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Out of the four Galilean moons of Jupiter, only Callisto is at a safe distance from its parent planet that the radiation is low enough so one is able to colonize it. Io is totally uncolonizable (from a realistic point of view) and Europa too, however one could survive below Europa's surface if one drills through fast enough.

Now what about Ganymede? Ganymede's orbit is between Europa's and Callisto's, and from a realistic point of view, one could visit Ganymede for several days. However, is there any realistic way to be able to settle on Ganymede (or an orbit around it) permanently? Ganymede has an own magnetosphere which surely helps, and maybe on the far side of Ganymede there's less radiation than on the side facing Jupiter? How thick would walls have to be to shield you well enough from Jupiter's radiation on Ganymede's surface?

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    $\begingroup$ And the far side of Ganymede is not a place to hide because the radiation associated with Jupiter is not emitted by Jupiter. $\endgroup$ Apr 13, 2020 at 21:26
  • $\begingroup$ @StarMan I know that value but did they take into account Ganymede's own magnetospheric shielding? $\endgroup$
    – user35272
    Apr 14, 2020 at 4:51
  • $\begingroup$ @user30007 I think it does take into account Ganymede's magnetosphere. Ganymede has a magnetic field strength of 719 nT at the poles. Compare that to Mars which has a strength of about 1,500 nT. Ganymede receives more radiation that Mars and has a weaker magnetic field. $\endgroup$
    – Star Man
    Apr 14, 2020 at 15:22
  • $\begingroup$ @StarMan Don't you mean Earth? Mars has no magnetosphere, just local magnetic fields. Do you mean one of them? Afaik, Ganymede has 3% the strength of the Earth's magnetosphere. $\endgroup$
    – user35272
    Apr 14, 2020 at 16:45
  • $\begingroup$ @user30007 Mars does have a magnetosphere, but most of it actually comes from the Sun itself, not from the core of Mars. Now obviously, it's a lot less weaker and less extensive than Earth's. $\endgroup$
    – Star Man
    Apr 14, 2020 at 16:56

2 Answers 2

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Wikipedia's page on Ganymede says

The radiation level at the surface of Ganymede is ... 50-80 mSv (5-8 rem) per day

The reference for that is Podzolko, M.V.; Getselev, I.V. (4–8 March 2013). "Radiation Conditions of a Mission to Jupiterʼs Moon Ganymede". International Colloquium and Workshop "Ganymede Lander: Scientific Goals and Experiments. IKI, Moscow, Russia: Moscow State University.

Roughly speaking, an unshielded human would be dead in a couple months. As to the type of shielding, that depends on the kinds of radiation incident. I believe the component quantities are available in that report or similar documents.

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A Couple Feet of Concrete

Starting with 5 Rem per day surface radiation quoted on Wikipedia / Carl Witthoft's answer, you can calculated that shielding required for prolonged exposure would be several feet of concrete.

5 Rem is the US Navy lifetime limit for radiation exposure - so Uncle Sam would send you home after a single day on the surface of Ganymede.

If we lowered the rad level by an order of magnitude (10x), we'd get 10 days. 100 times reduction, 100 days stay time - and so on.

100 years times 365 days gives you 36,500 days in a human lifetime. Since we're doing an order of magnitude estimate, we'll pretend 36,500 is equal to 10,000. So if we lower the radiation level on Ganymede by 10,000 times, we can live indefinitely on it's surface.

OSHA's example chart lists some reasonable values we can use for "tenth thickness" - the amount of material needed to lower radiation levels by 1 order of magnitude.

The tenth thickness of concrete for Co-60 Gamma radiation is 21 cm. Since we need a 4 order of magnitude reduction, we need 4 x 21 cm = 80cm or about 33 inches of concrete. You could instead use 11 inches of steel or 6 inches of lead, but that sounds heavy and I'm assuming you'd rather make concrete in-situ than haul tons of material across the Solar System.

Realistically, I think you'd build your base underground, since you'd need to do a bunch of digging to get concrete anyway.

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    $\begingroup$ Interestingly, OSHA let's you get 5 Rem per year but the Navy (years ago when I was a rad worker) wouldn't let us exceed 5 Rem lifetime. I'm rolling with the Navy limit because I'm queasy just thinking about 5 Rem a year. $\endgroup$
    – codeMonkey
    Jun 23 at 21:43

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