Long journeys into deep space, including a mission to Mars, could expose astronauts to levels of cosmic radiation harmful to the brain and accelerate Alzheimer's disease, said US research Monday.

The NASA-funded study involved bombarding mice with varied radiation doses, including levels comparable to what voyagers would experience during a mission to Mars, and seeing how the animals managed to recall objects or locations.

Mice that were exposed to radiation were far more likely to fail those tasks -- suggesting neurological impairment earlier than such symptoms typically appear.

"Galactic cosmic radiation poses a significant threat to future astronauts," said Michael O'Banion, a professor at the University of Rochester Medical Center and senior author of the study published in scientific journal Plos One.

"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."

NASA is planning manned missions to a distant asteroid in 2021 and to Mars in 2035. A round trip to the Red Planet could take as long as three years.

While space is filled with radiation, Earth's magnetic field generally protects the planet and people in a low earth orbit from such particles. But once astronauts leave orbit they are exposed to a shower of various radiation.

Over the past 25 years, NASA has funded research to determine the potential health risks of space travel, aiming to develop countermeasures and determine whether or not the risks might imperil extended manned missions to deep space.

Several previous studies have shown the potential cancer, cardiovascular, and musculoskeletal impact of galactic cosmic radiation.

But the University of Rochester study examined the potential impact of space radiation on neurodegeneration, and in particular the biological processes in the brain that contribute to the development of Alzheimer's.

The brains of the mice also showed signs of vascular alterations and a greater than normal accumulation of beta amyloid, the protein "plaque" that accumulates 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," said 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."