Ganymede is probably the most appealing location for the Jovian colonists of the future, in large part because it has a magnetic field. This field recently was discovered by the Galileo spacecraft, and it is the largest known magnetic field of any moon. This magnetic field could shield Jupiter's radiation, just as the Earth's magnetic field shields the solar wind for us. It also has a decent gravity; 1/7 of Earth's. In this story, there are many colonists living on Ganymede because of these factors.