Because of the possibility of subsurface liquid oceans on Jupiter 's moons Europa , Ganymede and Callisto , there have been great interest in studying the icy moons in detail . Funding difficulties have delayed progress . The Europa Orbiter was a planned NASA mission to Europa , which was canceled in 2002 . Its main objectives included determining the presence or absence of a subsurface ocean and identifying candidate sites for future lander missions . NASA 's JIMO ( Jupiter Icy Moons Orbiter ) , which was canceled in 2005 , and a European Jovian Europa Orbiter mission was also studied , but was superseded by the Europa Jupiter System Mission .