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GOING INTO SPACE in search of answers to medical and other questions is, of course, not without risk.
In the movie "2001: A Space Odyssey," dogged by the deranged computer "HAL," astronauts had little time to worry about derangements in body physiology. In reality, however, long-term space travel sequelae such as bone demineralization and muscle atrophy merit the intense study being given to them by the National Aeronautics and Space Administration (NASA).
Four major challenges impose limits on the duration of human space journeys: physiological deconditioning, ionizing radiation, psychological difficulties, and life support.
Deconditioning is a serious concern, says a senior NASA astronaut, William Thornton, MD, adding: "If you go into space for 6 months without exercise, you may expect to be carried off when you get back."
Thornton points up a basic fact: "The heart is nothing more than muscle. It will atrophy without the demands made on it
Smith J. Long Periods in Space Flight May Take Physiological, Psychological Toll Among Crew. JAMA. 1990;263(3):347-351. doi:10.1001/jama.1990.03440030015002