The International Space Station has recently played host to a rather important space experiment to see how long stays in space affect the human body.
NASA astronaut Scott Kelly just returned to Earth yesterday after 340 days in space, which is almost an entire year. The effects on his body will be compared to his twin brother, Mark Kelly, to see how space affects the human body over such a long period of time.
Some of the things we know space does to the human body are make us age faster, reduce our bone density, expose us to more radiation, and even cause bloating.
All of these effects may be pretty observable on the International Space Station, but it's thought that the effects could be worse on Mars.