2. The Effects of Space Travel on Humans
Human travel in space has required coping with not only a hostile environment but also the effects of space travel itself. In leaving the earth, the space traveler leaves a dense, life-sustaining atmosphere and ascends through increasingly rarified before entering the vacuum of space. During liftoff and reentry into the earth's atmosphere, the body is subjected to vibration and great forces of acceleration and deceleration. While traveling in space, the body is weightless. Also, the space traveler leaves the familiar social and physical environment of the earth and works in constant hazard, alone or in a small group, and is thus subject to psychological stresses not encountered on earth. These as well as other factors must be considered to ensure all of the components necessary for the traveler’s well-being.
Life Support System. Humans are accustomed to an atmosphere that is 21% oxygen, 78% nitrogen, and 1% other gases, at a total pressure of 760 millimeters (mm; 29.9 in) mercury. The earth's atmosphere is simulated for the space traveler by a support system that provides the necessary oxygen, and often an inert gas, at a suitable pressure. Oxygen is supplied at a pressure great enough to saturate the red blood cells in the lungs’ air sacs. The red blood cells carry oxygen and distribute it throughout the body. The partial pressure of oxygen entering the lungs' air sacs varies somewhat because of the exchange of oxygen for carbon dioxide, but it is generally about 150 mm (5.9 in) mercury. The life-support system pressure will ideally be that of earth's with mixed gases, but the total pressure may be reduced to 200 mm (7.9 in) mercury inside the cabin as the percentage of oxygen is increased.
Oxygen at partial pressures above 150 millimeters mercury may be toxic to the human body, and it is certainly so at partial pressures over 760 mm (30 in) mercury. Increasing the oxygen pressure also increases flammability, and in pure oxygen even human skin will burn. The body's need for nitrogen, which makes up 78% of the earth's atmosphere, has not been proved definitely. The presence of nitrogen, however, clearly reduces the burning rate of materials inside the cabin.
In addition to providing oxygen at a suitable pressure, the support system must also remove water and carbon dioxide, which are products of respiration; remove contaminants; and maintain the cabin temperature against the extremes of the space environment.
Acceleration. Launching a space vehicle requires sufficient acceleration to escape earth's gravity. For orbital flight, accelerations as high as 6g (6 times the force of gravity experienced at the earth's surface) are required. During reentry to the earth, peaks of 10g to 13g may be reached. Humans cannot withstand these forces either standing or sitting, but they are able to withstand them in the supine or prone position. For this reason the astronaut couch was developed particularly to ease the problem during takeoff or landing, and the spacecraft is specially oriented during reentry.
The body's tolerance to acceleration depends on the magnitude of the force and its duration. The primary effect of increased g is on the circulatory system. At 5g the blood has the weight of iron, and as g increases, blood fills the lower portions of the lungs, displacing air. The exchange of oxygen for carbon dioxide is impaired by an insufficiency of ventilated lung sacs.
Vibration. The rocket motors that launch spacecraft also introduce marked vibrations. Intense vibrations between 1 and 20 hertz (cycles per second) are the most detrimental to the body. Minimal tolerance of the body is at vibrations of from 4 to 6 hertz, which is the natural frequency of the major body cavities. Tolerance times of these vibrations are short at low-g levels. Prolonged subjection to such vibrations may cause tissues to be torn.
Weightlessness. Humans have evolved in an environment where body weight depends on the gravitational attraction between the mass of the body and that of the earth. On the moon, where the gravitational force is 1/6 that of the earth, a person's weight is only 1/6 of his or her weight on earth. On Jupiter, where the gravitational force is over 2.5 times that of the earth, an individual would weigh over 2.5 times as much as he or she did on earth. On Mars a person would weigh just a little more than 1/3 of his or her weight on earth. A person in a spacecraft with the engines turned off experiences the same gravitational acceleration as the spacecraft and is therefore weightless.
Many of the body's systems are adapted to earth's gravity. The musculoskeletal system is adapted to gravity for posture and for the power to move about and to move other objects. The circulatory system is adapted to move blood from the heart to the periphery of the system and back to the heart while the body is in an upright, supine, or prone position. Our sense of balance and sense of movement or position are also oriented to earth's gravity. These systems may adapt over a long period of time to a new environment of weightlessness. If they do make these changes, a "space-adapted" state will exist that will put space travelers at a considerable disadvantage when they return to their native planet following a long stay in space.
Radiation. The space traveler has to be protected against the high-speed atomic particles of cosmic rays and solar flares and the charged particles geomagnetically trapped in the doughnutshaped Van Allen radiation belt that surrounds the earth. These ionized particles are damaging to living tissues. Depending on the dose and the dose rate, there may be immediate effects ranging from nausea to death, the latter requiring a very high dose. Doses above 100 rads (radiation absorbed doses) cause changes in the digestive system and in the formation of blood, which may cause death. Death from these causes would occur within a period of one month. If the dose is small but protracted, the effect will be a significant life shortening.
Low-inclination orbits-orbits at a small inclination to the equator-at 370 km (230 mi) are subject to the least radiation, while polar and synchronous orbits and lunar/planetary expeditions occasion more risk. In a low-inclination orbit the cosmic ray dose would be about 0.01 rad per day with 1.0 gram/centimeter^2 (0.2 ounce/inch^2) shielding. In the Van Allen belt the dose might be 1 rad to 10 rads per hour. In a solar-flare period of three to four days, the dose rate might be 100 rads for the duration. Isolation. The problems of performing for a long time alone or in small groups in space are complex and difficult to assess in testing procedures performed on the earth. Information that is gained in simulation studies is inconclusive. As one astronaut has put it, "In simulation the friendly environment is outside the simulator-in the real flight it is the hostile environment that is outside." Psychological effects of the space environment on travelers will have to be determined and procedures to select psychologically stable astronauts will have to be set before humans can undertake a journey to a planet such as Mars, which would take more than a year.
Physiology. Early crewed flights such as Apollo and, more important, long-duration stays by astronauts and cosmonauts in the Skylab, Salyut, and Mir space stations, established a benchmark in understanding human capacity for long flights in orbit or in deep space. The data on in-flight and postflight physiological performance give strong assurance of human ability to endure in space. No firm evidence of long-lasting physiological impairment or deterioration has been reported. However, there are many temporary effects that have been observed, both during flights as well as on the return to earth.
About half of all astronauts and cosmonauts experience "space adaptation syndrome" in their first few days in space as a response to weightlessness, a condition that includes symptoms such as nausea, vomiting, fatigue, loss of appetite, and loss of knowledge of limb position. Loss of bone mass, bone minerals, and calcium; cardiovascular changes; decreased blood plasma; backache; muscle atrophy; and immune system weakness have been among the effects noted on long-duration flights. The return to earth usually leaves astronauts feeling weak, tired, and light-headed until they readjust to gravity.
Much research has been done on Spacelab life-sciences missions carried by the space shuttle to determine the extent of some of these effects and to find better ways for the human body to adapt to weightlessness and the return to earth. A prime objective of the International Space Station will be to continue this research and to develop countermeasures to the physiological effects of a long-duration spaceflight and return to earth.