Jupiter has a total of 63 known satellites. The four great moons of Jupiter were discovered by Galileo Galilei (1564?1642) in Jan. 1610, and are called the Galilean satellites after their discoverer. Their names are Io, Europa, Ganymede, and Callisto. Like our Moon, the satellites always keep the same face turned toward the planet they circle. Jupiter's four largest moons all have thin atmospheres. A carbon dioxide atmosphere envelops Callisto; Europa and Ganymede each have thin oxygen atmospheres; and Io's contains sulfur dioxide.