Charon: Pluto's Moon Discovered by Accident (6 photos)

On June 22, 1978, astronomers James Christy and Robert Harrington, working at the U.S. Naval Observatory (NOFS) in Flagstaff, Arizona, discovered Charon, Pluto's largest moon.





Charon, imaged by NASA's New Horizons spacecraft, July 14, 2015

Christy and Harrington weren't hunting for moons. They set out to refine Pluto's orbital parameters. But Christy, studying the images, noticed something strange: in some of the photographs, the dwarf planet (then still the full ninth planet of the Solar System) appeared slightly elongated, as if smeared. Even more intriguing was the fact that this "blur" changed position with a clear periodicity—every 6.39 days.



Photographic plates that proved Pluto has a massive satellite

The intrigued astronomer told a colleague about the oddity. They looked at archival images of Pluto and discovered that the anomaly was repeating systematically. This could only have one explanation: there was a massive body near Pluto that was its satellite.

Thus, humanity learned that distant Pluto isn't alone—it has its own moon. Today, we know of four more moons besides Charon: Hydra and Nix (discovered in 2005), Kerberos (discovered in 2011), and Styx (discovered in 2012).





Styx



Cerberus



Hydra



Nyx

All names are related to Greek myths about the underworld:

Pluto - god of the underworld;

Charon - ferryman of souls;

Nyx - goddess of the night;

Hydra - a nine-headed snake-like monster;

Cerberus - a three-headed dog guarding the exit from the underworld;

Styx - a river in the underworld, separating the world of the living from the world of the dead.