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What does it take to be an Astronaut?

Updated on March 9, 2010

It's not a matter of strapping yourself into a seat and being launched into space. The reason why astronauts are picked from the best of the best and then undergo further intensive training is because the human body is exposed to stresses, strains and experiences that is far from the normal conditions that we face on earth.

Aerospace medicine the branch of medicine concerned with the human problems resulting from flight in the earth's atmosphere and in space. It includes aviation medicine, which is concerned with the medical and biological aspects of flight within the earth's atmosphere, and space medicine. Space medicine is concerned with all the medical, physiological, and psychological problems associated with the exploration of space.

Photo by Kenn Kiser
Photo by Kenn Kiser

The Earth's Atmosphere

The atmosphere of the earth on which man evolved is both life supporting and protective. It furnishes man with the oxygen needed for the processes of life, and it carries away the carbon dioxide expired. Its water vapor furnishes water, the solvent essential for all forms of life, and heating and cooling provide the wind necessary for mixing and moving materials and gases. The atmosphere's mass and chemistry filter out and decrease harmful radiation arriving at the earth's surface from the sun and galactic sources. In addition, its pressure (14.7 pounds per square inch at sea level) restrains human body tissues and helps to force oxygen from the lungs into the bloodstream. In the very different conditions existing in space these functions are lost. As a result, designers of spacecraft, in consultation with space-medicine experts and human engineers, must in some way replace or substitute for their loss.

It is generally agreed that space begins at an altitude of 110 to 120 miles (180-190 km). At this altitude, the atmosphere is so thin that a spacecraft can orbit for long periods without atmospheric drag slowing it down and causing it to return to earth.


Near earth the temperature of the atmosphere decreases about 1° F. (0.6° C.) for each 300 feet (90 meters) of altitude. This region of changing temperature is called the troposphere. At an altitude of some 6 to 10 miles (10-16 km), depending on latitude and other factors, the temperature reaches a level of about 65° to 70° below 0° F. (-18° C.). It remains at that level to a height of 16 to 18 miles (26-29 km). This region of stable temperature is called the stratosphere. Above the stratosphere is the ionosphere, a series of layers in which many chemical and temperature changes occur.

As the border of space is reached, heat can no longer be transferred to or from an object by the atmosphere. As a result, the temperature of any object, including a human being, depends on whether it is in sunlight or in shadow. For example, in sunlight the temperature of a dark absorbing body may reach levels above +250° F. (120° C.). On the other hand, if an object is in the shadow of the earth or of another body or is turned away from the sun, its surface may register temperatures below -250° F. (-157° C.). Fortunately for space crewmen, the temperature of the side away from the sun and the temperature of the side exposed to the sun more or less neutralize one another. Thus, with a little temperature control, it has not been too difficult to hold the interior of the space cabin within a reasonable temperature range.


The earth has the characteristics of a magnet. Arching high above the temperate and equatorial .regions of the earth, from magnetic pole to magnetic pole, are magnetic lines of force. These lines of force capture and hold or, in some instances, repel high energy particles, such as electrons and protons, that are spewed out by the sun and other cosmic sources. In this way the earth is protected from excessive radiation. However, high above the earth the lines of force form a huge series of belts somewhat doughnut-like in shape. These belts, called Van Allen belts of trapped radiation, constitute a radiation barrier to manned space flight.

The position and shape of the Van Allen belts vary from hour to hour, depending on the amount of high energy particles coming in from space and on solar wind, which is the pressure of the particles spewed out by the sun. Solar wind presses toward earth the belts on the side nearest the sun and forces outward the belts on the side away from the sun. The lower borders of the belts vary from about 300 to 600 miles (480-1,000 km) of altitude. The outer borders vary with the position of the sun, but at times reach altitudes of many thousands of miles.

Spacecraft flying to the moon must pass through the Van Allen radiation belts. Most space-medicine experts believe that passing through the belts at high speed and with proper protection does not produce an intolerable dosage of radiation. A more disastrous event might take place if a large shower of protons from a solar eruption were to occur while the space voyagers were en route. One Soviet manned space flight was ended early for such a reason.


Although space is traversed by many materials, it still represents a vacuum more complete than can readily be produced in laboratories. In some regions, for example, the density is as low as one atom per cubic centimeter, or about a trillionth of that of the earth's surface atmosphere. A spaceman suddenly exposed to such an environment without an inflated pressure suit would suffer explosive decompression, and his life could be maintained for only a few seconds.

Sound waves are not conducted across a vacuum, and, for this reason, space is silent. In an orbiting vehicle the only sounds are those produced inside by man or machine.

Selecting Astronauts

It is the mission of space medicine to enable man to live and perform effectively in the space environment. Consequently, the first and probably the most important task is the proper selection of those who are to go into space. Both physical and psychological fitness have been required of U.S. astronauts and U.S.S.R. cosmonauts.

As nearly as possible the individuals selected for space flight must be "stress proof." Their reaction to all events must be rapid, but at the same time carefully thought out. They must also perform their tasks without being influenced by fear or other emotion. The individuals who seem best able to meet these requirements are test pilots or other pilots with many hundreds of hours of flight experience. These men were selected to enter flying training because of their excellent physical condition and their psychological stability. When Harrison A. Schmitt, a geologist, entered the U.S. astronaut training program, he was required to complete a flying course before he could be selected to participate in the Apollo 17 moon mission. However, when routine flights to earth orbit on the space shuttle begin in the 1980's, the requirements on the passengers will be much less rigorous.

Preventing Fire and Explosion

Among the hazards that may endanger the astronauts and their supporting personnel are fire and explosion. The rocket fuels and the oxidizing substances that make them burn are, in many instances, highly corrosive, explosive, and very poisonous. If the fuels and the oxidizing substances come together, the danger of fire is always great. The same is true if pure oxygen, especially under pressure, comes into contact with ignitable materials in the presence of heat or sparks. An electrical spark is thought to have ignited the oxygen atmosphere of an Apollo command module in January 1967, causing a flash fire that killed three astronauts. The astronauts, Virgil I. Grissom, Edward H. White III, and Roger B. Chaffee had been performing tests iri the module on a launch pad at Cape Canaveral, Fla. Following this tragedy, fireproof materials began to be used in the manufacture of astronauts' clothing and of all contents of command modules.

Before takeoff, the astronauts are perched atop thousands of gallons of rocket fuels and oxidizing substances, and therefore many precautions must be taken. Of major importance are ejection mechanisms that are built into the spacecraft. By means of these mechanisms the space capsule or the individual astronauts may be ejected away from an imminent fire or explosion. Other precautions include the use of special gloves, garments, helmets, and masks by supporting personnel who work with hazardous materials.

Life-Support Systems

When in space, astronauts and cosmonauts live in their own little substitute worlds, completely detached from earth. However, all of the functions of the earth and of its atmosphere, such as providing water sources, heat, and light, must be replaced or in some manner substituted for in a very small capsule, the spacecraft. Such replacement or substitution is made by life-support systems.

Because of the tremendous amount of fuel and structure required to get each pound of spacecraft into orbit, everything must be designed and made within the framework of minimum weight and minimum volume. Anything that can be conserved must be, and, if possible, it must be used over and over again. Instruments, tools, and materials are used for more than one purpose wherever possible.

For extended space travel, it would be ideal if carbon dioxide could be reconverted into oxygen, watery wastes converted into drinking water, and solid wastes, in some manner, converted into food. Such recycling of essential materials happens in the natural processes of the earth, and it is the aim of designers of space vehicles to duplicate these complex functions within the spacecraft. The process is called a closed ecological system.

Experts in space biology and medicine have directed much effort toward perfecting a closed ecological system, and considerable progress has been made. At the present, however, life-support systems must rely upon oxygen, water, and food that are carried along. Waste products are collected in containers and are ejected into space except in those instances when specimens must be returned to earth for studies. In such studies, unusual stress reactions, loss of important body chemicals, and other effects can be determined and rectified. At the same time, information concerning temperature, heart rate, and respiration is constantly telemetered to earth, and analyzed, thus providing a basis for medical decisions and research. In this way, each flight furnishes a laboratory for later nights.

Cabin Atmosphere

Man has evolved in an atmosphere of air composed of approximately 78 percent nitrogen and 21 percent oxygen. The remaining 1 percent consists of small quantities of carbon dioxide and of argon and other inert gases. At sea level the atmospheric pressure is 14.7 pounds per square inch (1 kg/cm2), spoken of as one atmosphere of pressure.

An airlike mixture of oxygen and nitrogen at one atmosphere of pressure is used in Soviet space cabins. This atmosphere permits the space crewmen to operate without restricting pressure suits, which can often be uncomfortable. A major disadvantage of the airlike atmo'sphere is that it requires a stronger, heavier cabin structure. Another disadvantage is the fact that if pressure is lowered, as would occur in the case of a leak, the occupants are much more likely to suffer decompression sickness, often called bends. Decompression sickness results if the pressure surrounding the body is lowered sufficiently so that dissolved nitrogen in the blood comes out of solution and forms bubbles in or around joints or in other parts of the body. If severe, decompression may. result in death. Decompression caused the death of three Soviet cosmonauts during reentry of their spacecraft into the earth's atmosphere in June 1971. The cosmonauts, Georgi T. Dobrovolsky, Vladislav N. Volkov, and Viktor I. Patsayev, were found dead in their seats when their spacecraft, Soyuz 11, landed after a flight of record-breaking duration, 24 days in orbit around the earth.

A pure oxygen atmosphere pressurized to only one-third of an atmosphere of pressure is used in U.S. spacecraft. Such an atmosphere supports life effectively, and it requires less cabin structure and weight to restrain the pressure. In addition, operation outside the spacecraft is less complicated and safer if the crews have breathed pure oxygen for long periods before their exit. However, the fire hazard is greater, as was learned in the accident mentioned above.

Noise and Vibration

During the launch phase of manned space flight, the levels of noise and vibration reach the extremes of human tolerance. It has been found, however, that adequate protection can be provided by the use of proper cabin insulation, helmets and ear protectors, and properly designed seats.


The acceleration of gravity at the surface of the earth is called one g. During the first few minutes of flight, accelerations of 4 g to 6 g have been experienced. At 4 g the astronaut is forced into his seat with a force four times his weight. During the slowdown of reentry the astronaut is subjected to slightly higher g force.


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    • profile image

      Shakuntala Shingari 

      7 years ago

      wow what a life..............

    • profile image


      7 years ago

      "However, when routine flights to earth orbit on the space shuttle begin in the 1980's, the requirements on the passengers will be much less rigorous."

      When was this artcle written?

      And isn't space around 60 miles or 100 kilometers?

    • technorican profile image


      8 years ago from Houston

      Great hub! I like your photo. :)


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