This is currently a massive question in the media following the 'Twin Study' in which NASA sent one twin to the International Space Station for one year and kept the other on Earth as a ground controller.
After 25 months of testing, scientists found no significant health differences between the twins but several changes in the twin who spent one year in space that continued beyond his time in space. According to Cosmos: The Science of Everything, there were changes to the shape of his eyeball and less than 5% difference in his DNA Methylation.
Now, the question is 'what effects does Space Travel have on the human body' and this is a question that i want to try and answer using this current study and my general thoughts.
This study talks about a few key differences but after doing more research I found that there are actually multiple potential effects that spaceflight can have on the human body.
The NASA website talks specifically about a Mars mission and the different gravity fields an astronaut would experience. The gravity on Mars is approximately one-third of Earth's gravity and transitioning between the two could effect your spatial orientation, head-eye and hand-eye coordination, balance and give you motion sickness. Long periods of stay on Mars can also reduce your bone density about 1% per month which is not guaranteed to be corrected in rehabilitation (Abadie, Lloyd and Shelhamer, 2018). The Space Between Us portrays this aspect of space travel if you want to see it accurately portrayed in fiction.
If you think about it, when travelling in Space astronauts are confined to small places with the same few people for months at a time. This can cause major behavioural issues. NASA has specific training to try and avoid this from happening but extend space stays can cause a decline in mood, cognition, morale, or interpersonal interaction (Abadie, Lloyd and Shelhamer, 2018). A long stay away from home is likely to make anyone lose their mind.
The final major effect on the human body is Space Radiation. Earth's magnetic field and atmosphere protect the planet from cosmic radiation but on the Space stations, astronauts are exposed to over 10 times the radiation than what occurs on Earth (Abadie, Lloyd and Shelhamer, 2018). This exposure can damage the human body's central nervous system and increase the risk of cancer. But these aren't the only effects that can be cause. Exposure to space radiation can result in a lengthy list of problems including nausea, vomiting, anorexia and fatigue (Abadie, Lloyd and Shelhamer, 2018).
NASA is currently working on solutions to all these problems and many more so maybe space travel will be effects free and safe in the future, but how long will this take? Will these problems be solved before the next mission? Who knows but let me know what you think.
If you want to check out more on this experiment here are some links and as always I would love to hear any feedback so feel free to comment below.
https://www.sbs.com.au/news/nasa-twins-study-explores-effect-of-space-travel-on-humans
https://singularityhub.com/2019/04/14/nasa-twins-study-provides-new-insight-into-how-space-travel-affects-human-health/#sm.000enwc1u15aufd7v2m1o5l31674r
Reference
Abadie, L., Lloyd, C. and Shelhamer, M. (2018). The Human Body in Space. NASA. 9 Apr. 2019
https://www.nasa.gov/hrp/bodyinspace