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'Space Brain': Mars Explorers May Risk Neural Damage, Study Finds (nbcnews.com)
An anonymous reader quotes a report from NBC News: Astronauts making a years-long voyage to Mars may get bombarded with enough cosmic radiation to seriously damage their brains, researchers reported Monday. The damage might be bad enough to affect memory and, worse, might heighten anxiety, the team at the University of California Irvine said. It's the second study the team has done to show that cosmic radiation causes permanent, and likely untreatable, brain damage. While their experiments involve mice, the brain structures that are damaged are similar, they write in the Nature journal Scientific Reports. NASA knows that astronauts risk physical damage from the radiation encountered in space. Earth is enveloped in a large, protective sheath called the magnetosphere, which deflects a lot of the ionizing radioactive particles that speed through space. Teams aboard the International Space Station are inside that envelope. But moon travelers were not, and this summer a study showed the cosmic radiation may have damaged the hearts of many of the Apollo program astronauts. A trip to Mars would expose astronauts to even more radiation -- enough to cause cancer, for sure, and now this research suggests brain damage, as well. They bombarded mice with the same type of radiation that would be encountered in space, and then looked at what happened to their brains. It did not look good. The changes were seen in the connections between brain cells and in the cells, as well. "Exposure to these particles can lead to a range of potential central nervous system complications that can occur during and persist long after actual space travel -- such as various performance decrements, memory deficits, anxiety, depression and impaired decision-making. Many of these adverse consequences to cognition may continue and progress throughout life."
It's even worse because there's potential compounding factors on Mars that could make psychological issues even worse. For example, here's one that's little studied: deuterium. Mars's deuterium levels are 5-7 times higher than Earth's (nothing like Venus's 150-240x, but still..). Animals and plants certainly can survive rather high deuterium levels, up to 50% (and bacteria can survive 98% deuterated water); in terms of survival, it poses no threat. However, in terms of effects on long-term health effects, it's much less clear. For example, one study found a 1,8% increase in incidence of depression for every 10ppm increase in deuterium in water (Earth mean = ~155ppm). So when you're talking an ~800ppm increase... the issue of long-term deuterium health effects really warrants more study. Furthermore, microbial food sources that may be used on Mars (either for direct consumption or producing feed for, e.g. aquaponics) can concentrate deuterium even further.
Unlike most isotopes, hydrogen isotopes have rather different properties. Deuterated drugs are a new field of interest, for example, as they can have lifetimes in the body an order of magnitude higher than their non-deuterated equivalents. Deuterated plastics are often dramatically more transparent (and significantly more radiation resistant) than non-deuterated plastics. However, mixtures of deuterated and non-deuterated versions of the same plastic, melted together, often yield an opaque result because the two versions have different melting points and densities, yielding an inhomogenous result.
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http://www.nasa.gov/feature/goddard/real-martians-how-to-protect-astronauts-from-space-radiation-on-mars
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Indeed. This effect was described 25 years ago by John Kricfalusi and dubbed Space Madness.
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Not exactly. Ideal shielding is relatively thin metal followed by lots of hydrogen-rich material, plus a small amount of neutron absorbers (boron, etc). The hydrogen-rich material should make up the majority of the mass. This can be hydrogen-rich plastics (such as polyethylene), liquid hydrogen propellant (great ISP, although storage is difficult), methane propellant (what SpaceX plans to use, albeit they don't call for much during coast), ammonia (coolant, easy hydrogen store), water (need it anyway, even easier to store), hydrazine (commonly used for RCS thrusters), etc. NASA has been looking at trying to make structural composite materials out of hydrogenated boron nitride nanotubes, which would be killing two birds with one stone (since they're strong as well).
The reason you need lots of hydrogen is that a lot of your high energy impacts will often kick off neutrons, and these are much harder to block than ionized particles (this is particularly of concern with GCR and high-energy solar flare protons). The best way to eliminate neutrons is to moderate them down to the thermal spectrum so that they can be readily absorbed by high cross section absorbers. Hydrogen is by far the best neutron moderator per unit mass; nothing else really even comes close. It has a fairly high scattering cross section to begin with, and scatters far more per event than other compounds due to its low mass (more energy transfers from the neutron to the hydrogen), and presents far more nuclei to scatter from per unit mass than other elements. Liquid hydrogen is even better because you're thermalizing to a very cold temperature, which dramatically increases absorption cross sections (whether from hydrogen itself, or elements specifically used as absorbers such as boron). But again, liquid hydrogen is more difficult to store than other forms....
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The estimated exposure for a 400 day round trip transit and 560 days on the surface with 5g/cm2 aluminum shielding is 1070mSv with a 4.2% increase in death rate for men / 5.1% for women, and for 20g/cm2 aluminum it's 960mSv with an increase of 3.4% for men and 4.1% for women. But there's caveats that make this effectively higher, see below. Additionally, both of these are for solar minimum with no solar storms. The big problems however come when you have major charged particle events. If you don't budget for the mass for it, then you're playing dice with your crews' lives, so you pretty much have to. Events that give enough dose to significantly increase the mortality rate (ex. the August 1972 event) are not rare. And while rare, some events are powerful enough to cause acute radiation poisoning and even death in short (30 day) timeframes (4x the August 1972 event). The probability of the former is estimated at 0.2% per week, while the latter is estimated at 0.01% per week.
Re: the comic: XKCD vastly oversimplifies the situation. Radiation risks are not limited to "100mSv: lowest one-year dose clearly proven to increase cancer risk" levels. Radiation is also tied to a wide range of other diseases beyond cancer (cataracts, cognitive decline, lung damage, heart disease, etc), and the level "clearly linked" to cancer does not mean "there is no cancer beyond this point". Specifically concerning cancer: The NCRP-98 / NCRP-132 recommended limits for blood-forming organs are 250mSv/mo, 500/yr, 400-2900/life (depends on age and sex; young and female = less, old and male = more). These limits are based around a calculated excess 3% risk of developing fatal cancer. However, they are misleading because the error bars are large, and the upper end of the error bars is much more likely to kill you than the lower end - so if you want a 95% confidence interval, the risks from such figures are about 3 times higher than the mean suggests. Additionally, wherein the odds of dying from cancer are 3%, the odds of contracting cancer are inherently higher, since - especially in the presence of modern medicine - not all cancer is fatal.
Furthermore, studies with astronauts and animal models keep suggesting more problems from radiation in space than had previously been assumed, and we know little of the effects of the radiation environment beyond LEO. Simplistic radiation models that treat all types of radiation damage from a certain category "grouping" as equivalent appear thusfar to be inaccurate.
You will not find any researchers working in the field of studying the radiation health risks to astronauts who feel that the case is overblown. It is very much considered a significant problem that remains to be solved. Unless you're fine with willingly compromising travelers health and risking their outright survival in the case of a severe solar event, wherein, there's no problem, you can go ahead and launch ;)
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