Exploration of Jupiter reach Europa after a 6.5 year cruise. The spacecraft would fly by the moon 32 times to minimize radiation damage. 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 lander missions. NASA's "JIMO" ("Jupiter Icy Moons Orbiter"), which was canceled in 2005, and
Discovery and exploration of the Solar System ("Venera 3"), Mars in 1971 ("Mars 3", although a fully successful landing didn't occur until "Viking 1" in 1976), the asteroid 433 Eros in 2001 ("NEAR Shoemaker"), and Saturn's moon Titan ("Huygens") and the comet Tempel 1 ("Deep Impact") in 2005. The "Galileo" orbiter also dropped a probe into Jupiter's atmosphere in 1995; because Jupiter has no physical surface, it was destroyed by increasing temperature and pressure as it descended. To date, only two worlds in the Solar System, the Moon and Mars, have been visited by mobile rovers. The first robotic rover to visit another celestial body was the