This image shows Pluto (lower left) and its large moon Charon (upper right). Notice how large Charon is compared to Pluto, and how close the moon is to the planet. This image was taken by the Hubble Space Telescope on February 21, 1994. Click on image for full size (11K JPEG) Image courtesy Dr. R. Albrecht (ESA/ESO Space Telescope European Coordinating Facility) and NASA.

Charon is by far the largest of Pluto's known moons. Charon was discovered by the American astronomer James Christy on June 22, 1978, while working at the U.S. Naval Observatory.

Charon is so large compared to Pluto that the pair are often referred to as a "binary planet" (double planet). The radius of Charon is 603.6 km (375 miles), which is about half that of Pluto. The moon also orbits very close to the planet, just over 1/20th the distance between Earth and the Moon. Charon circles Pluto once every 6.4 days at a distance of 19,571 km (12,161 miles). Like Earth's moon, Charon rotates exactly once per orbit, so the same side of the moon always faces the planet. Unlike the Earth-Moon system, this "orbital lock" phenomenon also extends to Pluto; Pluto's day is exactly as long as the orbital period of Charon. This means that Charon is always in the sky when viewed from one hemisphere of Pluto, while an observer standing on the other side of Pluto would never see Charon!

Some astronomers think Charon may have formed in a way similar to the process that created Earth's moon. They think a large object slammed into Pluto early in its history, and that the debris from this collision coalesced to form the moon. Other astronomers disagree, citing the nearly identical densities of Pluto and Charon as evidence which conflicts with this formation theory. According to the theory, Charon should be less dense than Pluto, since it should be made up of material from the outer layers of the planet and not from the denser material at the planet's core. Charon is composed of rock and ice; a little less than 2/3rds rock and slightly more than 1/3rd ice.

Charon is named after a character from Greek mythology. Charon was the ferryman of the dead, who conveyed the recently deceased across a river into the underworld. Hades, the Greek god of the underworld, was called Pluto in Roman mythology. Although the "ch" in the name Charon is pronounced like a "k" in Greek, most astronomers pronounce the moon's name with a "sh" sound at the start. That is because James Christy, the discoverer of Charon, chose to pronounce it that way in honor of his wife Charlene.

This picture shows Pluto (left) and its big moon Charon (right). Notice how big Charon is compared to Pluto. Notice how close Charon is to Pluto. This picture was taken by the Hubble Space Telescope in 1994. Click on image for full size (11K JPEG) Image courtesy Dr. R. Albrecht (ESA/ESO Space Telescope European Coordinating Facility) and NASA.

Charon is a moon of Pluto. Pluto has moons. Charon is much larger than Pluto's other moons. James Christy discovered Charon on June 22, 1978.

Charon is nearly as big as Pluto. Some astronomers call Pluto and Charon a "double planet". Charon is also very close to Pluto. Because Charon is so close, it goes around Pluto fast. It only takes Charon a little more than six days to orbit Pluto.

Did you know that we can only see one side of our Moon from Earth? That's because the Moon spins around in exactly the same amount of time it takes it to orbit Earth. The same thing is true for Charon and Pluto. Someone on Pluto could only ever see one side of Charon. Now here's the really strange part. Pluto also spins exactly as fast as Charon. So on one side of Pluto, Charon is always up in the sky. A person on the other side of Pluto would never see Charon at all!

Charon is made of rock and ice, like Pluto. Some astronomers think Charon formed when a large object crashed into Pluto. That is a lot like how scientists think Earth's moon formed. Other scientists aren't quite so sure that Charon was formed that way.

Charon is named after a character from Greek mythology. Charon was the ferryman of the dead. He took people who had just died across a river in a boat. He took them to Hades, the underworld. Hades was also the name of the god of the underworld. In Roman mythology, Hades was called Pluto.

This picture shows Pluto (left) and its big moon Charon (right). This picture was taken by the Hubble Space Telescope in 1994. Click on image for full size (11K JPEG) Image courtesy Dr. R. Albrecht (ESA/ESO Space Telescope European Coordinating Facility) and NASA.

Charon is a moon of Pluto. Pluto has moons. Charon is much bigger than Pluto's other moons. James Christy discovered Charon in 1978.

Charon is nearly as big as Pluto. Some astronomers call Pluto and Charon a "double planet". Charon is also very close to Pluto. Because Charon is so close, it goes around Pluto fast. It only takes Charon about six days to orbit Pluto. It takes our Moon a month to go around Earth!

Did you know that we can only see one side of our Moon from Earth? That's because the Moon spins around in exactly the same amount of time it takes it to orbit Earth. The same thing is true for Charon and Pluto. Someone on Pluto could only ever see one side of Charon. Now here's the really strange part. Pluto also spins exactly as fast as Charon. So if you were on one side of Pluto, Charon would always be up in the sky. If you were on the other side of Pluto, you would never see Charon at all!

Charon is made of rock and ice, like Pluto. Some astronomers think Charon formed when a large object crashed into Pluto. That is a lot like how scientists think Earth's moon formed. Other scientists aren't quite so sure that Charon was formed that way.

Charon is named after a character from Greek mythology. Charon was the ferryman of the dead. He took people who had just died across a river in a boat. He took them to Hades, the underworld. Hades was also the name of the god of the underworld. In Roman mythology, Hades was called Pluto.