The human body was not designed for space. Low gravity causes muscles to waste away and bones to lose their density. Without the protection of the Earth’s magnetic field, cosmic rays and solar radiation bombard tissue and cells, inflicting damage that could cause disease.
Living out there is psychologically taxing as well. Without the Earthly cycle of night and day, astronauts struggle to maintain their natural sleep schedule, which can lead to changes in mood and mental performance — never mind simply being so far from home, trapped in a living area the size of minivan, with only a few other folks to talk to for months on end.
Still want to volunteer for that nine-month trip to Mars?