- Jupiter is the giant of the Solar System and has something like 50 or more satellites. A satellite is something that orbits around a body in space.
- Of these satellites, or moons, there are about 16 large ones.
- The four largest are known as the Galilean moons because they were first seen and described by the great Italian scientist Galileo in 1610.
- They were later named Io, Europa, Ganymede and Callisto.
- These moons were not known before 1610 because they cannot be seen with the naked eye.
- Galileo was the first person to design a telescope and was using this to examine the night sky.
Figure 1. The 4 Galilean moons of Jupiter, using a small telescope, but a much better one than Galileo had.
- The discovery of Callisto and the other 3 Galilean moons was extremely important.
- Galileo showed that these moons orbited round the planet Jupiter. This later gave support to the new theory (at that time) that the planets of the Solar System orbited round the Sun.
- It is the 3rd largest moon in the Solar System, with a diameter of 2,985 miles (4,800 kilometres). Callisto is larger than the planet Pluto and almost as big as the planet Mercury.
- Although it is the size of a small planet, Callisto is called a moon because it orbits round Jupiter and not round the Sun, as the planets do.
- Callisto may be one of the oldest bodies in the Solar System.
Figure 2. The surface of Callisto, showing ancient craters
- Callisto is more cratered than any other moon or planet in the Solar System.
- Like Ganymede, Callisto has a rocky core which is covered by a thick layer of ice.
- Callisto has a very dark surface, which shows its great age.
- In the sky, however, when seen through a telescope, it is much brighter than our own moon, because of the thick layer of ice.
- Callisto is the furthest of these four large moons from the planet Jupiter and is not influenced by the pull of the other moons’ gravity.
- Callisto is over one million miles from Jupiter. It orbits round Jupiter at an average distance of 1,169,856 miles (1,882,700 kilometres) from the planet.
- It takes 16.5 days to complete a single orbit, although it has to travel over 7 million miles.
- Callisto’s orbit is 7,345,927 miles (11,829,191 kilometres) long.
- Callisto orbits at a speed of 18,400 miles (29,530 kilometres) an hour.
Figure 3. A NASA image of the very ancient and heavily cratered moon, Callisto
- Like the other moons that orbit Jupiter, Callisto was named after a character from Greek mythology who had been loved by the Greek sky god Zeus, known to the Romans as Jupiter.
- If the astronomers had been able to see the ancient and pitted surface of Callisto, they might not have called the planet Callisto!
- Callisto was a beautiful girl, whom Zeus (Jupiter) turned into a bear to save her from the jealousy of his wife, Hera (Juno to the Romans).
- Zeus (Jupiter) then put Callisto into the sky as the constellation of stars which is known as the Great Bear (it is also called the Plough).
- There is also a constellation called the Little Bear beside the Great Bear. This was the son of Callisto and Zeus.
Overview of Callisto from NASA
Callisto Big Splash