Using the particle accelerators at NASA's Brookhaven National Laboratory on Long Island, mice were exposed to levels of radiation that proportionally matched what humans could expect to encounter on a three-year round trip to Mars. The mice were then given a series of memory tests to track how their brains held up in the wake of such exposure.
The results weren't good. Those exposed to the radiation were significantly more likely than the unexposed control group to fail tests that depended on their ability to recall objects and locations. Worse, these mice's brains showed clear buildup of the protein beta amyloid — which, in both mice and humans, is one of the clearest hallmarks of Alzheimer's disease.