Environmental Factors The atmosphere, pressure, and temperature of
spacecraft interiors are always strictly controlled in
order to avoid serious or even fatal health hazards such as explosive decompression of the craft, the onset of decompression sickness (see bends), carbon dioxide narcosis, hypoxia, and other such problems. All materials used in or brought aboard
spacecraft are tested beforehand for the potential release of toxic substances when in the spacecraft environment. As of the late 1980s, seven human fatalities could be attributed to errors or malfunctions in life-support systems. Three were U.S. astronauts: Gus Grissom, Roger Chaffee, and Edward White, who died on Jan. 27, 1967, in the Apollo 1 spacecraft as the result of a fire during a ground simulation. As a result, the use of a pure oxygen atmosphere during launch and ascent was abandoned by the U.S. space program. A Soviet cosmonaut, Valentin Bondarenko, had died in 1960 under similar circumstances on the ground. Three cosmonauts, howeverÑGeorgy Dobrovolsky, Vladislav Volkov, and Viktor PatsayevÑdied in space, on June 30, 1971, when a valve for equalizing air pressure in their Soyuz 11 spacecraft opened during descent and all their air quickly leaked out. An important concern is the
radiation encountered in space, since excessive exposure to such radiation can result in a greater likelihood of developing certain kinds of cancer. A crew's exposure to radiation depends on many factors: the type and length of the mission, the amount of shielding on the spacecraft, the relative altitude of the craft's orbit, and activity on the Sun during the period of flight. The average skin radiation dose received on the Apollo missions ranged from 0.16 to 1.14 rads (see radioactivity), which is less than the dose received with some diagnostic X-ray procedures. The effect of altitude is illustrated by comparing the third, 84-day U.S. Skylab mission with the 365-day mission spent aboard Mir 1 by a Soviet crew. The Skylab crew received a dose of 17.85 rads to the skin, whereas on the low-altitude Soviet mission the crew received a dose of 10.81 rads to the skin. The career limit for radiation exposure placed on U.S. astronauts varies with age and sex, ranging from 100 rems for younger women to 400 rems for older men, but these limits may be revised sharply downward to half those amounts. (One rem roughly equals one rad, but it is a more biologically sophisticated expression of the amount of any ionizing radiation absorbed by the body that is equal to one roentgen of X-rays.) Another environmental concern is that of the natural circadian (24-hour) cycle of human body rhythms (see biological clock). Although it is best to maintain these rhythms by keeping U.S. crews on Houston time and Russian crews on Moscow time, special space activities, such as the launching of certain satellites, require that the crews do a "circadian shift" out of their accustomed sleep/wake cycles. To encourage their circadian rhythms to shift, U.S. crews use the "bright lights" method a few days before launch. This entails exposing themselves to bright, Sun-like fluorescent lights in their crew quarters for several hours, followed by complete darkness, to achieve the desired sleep/wake schedule.