James W. Christy, a U.S. Naval Observatory scientist, was studying plates to refine our knowledge of Pluto’s orbit and position, when the discovery of Charon was made. The discovery image of Charon and Pluto was taken on July 2, 1978. The new satellite was named after the ferryman who rowed souls across the River Styx to Pluto’s realm in the underworld of Greek and Roman mythology. Charon is informally pronounced "SHAR-on," similar to the name of the discoverer’s wife, Charlene.

The 1978 discovery of Pluto’s satellite, Charon, was a breakthrough in gaining understanding of Pluto. By measuring the distance between the two and knowing the orbital period, it was possible to calculate for their mass. Charon’s mass is about 9.6 x 10^-10 solar masses. This is about 1/7th the mass of Pluto.

During the 1980’s, Earth crossed the orbital plane of Charon and a series of mutual events were observed that significantly enhanced the study of the Pluto-Charon system. It was fortunate that Charon’s discovery was made prior to the mutual events, considering that the sun crosses the orbital plane of Charon only twice during a single Plutonian year.

The mutual events led to the first accurate determination of the sizes of Pluto and Charon. Charon’s diameter is 1,172 kilometers (728 miles), just over half the size of Pluto. Charon is the largest satellite relative to its planet in the solar system. Their average separation is 19,640 km (12,200 miles).

Charon’s orbit is gravitationally locked with Pluto, so both bodies continually keep the same hemisphere facing each other. Pluto and Charon's rotational periods, and Charon's orbital period are 6.387 days (6 days, 9 hours, 17 minutes).

Charon’s surface composition is different from Pluto's. The moon appears to be covered with water-ice rather than nitrogen ice. Charon’s density is 1.2 to 1.3 g/cm³, indicating it contains little rock. Pluto’s density is 1.8 to 2.1 g/cm³. The differences in density tell us that Pluto and Charon formed independently. This rules out the idea that they accreted together as a double planet.

The accuracy of Charon’s density provided by the HST information is still being challenged by observers from ground based telescopes. Some theorize that Charon was formed when a planetesimal slammed into Pluto long ago, similar to the idea considered as the origin event of Earth’s moon. "Pluto’s axis or rotation and Charon’s orbit are highly inclined, as we might expect from such an impact."

It is thought possible that Charon, Neptunes’ moon Triton, and Pluto are icy worlds of the outer solar system that were not swept up by Uranus and Neptune or ejected from the solar system. These three appear to be relatives. Charon and Triton could have become satellites and the largest could have become Pluto.

Although there is yet much to be learned, observation of mutual events and other studies provided new data that has greatly enhanced our understanding of Charon, Pluto, and the outer solar system.

Animations of Pluto and Charon

• Pluto, Charon, and USA Size Comparison.
• Charon rotation movie.

Views of Pluto & Charon

Four Views of Charon
This image shows four views of Pluto's moon Charon. The images are centered in 0 degrees latitude, and 0 (top-left), 90 (top-right), 180 (bottom-left), and 270 (bottom-right) degrees longitude. The images are based on photometric measurements of Marc Buie/Lowell Observatory. (Courtesy Calvin J. Hamilton, and Marc Buie/Lowell Observatory)

Hubble Telescope Image
This is the clearest view yet of the distant planet Pluto and its moon, Charon, as revealed by the Hubble Space Telescope (HST). The image was taken on February 21, 1994, when the planet was 4.4 billion kilometers (2.7 billion miles) from the Earth.

The HST corrected optics show the two objects as clearly separate and sharp disks. This now allows astronomers to measure directly (to within about 1 percent) Pluto's diameter of 2,320 kilometers (1,440 miles) and Charon's diameter of 1,270 kilometers (790 miles).

The HST observations show that Charon is bluer than Pluto. This means that the worlds have different surface composition and structure. A bright highlight on Pluto indicates that it might have a smoothly reflecting surface layer. A detailed analysis of the HST image also suggests that there is a bright area parallel to the equator of Pluto. However, subsequent observations are needed to confirm that this feature is real. The new HST image was taken when Charon was near its maximum elongation from Pluto (0.9 arcseconds). The two worlds are 19,640 kilometers (12,200 miles) apart. (Courtesy NASA/ESA/ESO)

Map of the Surface of Charon
This is the first surface map of Charon, the moon of the solar system's most remote planet, Pluto. The map is based on photometric measurements. It covers the entire surface of the moon. (Courtesy A.Tayfun Oner, based on pictures courtesy of Marc Buie/Lowell Observatory)

Pluto

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