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Effects of Microgravity
Microgravity creates a number of physical effects that can be considered one of the cons of space expeditions. Those astronauts and cosmonauts taking part in missions to low Earth orbit for long durations have been found to suffer from numerous problems. Among the most common effects of a weightless environment include bone density issues, a decline in muscular strength, and issues with aerobic ability. These can lead to higher potentials of injury among the personnel, especially in regards to long term spaceflights.
Scientists have also found that weightless environments create a feeling of disorientation, and even basic motion sickness. Upon a return to gravity, astronauts must readjust to its effects.
This can be considerably important in regards to potential space travel to other planetary bodies in the Solar System. For example, if NASA attempted a mission to Mars, the personnel would be subject to the weightless environment for a sustained period of time. Upon arrival, they could find themselves with a 25 percent loss in muscle strength as well as possessing fragile bones. An astronaut could take one step out of the landing vehicle and possibly break a leg, creating an emergency that could compromise the mission.
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The Threat of Radiation
Space expeditions also suffer from the overall threat of radiation damage. According to the Society of Leukocyte Biology, those exposed to long durations of radiation in space suffer from immune system problems. Cells in the bone marrow, responsible for creating the immunities in the blood, fail to properly develop. In a situation of confined spaces with multiple people, a virus or bacterial infection can spread rapidly throughout the spacecraft.
Further problems can also develop from extreme cases of radiation, most notably from solar flares. The protons developed in an extreme flare impacting a spacecraft could result in advanced radiation poisoning and possibly death.
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Considerations of Isolation
Perhaps one of the most pressing cons to space expeditions is the general psychological well-being of humans placed in confined spaces and isolated from society for long periods of time. Research conducted by NASA and the military show that a small crew in a space vehicle could easily suffer from cabin fever and depression over the course of a long expedition. If a small crew of astronauts were to journey for years to another planetary body, they would slowly find themselves detracted from their lives on Earth, as well as facing problems with long-term exposure to crewmates.
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Besides the physical effects to humans, space expeditions are also one of the most expensive projects undertaken by government agencies, both in terms of treasure and blood. During times of economic unrest, government bodies are wary of spending taxpayer dollars on missions that a considerable part of the public views as frivolous. Especially when the missions could possibly be conducted by robotic spacecraft. For example, some estimates put the Apollo program to land humans on the Moon to cost over $100 billion in today's dollars.
The safety of ground crews designing and building the technology, too, are a major concern. Since it's inception, NASA has suffered the loss of 71 ground personnel constructing and testing flight equipment.
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In an era when computerized technology can replace much of the duties of human crewmembers, there are many cons to space expeditions that are identified by organizations and individuals. However, supporters of the potential human space flights offer, believe these drawbacks are well worth the risk and financial sacrifice.
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"Project Apollo" NASA History: http://history.nasa.gov/Apollomon/Apollo.html
"Superflares could kill unprotected astronauts" NewScientists: http://www.newscientist.com/article/dn7142
"Spaceflight-associated immune system weakening" Journal of Leukocyte Biology: http://www.jleukbio.org/cgi/content/abstract/86/5/1027
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Buzz Aldrin from Apollo 11. (Supplied by NASA; Public Domain; http://upload.wikimedia.org/wikipedia/commons/9/9c/Aldrin_Apollo_11.jpg)