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5/27/2016 by mdc
Astronauts on a Mars mission will be exposed to galactic cosmic radiation (GCR) and increased radiation during periodic solar storms. GCR is highly penetrating but low intensity (a cosmic drizzle), while solar storms are intense, short lived, and infrequent (cosmic thunderstorms). Solar storms are most likely during a roughly 7 to 8 year period of increased solar activity (solar maximum) and much less likely during a 3-to-4-year period of reduced solar activity (solar minimum).

Radiation from solar storms can be effectively reduced by shielding thickness. The GCR is nearly twice as strong during solar minimum compared to solar maximum and is difficult to shield against. Doubling a nominal spacecraft shielding thickness reduces the GCR exposure by only a few percent.

The biggest threat from radiation exposure is the possibility of dying from radiation-induced cancer sometime after a safe return to Earth. NASA radiation limits today are set to constrain this life-shortening risk to less than 3%, incorporating our current uncertainties in how radiation exposure leads to cancer. Mars mission plans today exceed this risk limit, but improved understanding of the risk may lead to longer mission times by reducing the uncertainty.

Andy Weir indirectly addressed this challenge when he chose to use a continuous thrust trajectory to and from Mars, reducing the transit time outbound to four months and the return to Earth to eight months. NASA quantifies radiation exposure in units of Sievert (Sv) which is a measure of how effectively radiation induces cancer. On Earth, you are exposed to approximately 5 milliSievert (mSv) per year. Radiation workers are limited to less than 5 centiSievert (cSv) per year.

Mission doses for astronauts on the ISS have ranged from a tenth to ten cSv. A rule of thumb, if used with caution, is that twenty cSv increases your risk of a fatal cancer by 1%. NASA is working to expand on that research to answer theses critical questions.

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