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Hazards of Space Flight
Man In Space
Man has evolved in the sheltered environment of the planet earth, where a dense atmosphere protects and sustains him. The environment in space, however, far from being protective, is extremely hostile. Its vacuum, temperature extremes, and radiation will kill a man unless he has special protection. Furthermore, space travel itself creates adverse conditions, such as great acceleration, high temperatures, weightlessness, and vibration. The present section describes the space environment and its physiological and psychological effects on man, and discusses the various crew support systems necessary for coping with this environment.
The Emptiness of Space
On the earth, man lives at the bottom of a dense atmosphere that fulfills many functions. It contains oxygen, which sustains life. It provides a pressurized gas environment that balances the pressure inside each body. It forms a protective shield against most harmful radiation and meteoroids. Its circulation moderates the earth's temperature; without it the earth would experience intolerable extremes of heat and cold. It is an essential link in the water cycle, for it carries the water vapor that has evaporated from rivers, lakes, and oceans and from plants; the vapor condenses and falls back to earth, resupplying it with water and making the processes of life possible. Finally, it scatters sunlight, producing a diffused illumination.
In space, there is no atmosphere to fulfill any of these functions. There are almost no gases, and thus the pressure to which man is accustomed does not exist. Above altitudes of approximately 18,000 meters (59,000 feet) the external pressure is so low that the body fluids of a man exposed to it would boil, damaging the living tissues and rapidly causing death. The amount of oxygen in the atmosphere also decreases with increasing altitude. Above approximately 10,500 meters (34,500 feet) there is insufficient oxygen for man's body functions.
From the surface of the earth, the sky appears blue and evenly illuminated because the sun's light is scattered by the atmosphere and the dust particles it contains. Above the atmosphere, however, the sky appears black, even in the daytime. To an astronaut the sun looks like a bright disc and the stars shine more brilliantly and steadily than when viewed from the earth. In space, the stars do not appear to twinkle, because there is no atmosphere to cause the variations in light intensity that are seen by observers on earth. (Space photographs taken by astronauts do not show a starry background because the cameras are adjusted to the intense illumination of the objects being photographed. At such settings the stars (mere pinpoints of light) do not affect the film enough to produce an image.)
In space, external pressure can be provided within either a special space suit or a pressurized cabin. The cabin must, of course, be airtight, and equipped with certain safety features in case a leak, such as might be caused by collision with a meteoroid, should occur. There must be a device to detect any escape of cabin air and consequent loss of pressure, and devices to locate and repair the leak if it is in an accessible location. There must also be a reserve supply of air for increasing the cabin pressure after a cabin depressurization. In addition, pressurized, air-conditioned space suits must be available to ensure the survival of the crew even if the cabin pressure should be lost permanently. For entering and leaving a spacecraft, a double-door airlock arrangement is used.
Space suits are also necessary for excursions outside the spacecraft, for such purposes as repair and maintenance, data gathering, and lunar exploration. The space suits must be flexible, lightweight, and capable of withstanding high accelerations. They must be pressurized and air-conditioned, and the humidity of the air within them must be closely controlled. Astronauts can carry their oxygen supplies, pressurizing equipment, and air conditioners on their backs. They may also carry communications equipment and a small rocket device for propulsion during short walks in space.
Any lack of atmospheric pressure may also affect parts of the spacecraft itself. All liquids aboard a spacecraft, such as propellants, lubricants, drinking water, and refrigeration fluids, must be pressurized at all times to prevent their boiling. External surface coatings, certain paints, and the lubricants in external bearings also evaporate if exposed to the vacuum of space. Thus, great care must be exercised in the choice of materials and in the design of the spacecraft.
Several different kinds of radiation are found in space. Some of them, such as ultraviolet rays, gamma rays, and X rays, are true radiations that are propagated by means of wave motion. Others, such as the so-called cosmic rays, are not true radiations, but consist rather of streams of atomic particles moving at high speeds. However, such streams of particles sometimes have effects similar to those of radiations. Of these different kinds of radiation, some pose only slight problems for space flight. Others, however, constitute major hazards for spacecraft and their occupants.
Ultraviolet radiation is a form of short-wavelength radiation that is emitted by the sun. It causes sunburn, and can cause severe damage to the eyes. On earth, most of the sun's ultraviolet radiation is filtered out by the atmosphere. In space, however, exposure causes the human skin to burn from 10 to 50 times as fast. Fortunately, it is easy to protect the crew of a spacecraft against ultraviolet radiation. The normal cabin wall will be an adequate shield, and windows can be made of materials that filter out ultraviolet rays. Intense ultraviolet radiation can also cause serious damage to paints and other surface coatings, upsetting the heat balance of the spacecraft. Therefore, the engineers designing the craft must choose materials carefully.
A considerable amount of X radiation is encountered in space, although it is of low intensity and the walls of spacecraft cabins should, provide adequate shielding from it. Of greater concern are the secondary X rays that are generated by the impact of cosmic ray and solar-flare particles on the spacecraft.
Cosmic Rays and Solar Flares
The so-called cosmic rays are actually atomic nuclei traveling at very high speeds. Most of them are protons, the nuclei of hydrogen atoms. From one-tenth to one-fifth are helium nuclei, and a very few are the nuclei of heavier atoms, such as those of magnesium, calcium, or iron. The origin of cosmic rays is not known, but is believed to be outside the solar system. Since the nuclei have all been stripped of their planetary electrons, they carry a positive electric charge and therefore can be deflected to a certain extent by the earth's magnetic field. This magnetic field is such that cosmic rays can penetrate the atmosphere much more readily near the North and South poles than near the equator. However, the protective atmosphere in any case prevents most of the heavier particles from penetrating regions below an altitude of about 16 kilometers (10 miles).
The sun also emits large numbers of particles, consisting mostly of high-speed, high-energy protons. At certain times, during periods of sunspot activity, tremendous eruptions called solar flares occur on the sun, and the emissions become very intense. Between 1957 and 1960 there were 30 major solar flares, most of them having lifetimes of at least a week. Sunspot activity seems to occur in cycles of 11.5 years.
Cosmic rays and the particles emitted by large solar flares are similar in their effects on living creatures. Secondary particles are created by the impact of primary particles on a material. When a high-speed cosmic-ray or solar-flare nucleus enters the atmosphere, the wall of a spacecraft, or any other material, it collides with atoms in the material. In so doing, it ionizes the atoms by knocking electrons from them, thus creating a multitude of new charged, high-speed particles and also gamma radiations. These particles and gamma rays are called secondary radiations. The heavy primary nuclei of calcium or iron cause a great deal more secondary radiation than do protons, the nuclei of hydrogen atoms. Such secondary radiation can cause serious damage to the living cells.
The body apparently suffers no ill effects from short exposure to a small number of low-intensity cosmic-ray or solar-flare particles. However, as the radiation intensity and the time of exposure increase, there are various ill effects. Some of these, such as localized graying of the hair and the growth of tumors, take time to develop, becoming noticeable sometimes weeks, sometimes years, after exposure. Very intense radiation can have immediate effects, such as blindness or brain lesions. In extreme cases, death may result. This radiation may also cause long-term genetic effects, which may be noticeable only in the children or grandchildren of the person exposed. These effects are not yet well understood. In addition to their effects on living organisms, intensive radiations have a harmful effect on certain organic materials, such as plastics, electrical insulators, and paints, and on certain electrical components, particularly semiconductors.
No really practicable method of protecting astronauts against exposure to cosmic-ray and solar-flare particles is known at present. A very thick, massive shield made of carbon would afford relatively good protection but would be much too heavy for spacecraft of reasonable size and cost. Deflection of particles by a magnetic field is theoretically feasible, but the construction of a sufficiently powerful magnet within the limits of spacecraft weight requirements is beyond the capabilities of present-day technology.
The best method of protection at present seems to be the avoidance of space flight during periods of intense solar-flare activity. However, accurate and reliable methods for the prediction of such activity have still to be developed. The present network of detectors can give several days advance notice of a solar storm, however, which is sufficient time to recall lunar flights.
Van Allen Belt
Charged particles arriving from outer space in the vicinity of the earth are trapped by its magnetic field and then become aligned with the field's lines of magnetic force. This effect has produced a large, donut-shaped radiation belt surrounding the earth. It is called the Van Allen Belt, for its discoverer, the American physicist James A. Van Allen. It was at first thought to consist of two separate belts: an inner one, containing mostly high-speed protons, beginning at an altitude of about 1,000 kilometers (620 miles) and reaching maximum density at about 3,200 kilometers (2,000 miles) and an outer irregular belt, mainly of electrons. Further research has led to the belief that there is only one belt, the composition of which varies with time and altitudes. The causes of these variations are not yet fully understood.
The high-speed protons of the Van Allen belt have effects similar to those of cosmic rays, and prolonged exposure to them must be avoided. For this reason manned satellites usually orbit the earth at altitudes between 160 and 320 kilometers (100-200 miles) below the belt. The short exposure to Van Allen radiation that astronauts might encounter during escape and reentry is not considered likely to cause any serious effects. However, even this exposure could be minimized by setting the spacecraft's route over the polar regions, where the radiation is considerably less intense.
Particles in Space
On prolonged flights, space vehicles will probably encounter many microscopic dustlike particles. These particles could not penetrate the vehicle skin and are not expected to constitute a serious hazard. However, the continual impact of many such small particles would have a gradual pitting, eroding, or sandblasting effect that could become troublesome in the case of exposed windows or optical surfaces, such as a telescope lens. Collisions with a few larger particles, weighing more than one gram, are also expected. These particles could puncture the walls of a spacecraft. However, recent data collected from experimental spacecraft indicate that the chances of a spacecraft's being hit by meteors large enough to disable it are very small—much smaller than was anticipated a few years ago. Besides, various kinds of protective devices are available. These include a meteor bumper, or extra skin outside the main spacecraft skin, and several layers of thin-skinned, self-sealing materials. There is also a suitable alarm system whose purpose is to Indicate both the occurrence and the position of a puncture.
The weight of a body on the earth results from the gravitational attraction between the earth's mass and that of the body. On a planet having a different mass, the weight of the object will also be different. For example, a man weighing 72 kilograms (159 pounds) on the earth would weigh 12 kilograms (26 pounds) on the moon. On the surface of Jupiter, on the other hand, he would weigh 192 kilograms (423 pounds).
Obviously these variations present problems for interplanetary exploration. On the moon a man might be able to jump as high as a house with ease, but on Jupiter he could not even stand up.
A body in free fall or coasting space flight is not subjected to any resultant force, if the body itself is taken as the reference point. Such a condition is described as zero-gravity or zero-g, and the body is described as weightless. (For example, if an elevator fell freely down the elevator shaft, all objects within the elevator would float weightlessly during the fall.)
Experience to date with manned space flight indicates that man suffers no permanent ill effects when exposed to weightlessness for periods of time up to nearly three months.
Zero-g conditions pose considerable problems with regard to equipment for space travel. All objects in a spacecraft's cabin must be fastened to a rigid part of the cabin structure to prevent them from floating freely. Water and liquid foods cannot be poured and will not stay confined in an open container, and so must be stored in toothpaste-type tubes or plastic squeeze bottles. Under zero-g conditions it is not easy to withdraw a liquid, such as propellant, drinking water, or fuel for a fuel cell from a tank. On earth, it is necessary only to have an outlet located at the bottom of the tank, where the liquid is held by the force of gravity. Any vapor from the liquid rises to the top of the tank because of its lower density. Under the zero-g conditions of space, however, a liquid and its vapor are not separated by their different densities, but float around and mix inside the tank, and so the type of tank outlet used on earth is of no use. For example, in the case of a propellant tank, if the outlet were covered with vapor at the moment when a flow of propellant is desired, vapor rather than liquid would flow from the tank. This difficulty is overcome by specially designed tanks that ensure a flow of liquid that is unmixed with vapor.
The crew of a spacecraft is subjected to rapid changes in velocity, especially the acceleration during powered flight and the deceleration during reentry. The maximum acceleration occurs just before a rocket engine is shut off, when the vehicle stage has expelled almost all of its propellant mass and the remaining mass is small. The maximum deceleration occurs during reentry, when the spacecraft reaches the denser parts of the atmosphere, at an altitude of about 40 miles.
Man's ability to withstand severe accelerations or decelerations depends on his posture and on the duration, direction, and magnitude of the acceleration. A healthy astronaut, strapped into an upright pilot seat, can withstand a force of about 2 g, or twice the normal force of gravity, in a forward direction. In the opposite direction he can tolerate a force of about 3.5 g. He can, however, withstand much higher accelerations (6 or 7 g's~) in directions at right angles to his body. However, such forces can be tolerated for a limited time only. An astronaut could withstand the forces noted above for about 100 seconds without losing control of his vehicle. If the magnitude of the accelerations were doubled, he could probably maintain control for only 10 seconds.
In American spacecraft, the astronauts recline in special contoured chairs that enable them to tolerate accelerations much greater than those they are likely to encounter during normal takeoff and reentry. During periods of great acceleration, their bodies are forced back against this contoured chair, making movement of arms and legs extremely difficult. Under these circumstances, vehicle controls can best be manipulated by wrist or finger movements.
It is necessary to the life and well-being of the crew that the very delicate heat balance of a spacecraft in flight be maintained. The heat emitted by the bodies of the crew members and in the operation of internal machinery, such as electric motors, fuel cells, refrigeration equipment, rocket engines, and electronic equipment, plus the heat radiation absorbed from the sun, must equal the heat lost to space by radiation. Special paints and finishes and various radiators, together with such maneuvers as the "barbecue roll", a slow rotation of the craft to distribute solar radiation evenly, control heat balance.
If a substantial release of heat energy were to occur suddenly within the spacecraft, the cabin might become overheated. Such a sudden release could result from heating by air friction during the vehicle's reentry, the operation of rocket engines and the intense heat from their flames, or the operation of powerful transmitters. Man's tolerance of high temperatures depends not only on the length of his exposure to them, but also on the type of clothing he wears, the amount of air circulation, and the humidity. The air-conditioning system in a spacecraft is designed to prevent excessive heating from developing during a normal flight. In addition, each astronaut has his own individual air-conditioning system, in his space suit.
Vibration and Noise
The problem of vibration and noise in spacecraft is similar to that in aircraft. Because a manned capsule is crowded with such equipment as motors, pumps, and oscillators, the noise problem is more severe. Acoustic-insulation materials and quiet, vibration-free machinery are essential, not only for the comfort of the astronauts, but also so that they can converse and use their communications equipment and perform other functions without difficulty.
During the operation of large rocket engines, noise and vibration are severe. Fortunately the duration of powered flight is never more than several minutes, and the manned capsule of a multistage space vehicle is usually situated at a considerable distance from the engines. Consequently the noise heard inside the capsule is tolerable. The rocket noise outside and just below the launch vehicle is very intense, requiring even more protection and caution for those outside the capsule than in it.
The equipment, devices, and supplies necessary for the life and comfort of a space-vehicle crew constitute the crew support system. The principal functions of this system are the supply of food and water, the disposal of wastes, and the maintenance of the correct composition and pressure of the cabin atmosphere. In addition to these necessities, the crew members must have a reasonably comfortable cabin.
Food and Water
A man needs half a kilo of dry food and 2 liters, of water per day. However, since most food already contains a considerable amount of water, it is normally unnecessary to drink 2 liters per day. For short-duration nights of several days, these requirements can be met by the use of prepackaged foods and liquids that are concentrated or freeze-dried.
For short flights, simple leak-tight bags are adequate for catching and storing human wastes. For longer trips of several weeks, separate washing, laundry, and toilet facilities need to be provided, but the wastes can be stored in the spacecraft. On very long trips, the partial reuse of waste products will almost certainly be necessary. One solution to the problem would be the development of a closed ecological system in which the carbon dioxide exhaled by the crew members would be converted into oxygen by the process of photosynthesis of green plants, such as algae, under the action of bright artificial light. Plants would also assist in the breakdown of wastes.
Oxygen and Cabin Atmosphere
A cabin atmosphere-control system is necessary to furnish a continuous supply of fresh air to the crew. The system maintains the proper oxygen concentration, removes carbon dioxide, and maintains the correct pressure in the cabin. Since each crew member continually uses up about 2 pounds (0.9 kg) of oxygen a day, the cabin atmosphere must be replenished with oxygen at the same rate. On flights of short duration, this replenishment is done from stored compressed oxygen in cylinders. Also, the carbon dioxide exhaled by the crew members must be removed. A man exhales about 2Vi pounds of carbon dioxide a day; in concentrations greater than about 3 percent, this gas is poisonous. On long flights a regenerating system for the reclamation of oxygen from carbon dioxide will be necessary. The atmosphere-control system must also regulate the temperature and humidity of the air. Such regulation is achieved by using air-conditioning units. The air is dehumidified by condensing and removing the considerable amount of moisture exhaled by the crew members. The system must also remove unpleasant odors and any poisonous gases, such as might be emitted by electrical insulation and other cabin equipment over long periods. This task can be accomplished by passing the air through filters. Finally, the system must supply reserve air. The atmosphere-control system for a space suit is similar but smaller.
The crew of a spacecraft must live and work in a very small, cramped space, and they must work effectively with the many instruments and devices with which the vehicle is filled. The spacecraft must therefore be designed with careful attention to the crew's comfort and convenience. The lighting and all equipment in the cabin must be so designed and arranged as to enable the crew to function with maximum efficiency. Data provided by instruments and displays must be easy to read and evaluate. Controls must be conveniently located and so designed that one control is not likely to be mistaken for another, and communications devices must be simple to operate.
Medical and Psychological Factors
Living conditions in a spacecraft cabin are, at best, far from ideal, with little room for exercise, a lack of privacy, and, of course, isolation from society. These drawbacks can be overcome by picking healthy and well-motivated men as astronauts.