Charon (moon)

Charon is the largest moon of the dwarf planet Pluto. It was found in 1978 at the United States Naval Observatory in Washington, D.C..

Charon is very big. It has half the width and one‑eighth the mass of Pluto.

Because of its size, Charon and Pluto have a special pull on each other. They are tidally locked, which means they always show the same face to one another. This is rare in our Solar System.

Charon has a reddish-brown cap on its north pole made of special molecules. These might help us learn about how life could start. The New Horizons spacecraft flew close to Charon in 2015, giving us our first close looks at this interesting world.

Discovery

Charon was found by astronomer James Christy using a telescope at the United States Naval Observatory Flagstaff Station. On June 22, 1978, he saw a strange shape near Pluto in old pictures. He realized it was a smaller object moving around Pluto. This discovery changed what scientists thought about Pluto's size and weight.

Later, between 1985 and 1990, Charon and Pluto passed in front of each other many times as seen from Earth. This helped confirm that Charon was real and gave scientists more information about both Pluto and its moon.

Name

Charon was first called S/1978 P 1 after it was discovered. The person who found it suggested the name Charon to honor his wife, Charlene. Charon is also a name from ancient stories. In those stories, Charon helped people cross to the afterlife. This made it a good match because Charon orbits Pluto.

Before Charon was found, a science fiction writer named Edmond Hamilton had imagined moons of Pluto in his stories and named one Charon. Later, two more of Pluto’s small moons were named Styx and Kerberos.

People sometimes say the name Charon in different ways. Some say it with a /k/ sound, like the ancient story, while others say it with a /sh/ sound to match “Char” from Charlene. Both ways are used.

Orbit

Charon and Pluto orbit each other every 6.387 days. They are locked together, so each always shows the same face to the other. The average distance between them is about 19,596 kilometres.

The discovery of Charon helped scientists learn more about the Pluto system. By watching how Pluto's outer moons move, they found that Charon has about 12% of Pluto's mass.

Formation

Scientists think Charon may have formed from a big crash that happened about 4.5 billion years ago. One idea is that a large object from the Kuiper belt hit Pluto, creating Charon from the pieces that flew off.

Another idea is that Pluto and Charon crashed into each other and then moved into orbit around one another. This crash would have been strong enough to remove some of Pluto’s icy surface, like methane (CH₄), but not so strong that either body was destroyed. Because Pluto and Charon have similar density, they likely hadn’t fully separated into rock and ice when they crashed. After the crash, heat from the impact would have helped create a hidden ocean inside Pluto.

Physical characteristics

Main article: Geology of Charon

See also: List of geological features on Charon

Charon is a large moon that orbits the dwarf planet Pluto. It is about half the size of Pluto and is the twelfth-largest natural satellite in our Solar System. Charon is similar in size to some moons of Uranus, like Umbriel and Ariel. Because Charon is so large compared to Pluto, the two are sometimes called a double planet.

Scientists think Charon may have had a subsurface ocean long ago. This ocean might have helped shape the patterns and mountains on its surface. Charon's surface is mostly made of water ice, unlike Pluto's surface, which has nitrogen and methane ices. There are dark areas on Charon, especially near the north pole, which may be from gases from Pluto that freeze and change color. Charon also has deep valleys and tall cliffs, showing it has had an active geological history.

Observation and exploration

The first clear pictures of Charon were taken by the Hubble Space Telescope in the 1990s. These pictures showed Pluto and Charon as two separate objects. Later, better cameras on Earth and the New Horizons spacecraft gave us closer looks. In 2015, New Horizons flew by Pluto and Charon, giving us the best pictures of Charon ever seen. This was the first and only time a spacecraft has visited Charon.

Classification

The Pluto–Charon system has its center of mass outside either body. This means they both move around a common point in space. Because of this, and because Charon has about 12.2% of Pluto's mass, some scientists think Pluto and Charon should be called a binary planet, not just a planet and its moon.

The International Astronomical Union currently calls Charon a moon of Pluto. There has been talk about whether Charon might one day be called a dwarf planet on its own. Pluto's other moons — Nix, Hydra, Kerberos, and Styx — also orbit this same point, but they are much smaller and are just called moons of Pluto.