Beyond a few brief trips to the Moon over four decades ago, humanity has never ventured outside of Earth's protective magnetic field. And while it's natural to dream of exploring the stars, such voyages may carry impossibly high medical risks.
That's the finding of researchers at the University of Rochester Medical Center, who have undertaken the latest study examining how prolonged time spent in deep space could affect human astronauts. The big issue is radiation — without the planet's magnetic field to act as a shield, astronauts in deep space would be exposed both to the fierce energies of solar flares and the persistent hum of what's known as galactic cosmic radiation.
Now, solar flares can be dealt with — we might have to ground our deep space flights for a couple decades when they get really intense, but that's not the end of the world — but it's the other, more subtle radiation that is harder to overcome. Prolonged exposure to cosmic radiation has previously been linked to increased cancer risk, cardiovascular problems, and musculoskeletal issues.
But the Rochester study points to perhaps the greatest danger of such prolonged exposure: the degeneration of the brain itself. 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. The study suggests the exposed mice — who are a close if not perfect match for human astonauts — developed Alzheimer's in far greater numbers than would be expected, and much earlier in life as well.
Researcher M. Kerry O'Banion doesn't mince words in saying what this means for future astronauts:
"Galactic cosmic radiation poses a significant threat to future astronauts. 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... These findings clearly suggest that exposure to radiation in space has the potential to accelerate the development of Alzheimer's disease. 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."
But why wouldn't it be possible to shield astronauts from the harmful radiation? The challenge is that we're dealing with high-mass, high-velocity particles generated in the crucible of supernovas. Unlikely the stray hydrogen protons produced by our Sun's solar flares, these particles are so powerful that they can penetrate any standard shielding. Dr. O'Banion and his team used iron particles, and he provides a sense of just how far we would have to go just to give astronauts a half-decent shot of surviving a deep space mission without long-term cognitive problems:
"Because iron particles pack a bigger wallop it is extremely difficult from an engineering perspective to effectively shield against them. One would have to essentially wrap a spacecraft in a six-foot block of lead or concrete."
Admittedly, in the weightlessness of space, that amount of shielding wouldn't really preclude the effective functioning of a spaceship, but it would almost certainly mean we couldn't launch such a craft from Earth. If these neurological risks really are this severe, we might be committed to assembling such heavily-shielded craft in Earth's orbit — itself a huge engineering task that's probably at least a couple decades away from being doable.
Indeed, it's also possible that our understanding of Alzheimer's disease and various cancers could progress to the point that we would be able to effectively manage or perhaps even cure astronauts' later medical problems. But if these studies suggest anything, it's that our next giant leaps into space — including planned missions to an asteroid in 2021 and Mars in 2035 — will require several orders of magnitude more knowhow than our trips to the Moon, and not just in terms of spaceship-building. Otherwise, astronauts will be forced to face their greatest tests and make their biggest sacrifices long after they return from Mars.
You can check out the entire original paper over at PLoS ONE.
Artist's conception of the first astronauts on Mars via NASA JSC.