Ganymede and Callisto

...both provide abodes for earthlings, in Pamela Lee's image from, Out of the Cradle, but especially Callisto. It orbits so far from the giant planet that none of the radiation from the planet affects the activity there. Astronauts here move with as much impunity as on our own Moon. Excavating Callisto's ice-soil mixtures and finding the minerals there is a first big job.