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Archived by Raymond J. Noonan, Ph.D., Health and Physical Education Department, Fashion Institute of Technology of the State University of New York (FIT-SUNY), and SexQuest/The Sex Institute, NYC, for the benefit of students and other researchers interested in the human aspects of the space life sciences. Return to first page for background information on these pages.
Space Biomedical Research Institute


Can human beings live and work in space? The short answer, from the early Soyuz and Mercury missions, is yes. Those early missions proved that people can function in space, and that they could operate complex space vehicles under stressful conditions. This document examines the question in more detail and provides an introduction to the basic problems and theories of space biomedical research.

This document is divided into several sections. You can page through the document, or you can jump straight to the section you're interested in:


Long ago, the human imagination created a world of mythical beings to explain the wonders of the visible heavens. This world, peopled by an assortment of larger than life reflections of the human personality, such as Apollo in his fiery chariot and Diana, Goddess of the Hunt, provided a seemingly rational and colorful explanation of the mysterious and obviously powerful events taking place in the sky. The creation of myth was driven by the human need to understand and explain the universe; imagination was dominant during these ancient times when observation was limited.

All of this has changed. Over the last several hundred years, observational astronomy has progressed to the point where it is necessary to leave our planet Earth and travel into space to add to and refine our ideas of how the universe is put together. We have learned that the stars and planets are not great living beings, but, in fact, are great masses consisting of gases, liquids, and solids waiting to be explored and understood by the human mind. Today, space is an exciting part of our lives and it provides us with a unique laboratory, allowing scientific studies never before possible in the history of civilization. Results from experiments in the laboratory of space will touch and influence nearly every part of our lives, from computers and technology to medicine and psychology to business and enterprise. Future large scale laboratories, such as a permanently manned space station and/or a base on the moon, are already in the planning stages. Future exploratory missions, such as a trip to Mars and back, are also being actively discussed by scientists and engineers today, where only decades ago such voyages were the subject of science fiction. Many of these plans will become a reality in your lifetime; still, there is much to be done and much to be learned before we can accomplish these goals.


Future space missions will continue to involve sending humans into space. At one time, not very long ago, some people doubted whether any living things, much less humans, could even survive a journey into space. Over the years, we have learned that, with the proper protection, survival is not in question. The United States' Skylab Program and the Soviet Union's Salyut and Mir Programs have demonstrated that humans can survive space flights of several months to a year. But how long can humans live in space, and how effectively can they work in space? You see, living and working is far different from merely surviving. And after extended stays in space, can people return safely to Earth and lead normal, healthy lives? Such questions are much more difficult than questions of survival because they require sophisticated scientific experimentation in order to understand just how the human body changes during voyages into space. But, let us begin at the beginning with a discussion of basic survival in space and of some of the more obvious needs of the human body.

Human Survival: The Needs of the Body

To begin with, can you name the three main characteristics of the space environment that challenge human survival? That is, can you name the three main differences between the Earth's environment and the environment of space?

Environmental Factors of Space that Challenge Human Survival

There are a number of important environmental differences between space and Earth, and you may have come up with several interesting ones. The differences we will discuss here are atmosphere, radiation, and gravity.

The Atmosphere

The particular mixture of gases that make up the Earth's atmosphere is necessary to support life. Take a deep breath. Each time you inhale, gases are drawn into your lungs, and oxygen is absorbed into the bloodstream, carried to the cells and used to fuel the myriad of actions that your body performs every second. Meanwhile, carbon dioxide is being expelled from your system each time you exhale. This breathing cycle continues through the lifetime of all human beings. We depend on the correct mixture of gases in the atmosphere to sustain our lives.

The human being has evolved to be dependent upon the pressure of the atmosphere on Earth for breathing as well. Atmospheric pressure is a measure of the density and temperature of the gas molecules in the air. The atmospheric pressure allows us to inhale and exhale while breathing and also makes it possible to keep certain gases dissolved within the bloodstream. Breathing is an automatic body function and is essential to human survival. Thus, the correct mixture of gases in the atmosphere and the pressure of the atmosphere must both be maintained in order for us to breathe. On Earth, the gases which make up the atmosphere contain roughy 78% nitrogen, 21% oxygen, 1% carbon dioxide and other gases, mixed with a variable amount of water vapor (that amount is related to the temperature and the humidity of the air).

There is no atmosphere in space. There are gas molecules, but they are so few and far between that they do not produce a measurable pressure. The vacuum of space is more complete than any vacuum that can be produced in a laboratory on Earth. Therefore, space travelers must carry their own pressurized atmosphere with the correct mixture of gases in a leak-proof spacecraft cabin, or, when they venture outside the spacecraft, in suits pressurized by oxygen tanks.

In the spacecraft cabin and in the astronaut suits, the temperature must also be regulated and controlled because space is subject to extreme temperature variation. In general, it can be said that space is extremely cold! Temperature is a measure of energy and there is virtually nothing in open space that can absorb the energy from the sun and radiate heat. The energy from the sun passes freely through space. On the other hand, when there is a mass present in space (for instance, the Earth, the other planets, or a spaceship) the mass will absorb the energy from the sun and this energy translates into heat.

If a spacecraft, or an astronaut outside of a spacecraft performing what's called EVA (Extra-Vehicular Activity), lies in the path of the sun's rays, then the spacecraft or astronaut could absorb the direct, unshielded energy from the sun and become very hot. Thus, the spacecraft or the astronaut in his or her suit must be protected from overheating (or the loss of heat, if the direct sun's rays are not present) through the use of appropriate insulating material. But humans themselves have evolved to function on Earth with a body temperature of about 98.6 degrees F. and a rather narrow temperature band of comfort. Thus, the internal environment of both spacecraft and space suits must have active temperature regulation, involving both heating and cooling so that humans can survive space travel.

Select here to learn more about air in spacecraft.


The Earth's atmosphere serves another very important purpose in sustaining life: it filters most of the sun's ultraviolet (nonionizing) rays that can be harmful to the body, and it protects us from the even more dangerous ionizing radiation of space. While we may occasionally become sunburned after several hours outdoors at the beach or elsewhere, the atmosphere shields and protects us from the worst effects of the sun's radiation. It is true that, even on Earth, overexposure to the sun can and will occur without adequate additional protection from lotions or creams manufactured for that purpose. Skin cancers are a risk faced by everyone on Earth. However, can you imagine how much more dangerous the sun's radiation would be with no protection from the atmosphere at all? That is the situation in space, but protection against such nonionizing radiation is relatively simple. The real problem is created by the ionizing radiation of space.

There are several different ways that scientists describe a dose of ionizing radiation and such a description depends on two things. First, a dose measurement depends on the amount of radiation present. The basic unit of measurement for the amount of radiation is the roentgen. Second, a dose measurement depends on the medium the radiation is penetrating. For instance, is the medium in question a human being, a spacecraft, or a house here on Earth? The medium is the material receiving the radiation. Scientists usually describe radiation exposure to humans in units of REM (roentgen equivalent, man). Table 1 below compares different types of human exposure and the corresponding dose.

Table 1. Comparison of different types of human exposure to radiation and dosage.

TYPES OF EXPOSURES                                      REM
------------------                                      ---
Transcontinental Round Trip by Jet                    0.004
Chest X-Ray (Lung Dose)                               0.010
Living One Year in Houston, TX                        0.100
Living One Year in Denver, CO                         0.200
Living One Year in Kerala, India                      1.300
Highest Skin Dose, Apollo 14                          1.140
   (Mission to the Moon; 9 day mission) 
Highest Skin Dose, Skylab 4                          17.800
   (Orbiting Earth at 272 miles, 87 day mission) 
Highest Skin Dose, Shuttle Mission 41-C               0.559
   (Orbiting Earth at 286 miles, 8 day mission)
Maximum Allowable in 1 Year to a Terrestrial Worker   5.000
The findings from radiation research carried out on previous and planned space missions will help medical scientists determine if men and women can safely undertake long-term space voyages, such as a flight to Mars. Also, and perhaps of even greater importance, these findings will aid in determining the tolerance of man to the ever-present radiation (from natural sources, such as ultraviolet rays, and from industrial, scientific, and medical sources) now being encountered on Earth.

Radiation is hazardous because it kills body cells. The high energy of the radiation breaks chemical bonds in the cell's molecules and disrupts cellular metabolism.

(U. S. Government Standards) 
Radiation Workers:    5.0 REM/yr
General Population    0.5 REM/yr (including children)

Background radiation on surface of the Earth 0.1 REM/yr
REM = Roentgen equivalent, man

Figure 1. Ionizing radiation hazards of space.

Hazardous and continuous
Annual dose: 10 REM
No hazard and continuous

Protons, electrons, and other particles are low energy with a velocity of 500 km/sec

Very hazardous and intermittent but may persist for 1 to 2 days.

High energy protons travel at the speed of light so there is no time to get under cover

Protected dose     1O-100 REM/hr
Unprotected dose   Fatal
Select here for more information about space radiation.


So far, we have discussed two main differences between the environments of Earth and space. The first difference is that space has no atmosphere. This means there is virtually no pressure, very little gas molecular activity, and extreme temperature variation. Human beings could not survive under these conditions without taking their own atmosphere and temperature control system with them. The second difference is that space does not have an atmospheric filter to help shield and protect humans from the dangers of radiation exposure. A human being could not survive without adequate protection from the radiation of space. (Note: There is a certain amount of radiation protection in space due to the Earth's magnetic field if the spacecraft is in what's called low-Earth Orbit (LEO), which happens to be the case for most human missions today.) Now, we will discuss the third and perhaps most profound difference of all: the almost complete absence of gravity in space compared to the level of gravity on Earth. Can a human being survive without gravity? Let's investigate this question.

On Earth, our feet remain firmly on the ground, held there by the downward force of gravity. The magnitude of the force of gravity between two objects is directly dependent on the mass, or amount of matter, in each object, and inversely dependent on the (square of the) distance between them. The equation for the force due to gravity is:

F = GM m / (r^2)

     where F is the force due to gravity,
           GM is the universal gravitational constant, G, times
                the mass of the Earth, M
           m is the mass of the body in orbit
           r is the distance from the body to the center of the Earth
Since F = ma (force equals mass times acceleration, Newton's Third Law), this becomes:

a = GM / (r^2)

See how the mass cancels out? This means that pens and pencils experience the same acceleration in space as people and even spacecraft. On Earth, this means that a pebble falls as fast as a boulder.

Because of the gravity, we stay attached to the Earth's surface. However, the further we go from the surface of the Earth, the weaker that pull of gravity becomes. When an astronaut is in space, that pull of gravity becomes small enough that all it takes to counteract it is for the spacecraft to orbit, or circle, the Earth. This creates a centrifugal force which balances out the gravitational forces on the spacecraft and on the crewmember (the spacecraft and the crewmember are said to be in free fall). If the spacecraft were not orbiting the Earth with enough velocity to balance out the pull of gravity, it would fall back down to Earth.

Figure 2. The balance between centrifugal force and gravity.

Confused? Let's try it another way.

Let's pretend we have a cannon which can fire a cannonball horizontally. (For this example, we'll neglect air resistance.)

Figure 3. Cannonball trajectories, neglecting air resistance.

1.  With a low muzzle velocity, the cannonball flies a short distance.

2.  With a slightly higher muzzle velocity, the cannonball goes further.

3.  If it goes fast enough, the cannonball can go over the horizon.

4.  Faster still, it can go half way around the planet.

5.  Just a little faster, and it will orbit the planet, and impact the cannon!

6.  Exactly the right speed gives a circular orbit.

7.  A little more speed gives an elliptical orbit.

8.  Here, the cannonball has more than escape velocity.

A spacecraft travels the same way as our imaginary cannonball. The rocket engines of the spacecraft make it go fast enough to enter a low orbit around the earth -- usually a circular orbit.

As a spacecraft orbits the Earth, the astronaut experiences a condition known as weightlessness because nearly all of the forces on the body are balanced. If astronauts could somehow step on a normal bathroom scale in space (although they could not do this without tying themselves down), the scale would read zero. Because of weightlessness, many actions that are impossible on Earth become possible in space. For instance, the crewmembers can turn somersaults with ease as they float through the spacecraft. Very heavy objects that might take two or three strong adults to move even an inch on Earth can be moved with a "pinky" in space. If objects such as tools are not securely fastened (Velcro is often used), they drift hazardously about the spacecraft cabin. It is important to note that, on Earth, gravity can never be completely eliminated. Therefore, it is not correct to refer to this phenomena as "zero-g." The correct term is microgravity. However, the level of gravity is so small in space that only the most sensitive instruments are capable of measuring the accelerations present in an orbiting spacecraft.

Microgravity is a new experience for the human body and all the mechanisms that have evolved to cope with the constant tug of Earth's gravity (called "1-g") no longer function in the same way in space. But, as long as a spacecraft contains a carefully controlled atmosphere to enable normal breathing, adequate temperature regulation, and adequate shielding to guard against the dangerous radiation levels in space, the human being can survive under microgravity conditions.

Imagine that you are on your first ride on the Space Shuttle. After launch, huge thrusters provide enough power to carry the spacecraft quickly through the clouds and out of Earth's atmosphere. Welcome to weightlessness! Your feet rise from the floor and you are ready to turn somersaults in the cabin, walk along the walls and "ceiling," and balance bulky objects, even other crewmembers, on the tip of a finger. You have become an instant acrobat. In weightlessness, there is no "up" or "down" as we usually sense it. You don't even know the orientation of your body at first because it has no weight for you to feel and sense "where it is." In space, your body becomes confused by the sudden change in what it has learned to expect. It will now begin to tell you so.

Upon entry to weightlessness, nearly all astronauts are troubled to some extent by a condition called space motion sickness which is similar to car or sea sickness. Because the brain on Earth has learned how to process signals about the position of different parts of the body in relation to the world around it, this sudden input of confusing signals causes many astronauts to feel sick. Within the inner ear, there is a balance organ called the vestibular organ and information from it, together with information from your senses of sight and touch, and information from your muscles and joints, all integrated together, helps your brain focus on the position of your body relative to the downward pull of gravity. In space, with virtually no gravity, the signals from the vestibular organ combined with what is seen and felt by the other body sensors are all giving conflicting information to your brain.

Other body functions do not adapt as quickly and, in fact, the changes in certain other physiological functions may prove to be lasting and could cause serious problems, especially when astronauts return to the "normal" gravity of Earth. Sometimes it is not apparent when our Space Shuttle astronauts emerge upon landing, but you can believe that all of them feel shaky as they walk to the podium to receive their welcome home. Most Soviet cosmonauts, after spending months in space, are actually carried away from their spacecraft in a specially designed stretcher.

Astronauts suffering from space motion sickness may get headaches, lose their appetite, feel as though there is a "knot" in their stomach and find it very difficult to work efficiently. Some astronauts actually vomit. In space, microgravity is constant, and the brain and the body learn to adapt many of their functions relatively quickly. Fortunately, for most people, the symptoms of space motion sickness seem to last only through the first few days of the mission.

While in space, the body no longer experiences the downward pull of gravity to distribute the blood and other body fluids to the lower part of the body, especially the legs. In fact, the blood and fluids make what is called a headward shift, which means that these fluids are redistributed to the upper part of the body and away from the lower extremities. This phenomenon carries with it some interesting effects. While in space, the astronauts even look different: 1) they have a puffy face because they have more fluid in the upper body, filling the facial cavities which are normally dry; and, 2) they have legs that are much smaller in girth (and are called "bird-legs") because they have less fluid in the lower body; and 3) the spine straightens, lengthening the body about one inch.

Select here for JPL's Basics of Space Flight, where Chapter 3 discusses gravitation and basic orbital mechanics.

[For lessons on anatomy, take a look at the University of Washington, Radiology Teaching File, Anatomy Teaching Modules.]


  1. What purposes does the atmosphere serve? In space, what mechanisms are needed to maintain the atmosphere inside a spacecraft?
  2. If you type on a keyboard or push down on a mouse button, you will be pushed upward: Every action has an equal, but opposite, reaction. In space, how would this affect computer design?
  3. Imagine the room you are sitting in now is to be relocated on International Space Station Alpha. You have been designated as the chief architect in charge. What differences in construction, arrangement and freedom of movement would be required to make it usable in microgravity?
  4. List your three favorite hobbies. Predict how making your home in space might impact them.
  5. How important do you feel human exploration of and expansion into space is? Give examples of what you feel to be the benefits and drawbacks of such exploration and expansion.
  6. Describe how each of the three major environmental factors of space impacts the human body.
  7. Examine the Chart on Radiation Exposure (Table 1).
    What do you think might be the reason for the large difference in radiation exposure among Apollo, Skylab and Shuttle missions?
  8. Since the radius of the Earth is about 6,378 km, if the Shuttle orbits at an altitude of 400 km, how much is the force due to gravity in orbit as a percentage of the force due to gravity on the Earth's surface?

The next section is about the cardiovascular system.

Last modified: Mar 29, 1995

Author: Ken Jenks


Contact Info:
Raymond J. Noonan, Ph.D.
Health and Physical Education Department
Fashion Institute of Technology of the
State University of New York (FIT-SUNY);
SexQuest/The Sex Institute, NYC
P.O. Box 20166, New York, NY 10014
(212) 217-7460

Author of:

R. J. Noonan. (1998). A Philosophical Inquiry into the Role of Sexology in
Space Life Sciences Research and Human Factors
Considerations for Extended Spaceflight
Dr. Ray Noonan’s Dissertation Information Pages:
[Abstract] [Table of Contents] [Preface] [AsMA 2000 Presentation Abstract]
And the Outer Space chapter in IES4:
 Click on the button to buy it!  Click here to buy it!  New! Volume 4 of the International Encyclopedia of Sexuality (IES4), including 17 new countries and places, Robert T. Francoeur, Ph.D., Editor, and Raymond J. Noonan, Ph.D., Associate Editor, published in May 2001 by Continuum International Publishing Group: Includes my chapter on “Outer Space,” which highlights cross-cultural sexuality issues that will have an impact on the human future in space, based partly on my dissertation. For the table of contents or more information, see the IES4 Web site:, including supplemental chapters available only on the Web. Order from!

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