Houston, we have another problem:

As if space travel was not already filled with enough dangers, a new study out today in the journal PLOS ONE shows that cosmic radiation – which would bombard astronauts on deep space missions to places like Mars – could accelerate the onset of Alzheimer’s disease.
"Galactic cosmic radiation poses a significant threat to future astronauts," said M. Kerry O’Banion, M.D., Ph.D., a professor in the University of Rochester Medical Center (URMC) Department of Neurobiology and Anatomy and the senior author of the study. "The possibility that radiation exposure in space may give rise to health problems such as cancer has long been recognized. However, this study shows for the first time that exposure to radiation levels equivalent to a mission to Mars could produce cognitive problems and speed up changes in the brain that are associated with Alzheimer’s disease."
While space is full of radiation, the earth’s magnetic field generally protects the planet and people in low earth orbit from these particles. However, once astronauts leave orbit, they are exposed to constant shower of various radioactive particles. With appropriate warning, astronauts can be shielded from dangerous radiation associated with solar flares. But there are also other forms of cosmic radiation that, for all intents and purposes, cannot be effectively blocked.