There are many obstacles to making manned space travel to Mars a reality. We currently crawl through space at a measly 18,000 miles per hour making any but the closest planets and moons unrealistically distant for travel purposes. Space is a vast area but there is a lot of debris floating out there. There is a decent possibility of impact with foreign objects. Unfiltered solar radiation has the potential to quickly degrade high tech equipment necessary for the flight of spaceships. However, these problems occur in non-manned as well as manned travel. I will focus on the dangers of manned space travel on the human body and mind.
There are three major deterrents to long-term manned space travel concerning the human body and psyche. None of them have easy answers and each, by themselves, might be enough to shelve the idea of comprehensive space travel, even to a planet as close as Mars, for decades or even centuries. Colonization of a rock as distant as Mars is essentially impossible today. I will speak about the problems of bone loss due to lack of gravity, unfiltered radiation's effect on a living organism and the psychological effects of such a long trip cooped up with people you might not associate with if given the chance plus the isolation from anything or anyone new for months and years at a time.
Without gravity, bones, especially the lower extremities, atrophy as they are not needed to carry the weight of the human body. This process is called disuse osteoporosis. The lack of stress to the bones from the weight of a human being slows the formation of osteoblast cells which build bone. With the loss of the bone-building cells and the natural process of bones breaking down leads to the loss of mass. Most bones atrophy between one to one and one half percent per month in the emptiness of space. This may not sound too horrible at first glance but right now it would take approximately 15 months to reach Mars. Some bones would have lost 22.5% of their original mass. This, in effect, would equal the effects of growing up bed-ridden as an invalid from birth.
There is currently substantial research into countering the effects of disuse osteoporosis at NASA and certain health organizations. The consensus is that a specific diet mixed with pharmacological aids, such as certain hormones, and specialized exercise regimens may eventually be used to minimize bone loss in space travel, but it is clear they don't have the exact answer now.
Bone loss leads directly to discussion about the second major danger to human beings while traveling in space. Without an atmosphere filtering out radiation, like the Earth has, space ships would get a direct dose of many types of radiation. It is unclear the exact effects of these energies upon human bodies but it is likely many of them are severely damaging at the cellular level. A recent study on mice showed that mice receiving the equivalent of what a human would receive on a trip to Mars lost 39% of spongy bone tissue and as much as 64% connectivity of spongy bone tissue, which is in line with a diagnosis of osteoporosis. Add this loss due to disuse osteoporosis and the effects would be catastrophic.
It is believed among scientists that the radiation in space would likely mutate DNA in humans causing all kinds of cancers. It would cause cataracts in the eyes. It would cause loss of fertility in space travelers and genetic defects in their offspring.
This problem is not easily overcome. Shielding using water, hydrogen or plastics could help but the amount of radiation absorbed by a traveler would still be well above the recommended maximum by NASA's standards. Faster travel would help as there would be less time to absorb these particles. Certain drugs could also help mitigate the effects of radiation on the body. Many scientists believe that a certain amount of risk will always be inherent due to radiation no matter how many safety measures are taken unless we succeed at finding some previously unimagined breakthrough.
The third major obstacle to manned space flight to Mars is the least measurable. It is virtually impossible to study as it is mostly theoretical. There has been no 15 month space travel. Those that do go into space for any amount of time are the best and brightest and are specifically trained for the hazards of space, physical, physiological and psychological. Although this would not preclude them from developing the same problems as the general population it is probably not an ideal test group if we are going beyond travel and are thinking about colonization with a variety of people.
Psychologists have used somewhat similar cases to make hypotheses on the effects of space travel on the mind. Arctic research stations and submarines house people who are in a closed space for many months at a time. Typical responses to the stress of being so isolated in a confined space with a small number of people for an extended period of time are insomnia, anxiety and depression.
Living in such close quarters with people will magnify people's irritation levels at annoying mannerisms and habits. If they are already incompatible with each other the relationship could develop into disdain or even loathing. Living with these feelings about a person you must be with for months or years with no break is unhealthy and can lead to a lot of stress related disorders.
Another problem that has an effect on the mind of the traveler would likely be loss of motivation. Scientists have found that weightlessness of the human body causes a lack of vitality and an increase in fatigue causing lethargy and listlessness.
Keep in mind, these are just the effects observed on men and women that have passed rigorous psychological testing and are probably some of the most psychologically resilient people in the world. For the general public these issues would likely be extremely stressful and could lead to psychoses and neuroses in a portion of the population.
To me it is clear that the effects of space travel on human beings preclude us from comprehensive visits to even the closest planets and colonization of a planet such as Mars (even discounting the problems of terraforming such a place) would be virtually impossible at this time. Much research into propulsion, shielding, medicine and psychology is needed before even considering such a monumental journey.
References
Comins, Neal (2007). The hazards of space travel: A tourist's guide. Villard.
Hullander, Doug (2001, October 1). Space bones. Retrieved April 5, 2009, from Science@NASA Web site: HTTP://science.nasa.gov/headlines/y2001/ast01oct_1.htm
Lloyd, Robin (2006, July 18). Radiation and bone loss: Deep space mission concerns. Retrieved April 5, 2009, from Science.com Web site: http://www.space.com/scienceastronomy/060718_radiation_bones.html
Fornace, Albert J. (2008, April 16). Space radiation may cause prolonged cellular damage to astronauts. Retrieved April 5, 2009, from Science Daily Web site: http://www.sciencedaily.com/releases/2008/04/080415164332.htm
Edwards, Rob (2005). Cosmic rays may prevent long-haul space travel. Retrieved April 5, 2009, from NewScientist Web site: http://www.newscientist.com/article/dn7753-cosmic-rays-may-prevent-longhaul-space-travel.html
Atkinson , Nancy (2008, August 14). Research and technology to help psychological issues in space. Retrieved April 5, 2009, from Universe Today Web site: http://www.universetoday.com/2008/08/14/research-and-technology-to-help-psychological issues-of-space-travel/
Psychological effects of space travel. Retrieved April 5, 2009, from Astrobiology: The living universe Web site: http://library.thinkquest.org/C003763/index.php?page=adapt03
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