Human habitation on Mars deemed impossible due to four-year survival limit.
A recent study by UCLA has found that while a mission to Mars is still viable, humans should spend no longer than four years on the planet due to unsafe levels of radiation. The study also found that the best time to launch a mission to Mars is during the solar maximum, as it is possible to shield from Solar Energetic Particles. The spacecraft should provide enough protection during the round trip to and from Mars for the astronauts, but if the material the spacecraft is built with is too thick, it could actually increase the amount of secondary radiation.