect of cabin depressurization when astronauts go out into space or onto the surface of a celestial body. But when the single-gas cabin atmosphere is in use, the pressure of oxygen must be higher than its partial pressure in the earth’s atmosphere, and this increases the fire hazard. In addition, the temperature regulation system is made more complicated with a single gas. In cases of prolonged exposure (more than two to three weeks) to a single-gas atmosphere, some disturbances of human physiological functions are observed which reduce the organism’s resistance to the effects of space flight so that the use of such an atmosphere is inadmissible for a long flight.
A multiple-gas cabin atmosphere has a number of advantages at normal barometric pressure. However, during long space flights in such an atmosphere some deviations from normal earth atmosphere may occur. The permissible fluctuations of the total barometric pressure in a cabin are from 40 to 120 kN/m2. The partial pressure of oxygen should be between 20 and 40 kN/m2; if it falls below 20 kN/m2, it may bring on signs of oxygen starvation, a reduction of the organism’s resistance, an unfavorable reaction to the effects of space flight, and reduced working efficiency of the crew. A pressure increase above 40 kN/m2 may cause changes in the respiratory organs and may also reduce the organism’s resistance. The partial pressure of carbon dioxide should not be more than 1 kN/m2, corresponding to a volume concentration of 1 percent (at normal barometric pressure); increasing the concentration may cause a negative reaction in the organism. The physiological importance of nitrogen for the living organism has not yet been sufficiently explained. Eliminating nitrogen from the cabin atmosphere produces a reduction in the total barometric pressure, with corresponding harmful consequences for the organism.
Replacing nitrogen with another inert gas such as helium (which is seven times lighter and more heat-conducting), thus permitting an increase in the cabin temperature and a reduction in the power of the heat regulating system, seems to be a promising method. However, helium is more fluid than nitrogen, which complicates the control of cabin leaks. The possibility of a man existing for a short time (up to ten days) in a helium, or, more exactly, a helium-oxygen, medium, has been demonstrated experimentally. The relative humidity of a cabin atmosphere should be maintained within the limits of 30 to 70 percent at a temperature t = 20 ± 1° C, with a gas flow velocity of no more than 0.2 to 0.3 m/sec and a rate of change of pressure during the regulation and other processes no greater than 300 N/ (m2sec), or 2 mm Hg per sec. All the physical properties of a cabin atmosphere and its chemical composition are maintained by a life-support system.