Exploration of Jupiter disadvantages for colonization because of the severe radiation conditions prevailing in Jupiter's magnetosphere and the planet's particularly deep gravitational well. Jupiter would deliver about 36 Sv (3600 rem) per day to unshielded colonists at Io and about 5.4 Sv (540 rems) per day to unshielded colonists at Europa, which is a decisive aspect due to the fact that already an exposure to about 0.75 Sv over a period of a few days is enough to cause radiation poisoning, and about 5 Sv over a few days is fatal. Ganymede is the Solar System's largest moon and the Solar System's only
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