Astrodynamic considerations and existing propulsion technology limit the speed with which a crew can be delivered to and returned from the surface of Mars. A typical, energy efficient mission profile might involve six months of outward bound journey, up to a year and a half of exploration on the planet surface, and a return flight lasting another six months.1 All told this comes to nearly one thousand days, more than twice the length of any single mission in the history of human space flight and an order of magnitude longer than routine International Space Station operations.

Several hazards await the crews of Mars missions, including radiation exposure and the psychological stress of spending 30 months in a confined habitat, further from Earth than any human in history, with death no more than a hull's thickness away. This article focuses on the effects of prolonged weightlessness on the human body and our current understanding of the effects of microgravity on physiology.