 <unk> targets are Europa , due to its potential for life , and Callisto , due to its relatively low radiation dose . In 2003 , NASA proposed a program called Human Outer <unk> Exploration ( <unk> ) that involved sending astronauts to explore the Galilean moons . NASA has projected a possible attempt some time in the <unk> . In the Vision for Space Exploration policy announced in January 2004 , NASA discussed missions beyond Mars , mentioning that a " human research presence " may be desirable on Jupiter 's moons . Before the <unk> mission was cancelled , NASA administrator Sean O <unk> stated that " human explorers will follow . " 
