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The Sun's magnetic field now and then accelerates a gigantic amount of high velocity protons in different directions. Regardless of the types and causes, let's call it "proton storm" when it is dangerous enough to cause radiation acute sickness to astronauts outside of the useful natural protection of Earth's magnetic field and atmosphere.

What is the probability that a deep space mission gets hit by such a Solar proton storm? The Apollo missions all were lucky in this respect. But how lucky were they? As a benchmark, let's assume here the same radiation protection that the Apollo missions had, which I understand wasn't much at all.

What's the probability for a half-month Apollo type crew on the sunny side of the Moon being hit by an acutely unhealthy Solar proton storm?

What's the probability of a 26 months Hohmann conjunction mission to, and on, Mars having to experience such an event? If they did it the Apollo way in terms of radiation protection.

The further out from the Sun, the lower the impact, I suppose. Would it for example be a non-issue for humans at Saturn? Is there a neat probability distribution as a function of distance and time?

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  • $\begingroup$ There are Solar (Sun spot) seasons, 11 year cycles, to consider. To simplify a potential answer, this might be averaged out or taken at its best or worst if specified. $\endgroup$
    – LocalFluff
    Nov 3, 2016 at 17:28
  • $\begingroup$ Whilst this is an interesting question and important as a consideration for any interplanetary missions it is going to be difficult to answer without first making assumptions of a) shielding (which you have done by referencing Apollo) and b) the acceptable human impacts as a) and b) together define the size of storm that is important (otherwise it becomes prone to Carrington type answers where one is left wondering if smaller storms could also be a problem. $\endgroup$
    – Puffin
    Jan 20, 2021 at 12:46

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What is the probability that a deep space mission gets hit by such a Solar proton storm?

There's a paper by Riley et al. [2018] (https://doi.org/10.1007/s11214-017-0456-3) that estimates the occurrence rate of strong solar storms. So far as I recall, there is a ~10% of chance of a Carrington-level event happening once every decade.

The further out from the Sun, the lower the impact, I suppose. Would it for example be a non-issue for humans at Saturn? Is there a neat probability distribution as a function of distance and time?

Yes, one might assume the flux of energetic particles generated by a sun-originating shock wave would decrease with radial distance but this isn't always the case. Large and fast interplanetary coronal mass ejection (ICMEs) act like snowplows and sweep up particles, energizing them along the way. The longer the particles are in contact with the shock, the more energetic they can become. There is a peak radial distance though I don't recall off hand where it is (somewhere near or slightly beyond Jupiter if memory serves me) where the fluxes stop increasing and start to decrease for a variety of reasons. The size and speed of the ICME matters for all of these types of estimates and I do not think there is a clear, analytical expression or trend.

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    $\begingroup$ @Puffin - While magnetospheres are great for shielding from lower energy particles, strong storms tend to generate protons with energies >100 MeV, which tend to make it into LEO quite easily. Sure the Jovian magnetosphere will block higher energy particles than Earth's, but you will then be dealing with the much much harsher radiation belts of Jupiter. In general, if the sun gets upset there's not much to be done but brace for impact if you're in space. $\endgroup$
    – honeste_vivere
    Jan 20, 2021 at 15:05

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