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Out of the four Galilean moons of Jupiter, unfortunately 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. $\endgroup$ – Oscar Lanzi Apr 13 at 21:26
  • $\begingroup$ @StarMan I know that value but did they take into account Ganymede's own magnetospheric shielding? $\endgroup$ – user35272 Apr 14 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 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 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 at 16:56
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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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