![]() A third general feature of crew in space is an increase in height due to the removal of gravitational forces compressing the vertebral column, and the assumption of a fetal position when relaxed. Although some people adapt readily, others do not. In addition, crew show wide variation in their adaptability to conditions of spaceflight and in their subsequent recovery after return to earth. For example, symptoms of space motion sickness, caused by vestibular (inner ear) problems, may appear within the first few days of flight, whereas decrements in red blood cell mass or lean body mass gradually occur over a period of weeks to months. For instance, although some physiological shifts occur very soon after the initiation of spaceflight, others appear more gradually. This article will describe the effects of spaceflight on each system of the body and measures taken to counteract deleterious effects. Effects seen in some flights are not seen in others. Also, the data are often based on a small number of animals. Unavoidable elevated temperatures may have affected some experiments. In animal experiments, delays sometimes occurred between landing and analysis of cells, tissues, and fluids. Examples of such factors are the severe acceleration and vibrational forces that occur during ascent or descent, the psychological stress of a small habitat, the altered physiological patterns of sleep and work, the ionizing radiation of space, the atmospheric composition of the crew's cabins, and the different geomagnetic and electrical fields in space compared those on to earth. During spaceflight, although microgravity per se may have accounted for many of the physiological changes observed, crew experienced other stresses which either concurrently or synergistically could have had effects. Postflight data have readaptation to 1 g superimposed on 0 g effects, so that short-lived effects may be missed or obscured by recovery. Furthermore, with the some medical tests taken on-board Skylab, Mir, and the Space Shuttle, most data are preflight or postflight. In the following discussion, it should be remembered that true weightlessness is not reached during spaceflight microgravity or near weightlessness are probably more accurate terms. Several NASA technical memorandums and articles in the scientific journal Aviation, Space, and Environmental Medicine provide valuable summaries of experiments on these ventures. In the future, collaborative experiments on the International Space System will allow even better opportunities to study human physiology in space. The Skylab-3 mission, flown in 1989, lasted 7 days. The Cosmos series spanned the years 1975–1989 individual flights lasted from 7 to 19.5 days. The Apollo–Soyuz Test Project launched in 1975 was followed by a series of Soviet Cosmos unmanned biosatellites in which both Russian and American experiments have been flown. In 1972, the United States and the former Russia commenced a series of cooperative ventures in space. A few hundred people have now flown in space, for periods of time ranging from a few hours to more than 1 year. These increasingly complex programs resulted in achievements such as manned spaceflights in 19, extravehicular activity, rendezvousing and docking in space, lunar landing and exploration, and experimental orbital space stations. Through the National Aeronautics and Space Administration (NASA), the United States initiated projects Mercury, Gemini, Apollo, and Skylab, the Space Transportation System, and several Life Sciences Space Shuttles. Incredibly, one cosmonaut stayed in Mir for 439 days. In the past decade, the Mir space station enabled physiological studies on the effects of long-term spaceflight on humans. After orbiting several animals and plants in 1960, the former Russia embarked successively on the Vostok, Voskhod, Soyuz, Salyut, and Mir programs. ![]() Since the launching of the unmanned Sputnik in 1957, both the former Russia and the United States have pursued ambitious manned space programs. More than forty years into the space age, we are beginning to learn a great deal about space physiology: the adaptation of humans and animals to spaceflight. Caren, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 I Introduction
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