The gas giant planet, Jupiter has 79 moons. So far, 53 of the moons have been named, the rest are waiting for official names. Many of Jupiter's moons are recent discoveries - 61 have been found since 1999. Astronomers may find more moons around Jupiter in the future.

Io, Europa, Ganymede, and Callisto are the largest of Jupiter's 79 moons. These moons are also known as the Galilean moons. These 4 moons were given this name for the astronomer who discovered them, Galileo Galilei. Galileo first observed these 4 large moons in 1610. 

Io, Ganymede and Callisto are bigger than the Earth's moon. The Galilean moons are so big, you can see them with a good pair of binoculars! They will only look like small spots of light, a bit like a star. But you know they're the moons because they appear in a straight line that crosses Jupiter.

The other 74 moons of Jupiter are much, much smaller than the Galilean moons. If you added the total mass of those 74 moons, it would still be thousands of times smaller than Europa, the smallest Galilean moon. 

Many spacecraft have visited Jupiter and its moons. The Pioneer missions took photos of the Galilean moons in 1973 and 1974. The Voyager spacecraft discovered volcanoes on Io and water ice on Europa in 1979. The Galileo spacecraft orbited Jupiter from 1995 to 2003 and discovered Ganymede's magnetic field.

 

Facts and Figures	Io	Europa	Ganymede	Callisto
a 1 Earth Mass = 5,980,000,000,000,000,000,000,000 kg
Distance from Jupiter (km)	421,600	670,900	1,070,000	1,883,000
Radius (km)	1,815	1,569	2,631	2,400
Mass (Earth Masses) a	0.015	0.008	0.025	0.018
Time of Orbit (Earth Days)	1.77	3.55	7.15	16.69
Day Length (Earth Days)	1.77	3.55	7.15	16.69