As New Horizons speeds towards Pluto I get more and more excited because in my short life our knowledge of space has increased so dramatically. However, it saddens me as well because of the expectations, the lives we anticipated having in orbit or in colonies on other terrestrial objects, are dwindling.
There are noble plans such as “Mars One” which should have humans colonizing in 2026. Then there is the International Space Station which is not meant for long-term life, but has been functional since 1998. However, humans cannot thrive with gravities less than Earth’s, or if we are able to adapt we would quickly find these new humans would be unable to live on Earth.
The longest period of time a human has been in orbit was 14 months by Valeri Polyakov, a short time considering the lifespan of a human. The full range of effects of less gravity and a lack of atmosphere protecting us is not fully understood but scientists should know more soon because Scott Kelly; who is currently serving a year in space has an identical twin on Earth and upon his return extensive tests will be performed to try and identify all of the changes.
What we do know already ranges from minor inconveniences to long-term health effects.
Upon reentry astronauts will spend a couple of days with little activity because their bodies are readjusting to gravity. Then they are able to function mostly normally—except they will often drop objects because they are unaccustomed to gravity. That is an inconvenience, but their need to readjust is linked to a more serious issue.
Our bodies are designed to fight and use gravity—for example our walking is better described as a controlled fall. Our hearts and cardiovascular system specifically are designed to use gravity to aid in pumping our blood to the legs(so we can walk) and the system also works diligently pumping against gravity up to the brain. Without gravity our heart does not have to work as hard to send blood to our heads and without having to walk led goes to our legs. This causes puffy head-bird leg syndrome where our upper bodies and heads swell up with the extra blood and our legs shrink due to less blood being immediately in them. 
Pictured: Diagram of human proportions based on fluid dispersion.
Pictured: Astronaut before and during space flight.
With the extra pressure in our heads our brain signals the body to make less blood because it believes there to be an excess—so astronauts after adjusting will have about 1/5 the blood they would normally have. This is a major reason why the astronauts must rest after landing— with less blood they can be prone to fainting. Daily exercise also helps maintain a more normal physique and helps with blood levels.
Daily exercise is also a large factor in aiding the loss of muscle which occurs without gravity. The main muscles lost are those in the legs and some in the back that aid posture. Bones also have a similar issue in the legs—they weaken with time and possibly due to the extra radiation exposure. Calcium is reabsorbed into the body and released through urine and 1 to 1.5% of the total bone mass can be lost per month (opposed to a year on the surface of Earth).
The loss of bone and muscle mass can extend into long-term issues, along with the heart’s similar deterioration with less work. There are other issues that humans can face however, such as issues in the eyes. In one study 33% of astronauts showed exhibited expansion of the cerebrospinal fluid space surrounding the optic nerve, 22% showed flattening of the back of the eyeball, 15% exhibited bulging of the optic nerve, and 11% showed changes in the pituitary gland and its connection to the brain.
Brain damage is also a large concern thanks to galactic cosmic rays and solar flares that the atmosphere is normally able to protect us from my absorbing and deflecting many of the harmful rays. The same rays also increase the probability of cataracts, cancer, heart disease, and damage to the central nervous system.
Colonists on planets like Mars would have some defense, but not much. People presume that it’s very similar to Earth, but it’s much smaller and far less dense—boasting a gravity 38 percent of Earth’s. Then it’s atmosphere—because Mars does have one—is 1% the thickness of Earth’s. Magnetic fields surrounding the planet are also very important in diverting solar rays and Mars has a rather strong magnetic field however it is uneven and focused in part of the southern hemisphere—so it’s effects would be directly linked to the location of people on Mars.
Pictured: Earth’s magnetic field and Mars’ magnetic field.
The Mars colony established by Mars One is meant to be the first permanent colony on another terrestrial object, and one of the greatest hurdles(in my opinion) that they will make is having a child in an environment besides Earth’s. Presuming they can overcome the radiation lessening sperm counts and inhibiting fetus growth, the children will have to grow up dealing will all the aforementioned complications, and it would never be able to return to Earth safely.
Which raises the question: just because we can send people to another planet, should we? Can we subject people to living in conditions that cause irreparable harm? Or will the scientific advancements based on what we learn about them and our efforts to protect them justify the sacrifice?
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