 One of the main targets chosen by the <unk> study was Callisto . The possibility of building a surface base on Callisto was proposed , because of the low radiation levels at its distance from Jupiter and its geological stability . Callisto is the only Galilean satellite for which human exploration is feasible . The levels of ionizing radiation on Io , Europa , and Ganymede are hostile to human life , and adequate protective measures have yet to be devised . 
