So far to date, there have been 63 moons found orbiting Jupiter. The most famous of these are the Galilean moons: Io, Europa, Ganymede and Callisto, which were first seen by Galileo Galilei in 1610 using his crude 33-power telescope. When Galileo first saw these satellites of Jupiter, he realized that not everything in the Universe orbits around the Earth.
These first four moons are very large, in fact, Ganymede is the largest satellite in the Solar System.
The next moon discovered orbiting Jupiter was Amalthea, which was observed by E.E. Barnard in 1892. Moon after moon was discovered by astronomers, eventually totaling 13. Voyager 1 discovered 3 inner moons in 1979, bringing the total to 17. Since then, astronomers have turned up an additional 46 tiny moons orbiting Jupiter in eccentric orbits. These moons average a mere 3 km in diameter, and are probably all captured asteroids or comets.
The total currently stands at 63 moons; although, more will likely be discovered eventually.
The names for the moons of Jupiter are all taken out of Greek Mythology; specifically, lovers of Zeus (or Jupiter in Roman lore).
Io is the most volcanically active body in the Solar System, with dozens of volcanoes erupting at any time. The lava creates colorful bruises on the surface of Io. Io is the closest moon to Jupiter, and the intense gravity causes it to flex back and forth, driving the moon’s internal heat.
Europa is the second closest moon to Jupiter. Data gathered by NASA spacecraft indicate that the moon is surrounded by an ocean of liquid water, covered by a thick shell of ice. In fact, Europa may have twice as much water as Earth. Once again, the gravity of Jupiter is thought to be flexing the moon, and warming it up.
Ganymede is the next moon out, and is actually the largest moon in the Solar System – larger than the planet Mercury. It’s the only moon in the Solar System known to have an internally generated magnetic field.
Callisto is the 4th moon, and shows an extremely cratered surface. Unlike the other moons, Callisto’s surface is very ancient, with impact craters dating back billions of years.