But there were other questions raised by the unknowns of space environment. Man in space would be absolutely dependent upon an artificial environment.  One Soviet author described the life support system as "a set of engineering, physical-chemical and medical-biological resources" that will "satisfy the needs of man for oxygen, food and water" in order to create "normal living conditions for man in a flight vehicle."9 The atmosphere on earth is a mixture of 80 percent nitrogen and 20 percent oxygen, with small quantities of water vapor and carbon dioxide, plus traces of other gases. Since the astronaut would continually breathe oxygen and generate carbon dioxide and water vapor, the spacecraft needed devices to replenish oxygen and to eliminate excess carbon dioxide. While both the Soviet and American engineers removed carbon dioxide and humidity by using lithium hydroxide canisters, they approached the problem of oxygen supply differently.
The Soviets decided upon a cabin pressure equal to about one atmosphere (760 millimeters of mercury [mm Hg]) and an 80/20 nitrogen-oxygen composition, which would be essentially the same as on earth. The Americans adopted a cabin pressure of 258 mm Hg, or the equivalent of approximately 1/3 atmosphere, and elected to use a pure oxygen environment. While the Soviet system had the advantage of simplicity and minimal danger from fire (always present with oxygen), it had the disadvantage of exposing the cosmonaut to potential decompression should he have to switch to his space suit life support system in an emergency. American cabin and suit pressures were similar, so that a switch from cabin to suit system oxygen would not subject the crew to the "bends." Astronauts were required to prebreathe oxygen prior to launch to remove the nitrogen from their blood streams, reducing the possibility of decompression sickness, or aeroembolism. This absence of nitrogen in the atmosphere also generated the requirement for flameproofing all materials used in the cabin.* 10
Soviet and American technicians also differed in the manner by which they replenished spacecraft oxygen. There are three ways to store oxygen - as a high-pressure gas; as a cryogenic fluid; or as a solid, chemically combined with other elements. Storage as a gas requires strong, high-pressure tanks, which are heavier than the oxygen with which they are filled. Liquid oxygen can be stored in lighter and smaller tanks than those required for gaseous oxygen, but it must be kept very cold, below 90 kelvins (-297° F); this would require special thermal insulation. Chemical systems that release oxygen upon contact with carbon dioxide and water vapor have three drawbacks - weight; volume; and variable performance, based upon a number of factors,  such as the crewman's metabolic rate, cabin temperature, and humidity.11
To replenish cabin oxygen, Soviet environmental control system designers selected a "chemical bed" system based upon alkali metal superoxides, which liberate oxygen as they absorb moisture and form more alkali, which in turn absorbs carbon dioxide. Despite the lack of precision control and the amount of space required for the apparatus, the Soviets favored the chemical bed because it eliminated the problems encountered with high-pressure bottles for gas and the precise temperature controls required for liquid oxygen. In the U.S., John F. Yardley, John R. Barton, Richard S. Johnston, and Faget were successful in arguing for pure oxygen atmosphere at a pressure of 258 mm Hg, since it met the weight and volumetric requirements imposed by the design limitations of the Mercury spacecraft. Although the development of spherical pressure bottles for gaseous oxygen was a challenge, the American designers felt that the effort was justified by reliability.12 A key goal of Project Mercury engineering was reliability, to be established through use of proven concepts, redundant systems, and extensive testing. Soviet and American engineers selected an environmental control system that satisfied their respective design goals and criteria for reliability.
* Since life has evolved in the 80/20 nitrogen-oxygen atmosphere, over long periods of time breathing undiluted oxygen at sea level pressure (760 mm Hg) can be toxic. Toxicity diminishes as the pressure is reduced, and when pure oxygen is breathed at a pressure approximating the partial pressure of oxygen at sea level (181 mm Hg), there are no detectable adverse effects. For this reason, American engineers chose 258-mm-Hg pressure for use in Mercury, Gemini, and Apollo spacecraft.
9. Umansky, Chelovek na kosmicheskoy orbite, p. 49.
10. Ibid., pp. 50-54; Faget, Manned Space Flight, pp. 98-100; Swenson, Grimwood, and Alexander, This New Ocean, pp. 231 and 558, note 21; and Eugene B. Konecci, "Soviet Bioastronautics - 1964," paper, National Space Club, Washington, 15 Dec. 1964, pp. 4-7. Konecci summarizes the comparative advantages and disadvantages of sealevel and 258-mm-Hg cabin atmospheres.
11. Umansky, Chelovek na kosmicheskoy orbite, pp. 50-51; and Faget, Manned Space Flight, pp. 100-102.
12. Swenson, Grimwood,
This New Ocean, pp. 225-233, discuss
development of the Mercury environmental control system; Frank H.
Samonski, Jr., Technical History of the
Environmental Control System for Project Mercury, NASA Technical Note D-4126 (Langley, Va.,1967); and
interview (via telephone) Samonski-Ezell, 21 Jan. 1975.