Space Medicine Space medicine is the branch of medicine involved in protecting human beings from the environment of space and, at the same time, studying their reactions to that environment. The foundations of space medicine can be traced back to aviation medicine, and the term aerospace medicine has evolved to encompass activity in both areas. Aerospace medicine has been a certified subspecialty of the American Board of Preventive Medicine since 1953. In addition to physicians, however, engineers, veterinarians, dentists, nurses, physiologists, bacteriologists, toxicologists, pharmacologists, and biochemists also work in the field of space medicine. Specialists in space medicine are involved in all aspects of spaceflight, from spacecraft design and crew selection to flight operations and postflight review. EARLY HISTORY The early history of spaceflight was characterized by deep concern on the part of many scientists that humans would not be able to withstand the rigors of spaceflight, especially during launch and reentry, and might not be able to function usefully in space. As a result of this concern, the United States flew a number of monkeys aboard captured German V-2 rockets between 1948 and 1952. These early flights provided some launch and reentry data on comparable life forms and demonstrated the need for effective and reliable LIFE SUPPORT SYSTEMS. Between 1949 and 1956 the Soviet Union flew 15 similar flights using dogs, some flights reaching an altitude of 213 km (132 mi). A dog named Laika was then launched aboard SPUTNIK 2 on Nov. 3, 1957, and orbited the Earth for seven days until she was finally euthanatized. Between 1958 and 1961 the United States flew monkeys in several suborbital and orbital missions. These flights showed that pulse and respiration rates, blood pressure, and performance of specific tasks for which the monkeys had been trained were basically unaffected by spaceflight. On Apr. 12, 1961, the Soviet cosmonaut Yuri GAGARIN demonstrated for the first time that human beings could safely orbit the Earth. His flight confirmed a U. S. decision to use humans rather than animals on succeeding spaceflights. The U. S. program proceeded cautiously, however, by gradually increasing the exposure time of humans in space and by carefully observing and reviewing the effects of each flight. The six U. S. astronauts who flew on Project Mercury (see MERCURY PROGRAM) between May 1961 and May 1963 returned in good health, and as a result, many of the earlier medical concerns about spaceflight were dispelled. ENVIRONMENTAL AND PHYSIOLOGICAL EFFECTS The physiological functions of human crews in space have been measured by a variety of means over the years, ranging from simple sensors placed on the body to monitor heart rate and respiration, to the use of techniques such as echocardiography and electroencephalography. The many thousands of hours of information thus gained on human activity in a weightless environment have proved that people can perform assigned tasks well and need suffer no permanent postflight physiological changes, even after missions lasting several months. To maintain an acceptable state of health, however, space crews do require an appropriate atmosphere, adequate food and hygiene facilities, exercise, a proper balance between work and rest periods, and sufficient time to acclimatize to space and also to the return to Earth. 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, however-- Georgiy Dobrovolsky, Vadim Volkov, and Viktor Patsayev--died 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 84-day American Skylab 4 mission with the 237-day mission spent aboard Salyut 7 by a Soviet crew--a stay since exceeded in length. The Skylab crew received a dose of 17.85 rads to the skin and 7.29 rads to the blood-forming organs on their high-altitude flight, whereas on the low-altitude Soviet mission the crew received a dose between 3.08 and 5.5 rads. It is anticipated that on the planned U. S. space station, the radiation dose is likely to amount to a significant 35 rads a year. The career limit for radiation exposure that has been placed on U. S. astronauts is expressed in terms of rems. (One rem is roughly equivalent to one rad, but it is a more biologically sophisticated expression of the amount of any ionizing radiation absorbed by the body that is equal in biological effect to one roentgen of X rays.) This limit varies with age and sex, ranging from 100 rems for younger women to 400 rems for older men. Another environmental concern is that of the natural circadian (24-hour) cycle of human body rhythms These rhythms are maintained in the U.S. program by keeping the crews on Houston time, and in the Soviet program by keeping their crews on Moscow time. The Soviet Union has attempted to alter the circadian rhythms of their cosmonauts by having them awaken 20 minutes earlier on each day during a long-term flight, but such efforts have not proved successful and are no longer tried. Short-Term Physiological Effects In order to deal with the effects of increased gravity loading (g loading) during the acceleration of launch and deceleration of reentry, crews in both U. S. and Soviet spacecraft generally have been placed so that the g loads are experienced in the chest-to-back position rather than the head-to-foot position. The human body can withstand a much higher g loading in the former mode. The g loads themselves, which have varied from 3 to 8.2 times the gravity experienced at the Earth's surface, have created no problems. In the U. S. Space Shuttle, however, crews take a reentry decelerative force of about 1.5 g in feet-first position, which causes blood to push toward the feet. In order to prevent the pooling of blood, Shuttle crews are therefore provided with suits that produce pressure on the lower part of the body. Physiological changes start to occur from the first moment of achieving orbit. Body fluids redistribute themselves toward the head. As they do, the face puffs out, and some astronauts experience sensations similar to those of a head cold. The intestines also tend to float upward, and as a result an astronaut may lose as much as 10 cm (4 in) in girth. The curve of the spine also straightens out somewhat, so that persons may gain an inch or so in height while in a weightless environment. Many astronauts have experienced a mild lower backache as a result of the different stresses being placed on the back muscles in space. The vestibular system consists of the otolith and the semicircular canals of the inner ear which constitute the body's organs of balance. When they are disturbed, MOTION SICKNESS can result. In space, this is known as "space adaptation syndrome". Its symptoms include pallor, cold sweating, mild dizziness, stomach nervousness, nausea, and vomiting. The symptoms can occur as early as an hour after launch or as late as the second day of flight, and they can persist for as long as four days. Approximately half of the male astronauts and cosmonauts have experienced these symptoms, and about 10 percent of the females. The causes of the syndrome are not yet known, and treatments thus far appear to have been either ineffective or only marginally effective. Because the symptoms disappear shortly, however, the disorder is adaptive and self-limiting. Other physiological changes associated with short-term spaceflight include some cardiac deconditioning and a decrease in the volume of body fluids. These changes return to preflight norms within a short time. Effects of Long-Term Spaceflight Exposure to weightlessness for time periods exceeding about two weeks results in degenerative physiological effects similar to those of prolonged bedrest on Earth. If left unchecked, these changes could result in severe and perhaps even dangerous weakness. Because of this, several countermeasures are employed on long-duration flights to arrest the body's adaptation to the effortless weightless environment and to keep it fit enough to return safely to Earth. For example, certain muscles tend to atrophy through lack of use during weightlessness, and animal tests show that muscle tissue can also lose blood vessels and nerve associations. Studies also suggest that deterioration in muscle tissue may be accompanied by decreases in metabolic efficiency. An increase in food intake, combined with vigorous inflight exercise using a variety of equipment, can at least partially counteract muscle deterioration. The redistribution of body fluids during weightlessness causes the body to eliminate some fluids in the urine. Tissues dehydrate to some degree, and the volume of blood plasma drops by about 10 percent, and the shape of the cells also changes; their function, however, is not impaired. In fact, a red- blood-cell mass appears to stabilize and recover after about 60 days of weightlessness, in most cases. These changes appear to be normal body adaptations to space conditions; countermeasures include increased water intake and exercise during the flight. After the return to Earth, body fluids return to preflight norms rapidly and blood counts return to normal within a few weeks. The shift in body fluids and the decrease in blood volume, along with the absence of gravity, also reduce the size and pumping capacity of the heart. Exercise and adequate fluid intake are, again, the usual countermeasures used to avoid serious cardiac and circulatory problems. Most cardiovascular responses return to normal within a few weeks. Loss of calcium from the weight-bearing bones of the body is apparently continuous throughout a flight. The loss is significant, ranging in various individuals from 0.5 percent to 1.5 percent total body calcium per month. Although several countermeasures have been tried, none have halted the decalcification process; exercise, however, does appear to slow down the rate of loss. Postflight recovery of lost calcium is a lengthy process, taking at least as long as the flight itself. Some bone loss may be irreversible, particularly in the beamlike network (trabeculae) of spongy BONE. On very-long-duration Soviet flights, such changes have been observed as a rise in steroid hormones, damage to the T-lymphocytes, heightened sensitization to allergens, and increased vulnerability to staphylococcus and streptococcus infections. Such changes indicate possible changes in the body's immune system over long periods of time. Finally, some indications exist that drugs commonly used to treat illnesses on Earth may not be absorbed properly by the body in weightless conditions. In addition, many drugs can have unpredictable and unwelcome side effects and can even lose their effectiveness in space. Special Hazards and Emergencies Extravehicular activity (EVA), or spacewalking, poses a special health hazard. The astronaut or cosmonaut must rely on the perfect functioning of a spacesuit, and may have to experience changes of atmosphere and pressure between spacecraft cabin and spacesuit that can result in decompression sickness. In situations where cabin and spacesuit atmosphere and pressure are quite different, as in the U. S. Space Shuttle, lowered cabin pressure and prebreathing of oxygen prior to EVA is required. Few clear-cut medical emergencies have thus far occurred in space, although various medical problems have been indicated. For example, the U. S. astronaut James IRWIN experienced heart arrhythmias while walking on the Moon in 1971. Believed at the time to have been caused by an electrolyte imbalance, the arrhythmias may in fact have been an early symptom of the heart disease manifested by Irwin a few years following his Moon flight. The first known real medical emergency in space occurred when the Soyuz T14 crew cut short their mission and returned to Earth on Nov. 12, 1985. The commander, Vladimir Vasyutin, had developed an "acute inflammatory infection" several weeks beforehand and had not responded to treatment with onboard drugs. The infection was accompanied by very high fever, insomnia, and intense irritability, and Vasyutin was relieved of command shortly before the crew's return. Once on Earth, he was hospitalized for a month. The exact type of infection was not officially disclosed, but the widespread opinion is that it was prostatitis. More recently, on July 30, 1987, another Soviet cosmonaut, Aleksandr Laveikin, was returned to Earth earlier than planned as a result of unexplained electrocardiogram readings that were presumably arrhythmias. PSYCHOLOGICAL EFFECTS The psychological adaptability of humans to spaceflight conditions has been of great interest to the space programs concerned. Soviet scientists, in particular, have studied psychological matters in some detail during the lengthy occupations of their space stations. Their studies have included preflight crew-compatibility testing, voice stress analysis, and the elaborate psychological support of crews during actual flights. Several stages of psychological adaptation to spaceflight may be elaborated. They include the period of intense and lengthy preflight training and testing; the period of heightened anxiety prior to and during launch; the approximately monthlong period of adaptation to weightlessness and the establishment of an effective work-rest routine; the following period of midflight depression and fatigue that comes approximately four months into a flight; the anticipation of the flight's end (called "breakaway"), including excitement and uneasiness; the exhilaration and discomfort felt upon first return to Earth; and, finally, reintegration into the setting of family and accustomed routines. In a few cases--most notably some of the Apollo astronauts--crew members have exhibited a reassessment of their lives and changed direction after spaceflight. A LOOK TOWARD THE FUTURE Both the United States and the Soviet Union are seeking to develop permanent low-orbit space stations. Workers in space medicine are therefore focusing increased attention on the most crucial health issues involved: bone decalcification, changes in the immune system, heart muscle deterioration, and long-term exposure to radiation. Animal experiments and studies of drug effectiveness in space will form an important part of such research. In addition, since both nations have expressed an interest in the possibility of interplanetary flights in the farther future, health problems relating to colonization of the Moon and exploration of Mars and its moons will undoubtedly play a larger part in future space-medicine programs. Such problems include the development of crew-selection criteria for flights of very long duration, and the maintenance of mental acuity and physical well-being during such prolonged periods of relative SENSORY DEPRIVATION. Maintaining the social health of small-scale societies on space stations and on severely isolated extraterrestrial outposts will present unique challenges to all concerned.