With a diameter of over 4,800 km (2,985 miles), Callisto is the third largest satellite in the solar system and is almost the size of Mercury. Callisto is the outermost of the Galilean satellites, and orbits beyond Jupiter's main radiation belts. It has the lowest density of the Galilean satellites (1.86 grams/cubic cm). Its interior is probably similar to Ganymede except the inner rocky core is smaller, and this core is surrounded by a large icy mantle. Callisto's surface is the darkest of the Galileans, but it is twice as bright as our own Moon.
Callisto is the most heavily cratered object in the solar system. It is thought to be a long dead world, with hardly any geologic activity on its surface. In fact, Callisto is the only body greater than 1000 km in diameter in the solar system that has shown no signs of undergoing any extensive resurfacing since impacts have molded its surface. With a surface age of about 4 billion years, Callisto has the oldest landscape in the solar system.
Callisto was discovered on 7 January 1610 by Galileo Galilei. The discovery, along with three other Jovian moons, was the first time a moon was discovered orbiting a planet other than Earth. The discovery of the four Galilean satellites eventually led to the understanding that planets in our solar system orbit the sun, instead of our solar system revolving around Earth.
How Callisto Got its Name:
Galileo originally called Jupiter's moons the Medicean planets, after the Medici family and referred to the individual moons numerically as I, II, III, and IV. Galileo's naming system would be used for a couple of centuries.
It wouldn't be until the mid-1800s that the names of the Galilean moons, Io, Europa, Ganymede, and Callisto, would be officially adopted, and only after it became apparent that naming moons by number would be very confusing as new additional moons were being discovered.
Callisto was originally designated Jupiter IV by Galileo because it is the fourth satellite of Jupiter. Callisto is named for the beautiful daughter of Lycaon, who followed the chaste goddess of the hunt, Artemis. Unfortunately, since Callisto was seduced by Zeus (the Greek equivalent of the Roman god Jupiter) and became pregnant she was banished by Artemis. Zeus changed Callisto into a bear to protect her from his wife Hera's jealousy. Later, Zeus placed Callisto and their son in the sky, and mother and son became Ursa Major and Ursa Minor (Great Bear and Little Bear).