Another very important design factor in a manned mission to Mars is unique in space exploration : the medical effects of a long term space mission. Even in the shortest options for a Mars mission, total time is never less than two years This poses a serious health threat, for the human body can decondition severely during spaceflight.
Several dangers await the space traveler. All celestial bodies are sources of radiation, and Mars missions often imply perilous passages close to the Sun, that is, the most important source of radiation in the solar system. The spacecraft has to provide protection against background radiation and also in the case of extraordinary solar activity.
Astronauts returning from prolonged stays aboard space stations have clearly shown that the human body severely deconditions when exposed to microgravity conditions. On average, 1% of bone marrow is lost per month, which makes it practically unacceptable to consider a several years long Mars mission without providing for artificial gravity by means of rotating the spacecraft. This in turn generates a whole new series of problems derived from rotation (inner ear vestibular problems, motion sickness, etc.)