Discovery of Charon

Christy and Harrington: The Discoverers

Christy and Harrington
Christy (left) and Harrington in 1978
Credit: U.S. Naval Observatory

The New Horizons Science Operations Center at APL is named for James Christy — click here to read about it.

Charon was discovered in June 1978 by James Christy and Robert Harrington at the U.S. Naval Observatory Flagstaff Station in Arizona – only about six miles from where Pluto was discovered at Lowell Observatory. They weren't even looking for satellites of Pluto - they were trying to refine Pluto's orbit around the Sun!

Charon was discovered when sharp-eyed Christy noticed the images of Pluto were strangely elongated - it looked like Pluto had an irregular blob attached to its side. Perhaps the telescope was joggled when the picture was taken? No, that possibility was quickly eliminated by noticing that the other stars on the photo were round. Moreover, the blob itself seemed to move around Pluto; the direction of elongation cycled back and forth over 6.39 days - Pluto's rotation period. From this, Christy, after being checked by Harrington, concluded that Pluto either possessed a mountain thousands of kilometers high or a satellite in a synchronous orbit.

Searching through their archives of Pluto images taken years before, Christy found more cases where Pluto appeared strangely elongated. Working independently, Christy measured the angle (from north) where the elongations appeared while Harrington calculated what the answer "should be" if the elongation was caused by an orbiting satellite. When the anxious moment came for them to compare their answers, they found perfect agreement. Just to be sure, they waited for the Naval Observatory 60-inch telescope to make one more confirmation. And sure enough, on July 2, new images showed the elongation due to a satellite right where it was supposed to be. They announced their discovery to the world on July 7, 1978. Christy proposed the name "Charon," after the mythological ferryman who carried souls across the river Acheron, one of the five mythical rivers that surrounded Pluto's underworld. Apart from the mythological connection for this name, Christy chose it because the first four letters also matched the name of his wife, Charlene.

The Discovery Image


Charon's satellite status was finally confirmed when Pluto and Charon began a series of mutual eclipses in 1985. Later, Hubble Space Telescope and even advanced ground-based telescopes were able to spot Charon orbiting nearby — just 1/4,000 of a degree from Pluto!

Click here for more recent images of Pluto and Charon.

This 1994 Hubble Space Telescope/Faint Object Camera image of Pluto and Charon was the first to show Charon as a separate object.