Charon (moon)

Charon is the largest moon of the dwarf planet Pluto. It was discovered in 1978 at the United States Naval Observatory in Washington, D.C.. With a mean radius of 606 km (377 mi), Charon is very big compared to Pluto—it has half the diameter and one-eighth the mass of its parent body.

Because of its size, Charon and Pluto share a special gravitational relationship. They are tidally locked to each other, meaning 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 organic molecules. These might be important for understanding how life could begin. The New Horizons spacecraft came very close to Charon in 2015, giving us our first detailed views of this fascinating world.

Discovery

Charon was discovered by astronomer James Christy using a telescope at the United States Naval Observatory Flagstaff Station. On June 22, 1978, he noticed a strange shape near Pluto in old pictures and realized it was caused by 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 really there 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 the name of a figure from ancient stories who helped people cross to the afterlife. This made it a good match since 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 gravitationally locked, meaning 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.

But this crash would have made Charon colder and rockier than what we see today. 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 and their movement 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 big 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 together are sometimes called a double planet.

Scientists think Charon may have had a subsurface ocean in the past. This ocean might have helped create some of the interesting patterns and mountains we see on its surface today. 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 around the north pole, which may be caused by gases from Pluto that freeze and change color over time. Charon also has deep valleys and tall cliffs, showing that it has had an active geological history.

Observation and exploration

The first clear images of Charon were taken by the Hubble Space Telescope in the 1990s. These images showed Pluto and Charon as two separate objects. Later, better cameras on Earth and the New Horizons spacecraft gave us even 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 center of mass for the Pluto–Charon system is outside either body, meaning 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 considered a pair of worlds, called a binary planet, rather than a planet and its moon.

The International Astronomical Union currently classifies Charon as a moon of Pluto. However, there has been discussion about whether Charon might someday be called a dwarf planet on its own. The other moons of Pluto — Nix, Hydra, Kerberos, and Styx — also orbit this same center of mass, but they are much smaller and are simply called moons of Pluto.