 Because of the possibility of subsurface liquid oceans on Jupiter 's moons Europa , Ganymede and Callisto , there has 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 <unk> missions . NASA 's <unk> ( Jupiter Icy <unk> Orbiter ) , which was canceled in 2005 , and a European Jovian Europa Orbiter mission were also studied , but were superseded by the Europa Jupiter System Mission . 
