On June 21st, 1978, Pluto's moon Charon was discovered by the American astronomer James Christy. In mythology, Pluto was god of the underworld. Charon was his ferryman, who transported souls across the river Styx to the other side. Styx is another, more recently discovered moon, along with three more – Hydra, Nix and Kerberos. Charon is the biggest one though, it’s about half the size of Pluto. So when it orbits our outermost planet, Charon's mass has a substantial effect on Pluto, pulling it first one way, and then the other. The two are often referred to as a double planet, because their common center of gravity lies between them. Two summers ago, a space probe flew past Pluto and Charon, and sent back incredible pictures and information – ice mountains two miles high, vast nitrogen ice plains, and mysterious dark patches on Pluto’s farside. If you visit the website NASA dot gov, and enter the word “Pluto” in the search box, you can see these pictures for yourself.