One Giant Leap For Mankind...
Man In Space
Man's conquest of space has required coping not only with a hostile environment but also with the effects of space travel itself. In leaving the earth, the space traveler leaves a dense, life-sustaining atmosphere and ascends through increasingly rarified air before entering the vacuum of space. During lift-off and reentry into the earth's atmosphere, his body is subjected to vibration and great forces of acceleration and deceleration. While traveling in space, his 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 therefore subject to psychological stresses not encountered on earth. These and other factors must be taken into account to ensure his well-being.
Life Support Systems
Man is accustomed to an atmosphere that is 21% oxygen, 78% nitrogen, and 1% other gases, at a total pressure of 760 mm (29.9 inches) mercury. The earth's atmosphere is simulated for the space traveler by a support system that provides him with 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 inches) 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 inches) mercury inside the cabin as the percentage of oxygen is increased.
Oxygen at partial pressures above 150 mm mercury may be toxic to the human body, and it is certainly so at partial pressures over 760 mm mercury. Increasing the oxygen pressure also increases flammability, and in pure oxygen even human skin will burn. The body's need for an inert gas such as nitrogen, which makes up 78% of £he earth's atmosphere, has not been proved definitely. However, it is clear that the presence of nitrogen 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.
Launching a space vehicle requires sufficient acceleration to escape earth's gravity. For orbital flight, accelerations as high as 6g, or 6 times the force of gravity experienced at the earth's surface, are required. During reentry to the earth, peaks of lOg to 13g may be reached. Man cannot withstand these forces either standing or sitting, but he can withstand them in the supine or prone position. For this reason the astronaut couch was developed for 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.
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.
Man has evolved in an environment where his body weight depends on the gravitational attraction between the mass of his body and that of the earth. On the moon, where the gravitational force is Ve that of the earth, a man's weight is only Ve of his weight on earth. On Jupiter, where the gravitational force is over 2.5 times that of the earth, a man would weigh over 2.5 times as much as he did on earth. On Mars a man would weigh just a little more than % of his weight on earth. An orbiting body experiences no gravitational force at all 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. Man's 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, a new "space adapted' state will exist that will put the space traveler at a considerable disadvantage when he returns to his native planet after a long stay in space.
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 doughnut-shaped Van Allen 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, which requires a very high dose. Doses above 100 rads cause changes in the digestive system and in the formation of blood, which may cause death. Death from these causes would occur within one month. If the dose is small or protracted, the effect will be a significant life shortening.
Low-inclination orbits (orbits at a small inclination to the equator) at 230 miles are subject to the least radiation, while polar and synchronous orbits and 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/cm2 (0.2 ounce/inch2) 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.
The problems of performing for a long time alone or in small groups are complex. Information gained by simulation of the space environment is indefinite. 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 will have to be determined and procedures to select psychologically stable astronauts determined before man can undertake a journey to Mars, which may take a year.
The Mercury and Gemini and Apollo missions provided a significant bench mark in the step-by-step approach to building the capability for long flights in orbit or in deeper space. The data on inflight and post-flight physiological performance, in particular, lend a note of optimism concerning man's ability to survive in space for prolonged periods of time. No firm evidence of long-lasting physiological impairment or deterioration has been reported. The records of heart rate and blood pressure during the flights show no significant deviation from expected results or from simulation data. However, some questions concerning certain aspects 'of heart action and blood circulation have been raised in reports of Russian flights. These reports have also expressed concern about the possible effects of dizziness and loss of equilibrium experienced while the body is weightless. These physiological effects of space travel have not been confirmed by American flights.
During flight and especially during extravehicular activity there is evidence of physiological changes such as marked weight loss, fast heartbeat, lowered blood pressure when changing from a reclining to an erect position, and changes in the blood. The effects on the circulatory system are reversible, and the return to normal occurs within five days. There is also an apparent loss of calcium, fluid, and muscle mass. These physiological changes have been termed deconditioning phenomena, but more appropriately they might be termed aspects of a space adaptive state.