1978

Charon

James W. Christy (b. 1938), Robert S. Harrington (1942–1993)

At nearly 40 times farther away from the Sun than Earth, Pluto was a challenging object to observe in the decades after its discovery by Clyde Tombaugh in 1930. Nonetheless, astronomers continued to observe Pluto to learn more about its origin and properties. Many of these observations were performed from telescopes at the Lowell Observatory in Flagstaff, Arizona, where Pluto had been discovered.

While analyzing a series of such observations in June, 1978, Lowell astronomer James W. Christy noticed a bulge in some of the photographs of Pluto. Christy noted that the bulge appeared to move around the main image of Pluto in a period of about 6.4 days. After some further analysis, he and colleague Robert S. Harrington published their discovery of a satellite orbiting Pluto. Christy named the moon Charon, after the mythological Greek ferryman of the dead, but also in honor of his wife (Charlene, or “Char”), which is why the name is pronounced “Sharon,” not “Karon.”

Charon turns out to be quite large—750 miles (1207 kilometers) in diameter, about half of Pluto’s diameter—making Pluto-Charon more like a double planet system. Charon also turned out to have a surface dominated by water ice and possibly small amounts of hydrated ammonia—a combination quite different from Pluto’s surface, which is dominated by nitrogen and methane ice.

Hubble Space Telescope photograph of Pluto and Charon (bright pair) and the fainter moons Nix (closer) and Hydra (farther).

Observations of Pluto and Charon by the Hubble Space Telescope in 2005 revealed another surprise—more moons! Two additional but much smaller satellites, named Nix and Hydra after other Greco-Roman mythological figures, were discovered to be orbiting the center of mass of the Pluto-Charon system; two more, called simply P4 and P5, were discovered in 2011 and 2012. Excitement is mounting to see what these distant worlds look like when the NASA New Horizons probe flies past them in July 2015.

An artist’s conception of Pluto (larger) and Charon (smaller), viewed from the surface of one of the system’s smaller moons, discovered in 2005.