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There are significant health complications that come with long term space travel.

• Bone density loss. From the lack of daily impact on the skeleton from walking, moving around, and lifting things.
• Loss of muscle mass, also from the lack of use. ISS residents need to adhere to a pretty strict workout regimen, and still have issues with significant muscle loss when they return to earth.
• Radiation exposure. The atmosphere protects us from a lot of radiation, simply due to the fact that air has mass, and is able to stop a lot of radioactive particles before they reach the surface. Airline flight crews (like pilots and attendants) are considered an increased radiation risk, because they spend so much time at high altitudes. It’s even worse in the ISS, where the only thing between them and radiation is some thin glass and aluminum.
• Loss of coordination. Returning to the surface is more disorienting the longer you have been in space. Basically, you get used to the way microgravity feels, and suddenly experiencing gravity again is a shock to your system. You’ll constantly drop things because you’re used to things just floating when you let go of them. You’ll fall over, because you’re not used to actively keeping yourself upright. The same way long-term respirator patients need to re-learn how to breathe after being taken off the machine, long-term microgravity means you need to re-learn how to exist in gravity.