Space travel has always been portrayed as risky -- no air or water, extreme temperatures -- a place where even a small miscalculation can be fatal. It can also be hazardous to your brain health, particularly on a three-year-long mission to Mars, according to a study published this week in the scientific journal PLOS ONE.
The eight-year long study, conducted at the NASA Space Radiation Laboratory at Brookhaven National Laboratory on New York's Long Island, found that the cosmic radiation on such a mission could accelerate the onset of Alzheimer's disease.
NASA is working on sending astronauts to a passing asteroid in the 2020s, and talks of a trip to Mars in the 2030s. It would take three years, with current technology, to get there and back. Current spacecraft are not heavily shielded from the cosmic radiation crew members would encounter beyond Earth's protective magnetic field.
Researchers used mice that were genetically engineered to be predisposed to Alzheimer's disease. They exposed them to cosmic radiation that was simulated in the lab.
"Galactic cosmic radiation poses a significant threat to future astronauts," said Dr. M. Kerry O'Banion, senior author and professor of neurobiology and anatomy at the University of Rochester Medical Center.
The study team wanted to see if radiation had the potential to accelerate Alzheimer's in those who were genetically vulnerable. Mouse models have been used extensively in this type of research and the rate at which they develop the disease is well understood.
Scientists have long worried about the potential dangers of working and living in deep space. Cosmic radiation beyond low Earth orbit, researchers say, could lead to cancer, cardiovascular disease, even cataracts.
Radiation exposure can cause acute effects such as nausea, vomiting, fatigue, skin injury and changes to white blood cell counts and the immune system, according to the National Space Biomedical Research Institute. Longer-term radiation effects include damage to the eyes, gastrointestinal system, lungs and central nervous system.
On Earth, humans are protected by the planet's atmosphere and magnetic field. Crew members on the International Space Station, at an altitude of 200 miles, are still within the magnetic sheath that surrounds us. The 24 Apollo astronauts who flew to the moon between 1969 and 1972 were not protected, but the longest missions lasted less than two weeks.
Once out of low orbit, astronauts are exposed to showers of different radioactive particles. Though engineers say they can protect themselves from the radiation associated with solar flares, so far, they cannot block other forms of cosmic radiation.
The longer astronauts are in deep space, the greater the exposure to this low-level radiation.
This is the first such study to explore effects of radiation on the nervous system, a phenomenon known as neurodegeneration, according to the authors.
"The possibility that radiation exposure in space may give rise to health problems such as cancer has long been recognized," said O'Banion. "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."
O'Banion has spent the last 20 years studying Alzheimer's disease.
He and his fellow researchers studied a form of radiation from so-called high-mass, high-charged particles, which come in various forms and fly through space at high speeds. Some come from distant stars that have exploded.
At Brookhaven, where a portion of the research was conducted, particle accelerators were able to recreate some of the radioactive particles found in space.
"It is extremely difficult from an engineering perspective to effectively shield against them," said O'Banion. "One would have to essentially wrap a spacecraft in a six-foot block of lead or concrete."
For the study, researchers used mice that had been engineered to have two human gene mutations associated with familial Alzheimer's.
"Essentially, they were healthy mice with nasty genes," said O'Banion. Under natural conditions, mice do not get the disease.
The mice were exposed to the radiation for several minutes. Six months later, researchers evaluated the exposure to see if it had any effect.
"A minute or two [of exposure] is like three years in human life," O'Banion said. "It's apples and oranges, a very different kind of exposure, but the total dose is equivalent to what an astronaut would receive [on a three-year Mars mission]."
Cosmic radiation is unlike anything on earth, according to O'Banion, similar only to what might be experienced in a nuclear accident.
"The big problem NASA faces is that shielding is not effective, and a spacecraft is not bulky enough to protect them over long periods of travel," said O'Banion. "At least not now. In scenarios I have heard of sending people to Mars, they would then build an underground shelter on a long-term mission in order to protect them from radiation."
While the research does not solve the problem of radiation in space or explain why humans develop Alzheimer's disease, it is important, according to O'Banion.
"My own bias is this," he said. "It's just another example of how the environment can influence diseases. The mice had a genetic predisposition and you have added an environmental injury -- an insult to their system. And now they show exacerbation of the disease."
There may even be parallels with the development of the disease after brain injuries in football players, he said.
Jeff Chancellor, a scientist in radiation physics at the National Space Biomedical Research Institute, said the Rochester team's study is solid.
"I know most of the authors and they are very well respected, and I have full confidence in their results," he said. . "The main benefit of their research is being able to further enhance the field and perhaps provide justification for more research."
"Any time you identify a mechanism for how a disease or a condition is induced, you further the ability to mitigate it," he said. "It provides more tools for the researchers and for M.D.s ... There are great benefits to all these studies."