Re: Calisto is a better location than Ganymede
"G's mag field is way too weak to shield living things on the surface. You're right about Callisto being the only place for man - until we develop portable super-strength mag field radiation shields."
Or portable super-strength rad hardened genome hacks.
More near term though, these are both icy (surfaced) moons. If a tunnel system could be excavated in a hurry just after (or prior to) arrival, shielding problem solved. Of course, then I guess you just hide out in the ice warrens and explore roboticaly?