�From all their exceptional journeys, they all came back with the revelation of beauty. Beauty of the black sky, beauty and variety of our planet, beauty of the Earth as seen from the Moon, girdled by a scintillating belt of thunderstorms. They all emphasize that our planet is one, that borderlines are artificial, that humankind is one single community aboard spaceship Earth. They all insist that this fragile gem is at our mercy and that we must endeavour to protect it�
Human beings were obviously not meant for living in space. Several things would happen to a person exposed to them, and certainly none of them pleasant.
If our unsuspecting traveler were exposed to sunlight, he will certainly suffer temperatures of over 100 °C. But if anything shielded him from the light, like for example his spacecraft, he would find little relief in temperatures well below -100 °C.
Of course he wouldn't be able to breath, there being no air in space. And even if he could, his blood would boil away practically instantly through lack of atmospheric pressure.
The first very obvious conclusion is that the projected space colony must not be open to outer space. It must supply the dwellers with air, atmospheric pressure, relatively pleasant temperatures and all the comforts our frail bodies are used to.
"The musculoskeletal system has evolved on Earth according to the Earth's physical features (water, land air) and its gravitational field. These factors determine the physical demands which are imposed on all living animals. The construction rules of developmental mechanics have evolved over hundreds of millions of years to produce organisms which are designed specifically for this planet. The advent of space travel and the possibility of transporting animals to planets with different gravitational fields, present interesting questions in terms of immediate biological effects, the development of offspring, and the future evolution of species." -- Dennis R. Carter "Musculoskeletal Ontogeny, Phylogeny, and Functional Adaptation." J. Biomechanics, v. 24,suppl. 1, p. 13.
On Earth, our bodies are subject to the pull of gravity, which has been already discussed extensively. For reasons that have also been covered in previous chapters, astronauts in spacecraft in orbit and potential colonists would be exposed to microgravity conditions, that is, to the near absence of gravity.
As we shall see, even if the elementary physical protection of a pressurized spacecraft or habitat prevent us from perishing immediately, if stays in space are long, microgravity attacks us subtly to decondition our bodies.
Microgravity was of great concern to space pioneers in the early days of space travel. Nobody knew the effects of the absence of gravity on human bodies, and they were logically afraid of what would happen to human subjects during spaceflight.
This was so much so that both the U.S, and Russian space programs initiated their biomedical investigations by sending up animals, much to the humiliation of the astronauts who were outraged at having animals commanding their machines.
When animals performed normally, even during high G forces during reentry, humans had their first go.
Initial results for short missions were reassuring. Negative effects that will be described subsequently were minor, but they still posed a hazard to missions and to the astronauts' health.
Real problems arose with biomedical results from long duration flights. The Space Stations Sayut, Skylab and Mir were manned for long periods of time and yielded distressing information. Some of the effects encountered were not minor, and in some cases not completely reversible.
The space settlement is a very long range mission, in which families, that is, individuals with no special exceptional physical requirements, are expected to live for years on end. We must make sure that if microgravity is to be allowed in the colony its negative effects must be definitely attenuated.
In a normal environment the heart pumps blood upwards and doesn't need to pump down since gravity pulls it in that direction. When the body is exposed to microgravity, blood goes to the upper limbs and the lower limbs get very little blood. In this case the heart continues to pump blood in the same direction it does on earth, so the fluid accumulates in the upper limbs.
This leads to the " Puffy-head, Bird-legs" syndrome, which consists in the reduction of the legs mass and the increase of the head size. This has been proved by measuring before, during and after the flights and have concluded that this change actually happens but after a few days on earth, the body recovers its usual shape.
The fact that the legs become smaller is not only due to the accumulation of fluid in the head, but also to the deconditioning that the muscles experience. This happens because the legs don't have to support weight and there is no need to walk. In short term missions astronauts exercise with bicycles and others, but this can't be expected to happen in 2 year missions.
This negative effect could be counteracted by daily exercising for over 1 hour, like astronauts usually do in treadmills or fixed bicycles. However, it would be hardly desirable for the whole population to have to exercise constantly.
On Earth, being in an upward position, gravity distributes the ventilation process. This means that it controls the rate at which air is renewed each minute. So, when the body is exposed to microgravity, this process does not take place, leading to the accumulation of air, contrary to what happens on earth, at the top of the lungs. This on its own is not that bad, but the problem lies on the fact that the organism is not used to it, resulting in other problems. It should also be taken into account that when the body returns to Earth it will have to get used to gravity again what might lead to new problems.
The same happens with perfusion (blood flow). In microgravity the distribution of blood is not the same to the one in earth what might also bring problems since the body is not used to it.
On earth, nerves perceive gravity as muscles relax and contract, the eyes can see the surroundings and identify where the body is. The inner ear also senses and by means of this three sensory organs the person knows where he is standing. In microgravity the information sent by these organs to the brain conflicts with the information that the brain is used to receiving, leading to the "Space Motion Sickness".
This sickness has symptoms such as nausea, loss of appetite or loss of productivity
This is not that much of a problem because after some time the body gets used to it. Astronauts in short missions have to take special pills for motion sickness.
When the body is exposed to microgravity there is a considerable loss of calcium and other essential materials for a correct bone development. This implies a loss of bone marrow too, to an extent of up to 1% per month. This data was revealed by the studies made by the Russian's and American's space stations: Mir and Skylab respectively.
Some effects are puzzling : the bones in the upper body don't seem to lose minerals at all, while the weight-bearing bones in the legs and lower back lose can lose a large percentage of their bone mineral content over several months in space (up to 20% loss has been observed in some bones of some space travelers)
Together with the decrease of bone mass comes the loss of muscle mass. As it was previously mentioned, in microgravity legs are not needed as much as in earth. The fact that there is 0 G contributes to the deconditioning experienced by muscles: As bodies "float" legs don't have to support the body's weight.
These two issues are the main reasons for the reduction of red blood cells or the increase in their destruction (It is still uncertain what actually happens). Red blood cells are the most abundant of all the body cells. They are formed at a rate of 480000000 cells each minute in the bone marrow. Their major function is to carry oxygen from the lungs to the tissues of the body. So if the bone marrow decreases, red blood cells production can't take place, and, because muscles are not used, less oxygen is needed and therefore less red blood cells are needed to carry oxygen.
Loss of bone marrow is nowadays a critical issue in long term missions. Diets rich in calcium do not appear to solve the problem.
The immune system is in charge of defending the body against diseases. Lymphocytes are one type of white blood cells that helps the body resist infections by recognizing harmful foreign substances, such as bacteria, and eliminating them.
Analysis of blood from crew members from Shuttle missions revealed decreases in the number of circulating lymphocytes, and post-flight studies showed that the lymphocytes were not as effective in responding to challenges. However, astronauts have shown no increased susceptibility to disease, and lymphocyte counts returned to normal a few weeks after landing. In spite of this, the changes in the immune system must be understood and controlled, if possible, because such changes could have undesirable consequences on longer space missions.
Space flight may reduce white blood cell counts for several reasons. First, microgravity itself might cause a decrease in lymphocyte production or an increase in lymphocyte destruction. Second, the stress associated with the mission, particularly the launch and landing phases of the mission, might alter lymphocyte count or function. Studies of stress on Earth suggest that the second reason is plausible. It has been extremely difficult to separate the direct effect of microgravity from the indirect effects of stress. Third, space radiation may suppress the immune system by destroying lymphocyte cells.
Though it has been studied, scientists have not discovered the reason for the increase of the destruction or the decrease in the production of white blood cells.
This problem can have serious consequences in a space colony where lots of people are confined in a close place. This is so because if there is an epidemic and people have lowered their immune defenses a common disease on Earth could spread around and become fatal.
In Microgravity hygiene becomes quite complicated because, as everything, water "floats", so astronauts take showers with sponges. It also becomes very difficult to maintain the place clean. It can even be dangerous in confined places because lots of illnesses can appear as a result of the difficulty to be clean.
Another problem associated with microgravity is how to manage wastes: space studies have shown that microbes can multiply quickly in a small weightless area, and since germs can transmit illnesses, this could endanger the health of anyone on board.
In the current state of biomedical investigations, the problems outlined above point overwhelmingly to the need to generate artificial gravity by means of rotation if humans are to thrive and prosper in the space settlement. It could be argued that most of the problems become dangerous when the subject returns to normal Earth gravity and that if they were to stay in the colony indefinitely they would hardly be noticed.
Even so, colonists should not be condemned to not being able to return to their mother planet for medical reasons.
This conclusion does not invalidate differentiated gravity areas. Microgravity can be extremely useful for certain processes and lots of fun for humans. As long as they don't live all of their normal lives in weightless conditions, humans can tolerate and even enjoy microgravity.
Another hazard awaiting the space traveler is the again unfriendly present of radiation.
Radiation is made up of high energy subatomic particles, from neutrons to electrons and even the tiniest parts of the central cores of atoms called pi mesons. These particles exist in space from stars whose thermonuclear reactions spewed them out or they are ejected from the radioactive materials. Radiation particles all travel with different energies so they are classified according to their structure and their energy.
For this discussion we will consider only the radiation found in space. Nearly all of it is gamma rays. This kind of radiation is also frequently referred to as ionizing radiation and x-rays. The general energy range of these particles is from 10 to 10 million electron volts. These rays penetrate anything in their path and cause a pair of positive and negative ions to be formed along that path.
When that same ionizing radiation is intercepted by our bodies it can affect and kill human cells. Thus, radiation can cause nausea, vomits in mild quantities and produce severe mutations and even cancer when absorbed in great quantities.
There are three primary sources of radiation: the stellar primary or local sun, the belts of geometrically trapped radiation around a planet and cosmic or background radiation.
The effects of radiation in space are highly variable. For instance, each solar flare will exhibit a radiation profile unique to the single event, and each orbit around the earth may exhibit a different radiation profile.
Solar Flares: solar flares typically occur at times of peak solar cycles -once in 11 years. When the sun emits a solar flare, a huge, visible prominence erupts from the surface of the sun resembling an explosive, flaming tongue of gas, often hundreds of times larger than the earth and that can last as long as 30 - 50 minutes. Such a violent occurrence is accompanied by several consequent radiation events experienced in the solar system:
Electronic disturbances: The electromagnetic event lasts only as long as the prominence itself is visible and is propagated at the velocity of light.
Proton ejection: high energy protons are also released from solar flares in a wide range of energies and constitutes the major particle hazard from solar flares events.
Two primary classes of protonS are emitted during a solar flare defined by their energies. The highest energy protons arrive first, within 5 - 20 minutes following the visible event (13-28 minutes following the actual event as the sun is 8 light minutes from earth.) traveling at 0.6 the velocity of light (C). Their energies range from 200-300 million electron volts (MeV). The second class of protons arrive after half an hour with a energy at or less than 100 MeV. These particles continue to stream outward at decreasing energies for several days following the event. The Flux density is entirely unpredictable.
Solar Flare Radiation Risks: Between 1956 and 1962 studies have been made and they revealed that in 30 day space flights occurring during maximum solar activity, the risk of an exposure greater than 100 rads was between 5 and 10 percent.
Radiation can be measured in different ways:
Roentgen: the basic unit of radiation quantity. the amount of radiation required to produce 0.001293 grams of air ions carrying one electrostatic unit of electricity of either sign.
RAD: A biological definition of radiation dose. An acronym meaning- Radiation Absorbed Dose. The absorption of 100 ergs per gram of any medium. The medium must be stated.
REM: A term meaning -Roentgen Equivalent Man. The term used most often to refer to human exposure to radiation. It is equivalent to the absorbed of any ionizing radiation which produces the same biological effects in man as those resulting from the absorption of one roentgen of x-rays.
Relative Biological Effectiveness (RBE): The biologic effect of radiation exposure is related to a specific energy range and is called RBE. Radiations that have this effect are said to have an RBE of unity. Higher energy radiations are radiations that affect the biological tissues with more relative damage are said to have an RBE grater than unity.
Equivalent residual Dose (ERD): The amount of radiation injury that persists after the radiation exposure. 10 % of any damage done by radiation exposure is irreparable. The body repairs the remaining 90 % at a rate of 2.5 % per day.
There are radiation limits set by the U.S government that affects not only regular people but also military personnel, x-rays technicians and radiation workers. The amount of allowable occupational exposure is of 500 milirems (0.5 rem) per year.
The average person is exposed annually to about 0.08 rems of natural occurring radiation from radon gas to soil deposits. Another 0.1 rems are absorbed from medical and other man-made sources each year. An airline crew can log another 0.9 rems in high altitude flights.
Current industry and government have adopted what is called the ALARA standard, which is an acronym meaning "As low as reasonably achievable" Which means that no exposure is thoroughly safe or without risk so individuals should avoid unnecessary radiation exposure as a standard. Astronauts are inevitably exposed to greater radiation doses as they leave the atmosphere and the magnetic field of the earth. An example of this can be the Apollo astronauts records.
|MISSION||LENGTH (days)||EXPOSURE (rads)||AVG. EXPOSURE (rads/days)|
Note from this table than the average exposure per day was highly variable and relates to the radiation environment on a given day, the solar output and the proximity of the orbit to the geomagnetic radiation belts.
In considering long term exposure, it is estimated that at peak solar cycles, a crew could be exposed during any given flare to a lethal dose of 207 rads per day. On a very long term mission (2-4 years) conducted during a peak solar cycle, the average number is still alarming - 1.67 rads per day - some three annual federal exposure limit per day.
|10-50 rads||no effects|
|50-100 rads||vomiting and nausea|
|200-300 rads||20 % fatal|
|300-550 rads||50 % fatal|
|550-750 rads||up to 100 % deaths|
|1000 > rads||total crew loss|
It is assumed that all spacecraft will employ some kind or aspect of shielding just due to the requirement to provide a life support envelope. Because the launch requirements are opposite to the requirements of a high density material such as lead necessary for radiation shielding, it is unlikely that such high density materials will ever be used in space. But any material used for shielding produces secondary radiation effects when struck with high energy particles.
This effect is called bremsstrahlung radiation: When high energy particles strike the atomic nuclei of stable atoms, they emit a shower of secondary particles- which are, by definition, of less energy, but consist of a shower of many additional high energy particles, gamma rays and x-rays. Secondary radiation is produced when the primary particle has an energy of 300 MeV or greater.
Secondary radiation effects are important in relatively less dense materials, like aluminum or titanium. As it shall be discussed in the following section, secondary radiation is not a problem when the materials used to shield habitats are more dense, like in the case of our colony.
In order to protect the colony's inhabitants from a lifetime's exposure to cosmic radiation, the habitat must be fully shielded.
This method of protecting the habitat is called passive shielding, as opposed to active shielding where some electromagnetic contraption tries to stop incoming ionizing radiation.
Radiation protection constitutes, in fact, the key requirement that determines what the colony's shell will be built of.
Integral shielding was also a key factor in tracing the sunlight's path into the colony, for no areas must be glassed and thus exposed with very little protection to radiation.
Light resistant metals like titanium or aluminium are readily available in the Moon. Another option woud be to use a regolith (lunar soil) based composite material.
The following tables show the relative shielding supplied by different construction materials :
|0 cm depth||5 cm depth||g/cm|
|0 cm depth||5 cm depth||g/cm|
|0 cm depth||5 cm depth||g/cm|
(Annual Dose Equivalents for Galactic Cosmic Rays at Solar Maximum in Free Space at 1 Astronomical Unit from the Sun)
From the above table several conclusions can be drawn. Differences of shielding capacity between the construction materials studied are not highly significant. Then, although water appears to protect better then regolith and aluminium, it is a very scarce element in space. Aluminium presents the problem of secondary radiation, as it is a light metal.
So the default option is using some sort of regolith based composite material to shield the habitat. If some sort of backbone structure is made up of lighter and more resistant metal the regolith blocks can be used to shape and shield the structure.
Although the above study does not yield further values (more g/cm2), and the exact composition and thus the density of the regolith based naterial are not known, an estimation of the thickness of the shell could be made.
In order to have, for example, not 75 but 500 g/cm2, which would presumably satisfy shielding requirements, and taking a density of 2400 g/cm3 (density of concrete), the required thickness would be :
x= 2400/500 cm = 5 cm.
This very low value indicates that probably the stresses will be determinant factors in the thickness of the structure. Whatever thickness results from the stress analysis will surely adequately shield the structure against radiation.
�There appears to be a number of important psychophysiological variables that relate to an individual�s ability to adapt to spaceflight. For example, space sickness or space adaptation syndrome affect about one- half of all space travelers, primarily during the early days of a mission. Earth-based studies, although far from conclusive, suggest that age, gender, and personality traits, could be predictive of an individual�s susceptibility to space sickness. Sensory conflicts theory continues to be helpful in directing research efforts in general understanding of why space sickness occurs and how to simulate it on earth� � one intriguing line of evidence suggests that the highly athletic individual may present no advantage in withstanding the effects of weightlessness and may even be at a disadvantage compared with an average , healthy individual. Similarly, there is evidence to suggest that older individuals may handle certain types of physiological stress better than younger ones. These are related findings could have important selection implications.
There is certain biomedical evidence that deconditioning can be dangerous to the safety and survival of the astronaut, if measures are not taken to limit or reverse the deconditioning process. The successful completion of two 6-month-duration missions within a 18-month period by a soviet cosmonaut �does allay many of the fears for the future missions. However, the capacity of different groups of humans to resist pressures will continue to be an area of obvious concern.
Traditionally, our approach to reducing physiological deconditioning in space has been to select men in top physical condition and maintain this condition through exercise. Rigorous preflight and in-flight conditioning programs have been maintained under the assumption that the better the astronaut�s physical condition, the greater is his overall resistance to the stresses of spaceflight. In-flight exercise does appear valuable in reducing muscle deconditioning. The comprehensive exercise program used during Skylab missions was effective in preventing loss of weight, maintaining leg strength and leg volume and maintaining the integrity of muscle system in general�.However, in-flight exercise by no means offers complete protection.
Cosmonauts Berezovoi and Lebedev return from their 211-day flight aboard Salyut 7 in obviously debilitated condition.... Although they had exercised daily, their muscles were so flabby that they were barely able to walk for a week, and for several weeks after required intensive rehabilitation. Although in-flights exercise has been shown to benefit external muscles, the benefits to other physiological systems must be questioned. The Skylab exercise program did not deter decalcification or related problems of the skeletal system. �
Mary M. Connors, Albert A Harrison and Faren R. Akens Living Aloft, NASA publication 483
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