Charon, the Binary Companion of Pluto

FROM THE LECTURE SERIES: A Field Guide to the Planets

By Sabine Stanley, Ph.D., Johns Hopkins University

New Horizons’ mission wasn’t just a mission to Pluto. It was also tasked to study Charon, Pluto’s binary companion. Why is Charon called a binary companion? And what impact does it have on other smaller moons of Pluto?

An illustration of Charon and Pluto.
The moon, Charon, was discovered by James Christy in 1978. (Image: Elenarts/Shutterstock)

Charon: The Basics

The moon, Charon, was discovered by James Christy in 1978. He proposed naming the moon ‘Charon’ because it was similar to his wife’s nickname ‘Char’, short for Charlene. He wanted the soft ‘ch’, but the mythological connection of ‘Karon’, the ferryman of the dead, to Pluto, the god of the underworld, made hard ‘k’ the more obvious choice. So one will hear both pronunciations used. NASA and the New Horizons team members generally use Christy’s choice of ‘Charon’.

With a diameter of about 1200 kilometers, Charon is big enough for gravity to have made it spherical. Charon has a similar bulk composition to Pluto—ice and rock—but its lower density of 1700 kilograms per cubic meter tells us that it’s more ice-rich than Pluto. Data from the New Horizons mission determined that Charon’s surface is mostly water ice. This is a big difference with Pluto, whose surface is mainly nitrogen and methane ices.

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Why Is Charon a Companion, Not a Moon?

We usually think of moons as orbiting an object at the center, similar to how we think of planets as orbiting the Sun at the center. But this isn’t exactly right. What really happens is that two gravitationally bound objects will both orbit their center of mass.

The center of mass is the location that represents the average position of the mass in the system. If one of the objects is much bigger than the other, then the center of mass is very close to the center of the large object and it looks like the small object is orbiting about the center of the large object.

An illustration of Pluto's five moons, including Charon.
The binary system of Pluto and Charon is orbited by a system of four other moons. (Image: NASA/JHUAPL/SwRI/Public domain)

This is what happens in the case of all of the moon systems orbiting their planets. But Charon is not much smaller than Pluto. The center of mass of the Pluto/Charon system is actually outside of Pluto. And Pluto and Charon both orbit around this point, both with orbital periods of about 6.5 Earth days. Pluto and Charon are also in a mutual gravitational tidal lock, hence they both keep the same side facing the other.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

The Mordor and the Tallest Cliff in the Solar System

Two features on Charon’s surface immediately jump out at you. First, its north pole is dark red due to a dusting of complex organics, similar to Pluto. Once methane is on Charon, sunlight breaks down the methane to create the same complex organics at Charon’s north pole, just like on Pluto itself. This area on Charon has been informally named Mordor, from the Dark Lord Sauron’s home in J. R. R. Tolkien’s Lord of the Rings.

We see a series of canyons or chasmata in Charon’s equatorial region. These are all named for vessels in fiction. For example, Serenity Chasma is named for the spaceship in the Firefly series, and Nostromo is named for the spaceship in the Alien films.

Argo Chasma, named after the mythological ship of Jason and the Argonauts, can be as deep as 9 kilometers. The cliffs surrounding it put Argo in the running for the tallest cliff in the solar system. The only other contender is Miranda’s Verona Rupes in the Uranus system, where estimates range between 5 and 20 kilometers.

New Horizons had a terrific position for looking directly into sheer cliffs that seem to be several miles deep. The full length of Argo Chasma is longer than the Grand Canyon, and it’s 5 times deeper.

Learn more about how our Sun defines our solar system.

The Other Moons of Pluto

The binary system of Pluto and Charon is itself orbited by a system of four other moons, all very small. These small moons are all non-spherical, with longest dimensions of only tens of kilometers across. Nix and Hydra are about twice the radius of Mars’s larger moon, Phobos. Styx and Kerberos are closer in size to Mars’s smaller moon, Deimos.

A representation showing different orbits of the Pluto's moons.
Charon is more of a binary companion than a moon of Pluto. (Image: NASA, ESA, and L. Frattare (STScI)/Public domain)

Styx is about twice as far from Pluto as Charon and takes around 20 days to complete one orbit. Styx is only about 7 kilometers in its longest dimension. Next out is Nix, whose longest dimension is about 50 kilometers. We have a bit better of an image of Nix from the New Horizons mission than we have of the other 3 moons, and it reveals several impact craters on the surface.

Kerberos is about 3 times farther from Pluto than Charon and has the longest dimension of about 12 kilometers. Finally, there is Hydra, the outermost moon, at about 4 times Charon’s distance from Pluto. It’s also the biggest of these small moons with a long dimension of about 51 kilometers.

Interestingly, despite all the chaos in the direction of the spin poles, all of Pluto’s moons do orbit in its equatorial plane in the prograde direction on circular orbits. This suggests that the moons formed around Pluto, rather than being captured. One possible explanation is that all five of Pluto’s moons, including Charon, are the result of a giant collision, similar to the impact that created the Earth-Moon system.

Common Questions about Charon, the Binary Companion of Pluto

Q: Does Pluto orbit Charon?

Charon is half as big as Pluto and its size is one of the major reasons why Charon is considered as a binary companion. These two binary companions orbit each other around a common center of gravity located between the two.

Q: What is Pluto’s moon Charon named after?

The moon or binary companion of Pluto is named after a ferryman of the Underworld called Charon. It also happened to be named after Charlene, the wife of James Christy, who is the discoverer of Charon.

Q: What are the five dwarf planets?

The five dwarf planets are Pluto, Eris, Ceres, Haumea, and Makemake.

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