Even with shielding, the amount of cosmic radiation involved in a trip to Mars could put astronauts on the path to Alzheimer's disease.
The three-year trip, say scientists, would mean that they built up enough exposure to experience cognitive problems and permanent changes in the brain.
"Galactic cosmic radiation poses a significant threat to future astronauts," says Professor M Kerry O'Banion of the University of Rochester Medical Center (URMC).
"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."
The researchers studied the impact of a particular form of radiation called high-mass, high-charged (HZE) particles, which can penetrate solid objects such as the wall and protective shielding of a spacecraft.
They used particle accelerators at the NASA Space Radiation Laboratory at Brookhaven National Laboratory on Long Island to collide matter together at very high speeds and reproduce HZE particles suchn as iron.
"Because iron particles pack a bigger wallop it is extremely difficult from an engineering perspective to effectively shield against them," says O'Banion. "One would have to essentially wrap a spacecraft in a six-foot block of lead or concrete."
The researchers exposed mice to various doses of radiation, including levels like those that astronauts would experience during a mission to Mars, and then gave them a series of experiments during which they had to recall objects or specific locations. They found that mice exposed to radiation were far more likely to fail these tasks - suggesting neurological impairment - earlier than Alzheimer's symptoms typically appear.
The brains of the mice also showed signs of vascular alterations and a greater than normal accumulation of beta amyloid, the protein 'plaque' that builds up in the brain and is one of the hallmarks of the disease.
"These findings clearly suggest that exposure to radiation in space has the potential to accelerate the development of Alzheimer's disease," says O'Banion. "This is yet another factor that NASA, which is clearly concerned about the health risks to its astronauts, will need to take into account as it plans future missions."