 One of the main targets chosen by the HOPE 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 . 
 One of the main targets chosen by the HOPE 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 <a href='drop_aux' title='1'><mark>is</mark></a>the only Galilean satellite for which human exploration <a href='drop_aux' title='2'><mark>is</mark></a>feasible . The levels of ionizing radiation on Io , Europa , and Ganymede <a href='drop_aux' title='3'><mark>are</mark></a>hostile to human life , and adequate protective measures have yet to be devised . 
One of da main targets chosen by tdaHOPE 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 the only Galilean satellite fa which human exploration feasible . The levels of ionizing radiation on Io , Europa , and Ganymede hostile to human life , and adequate protective measures have yet to be devised .