Charon

What is Charon?

Moons· Diameter 1,212 km. Pluto's largest moon — half Pluto's diameter, so the two circle a shared center of mass while forever facing each other, almost a double dwarf planet. Its dark red polar cap is methane that escaped from Pluto and froze. Watch the pair spin every 6.4 days in the orbit view.

The orbit of Charon at this moment: https://nowsky.org/solar/orbit?fw=moon:pluto:charon&sp=0.0006 — replayed live, no login.

Orbit and size

Figures are representative approximations. Source: compiled from NASA and IAU public data.

Parent planet and sibling moons

Frequently asked

How big is Charon?

1,212 km (half of Pluto).

How far is Charon from Earth?

19,600 km.

What is notable about Charon?

mutually tidally locked pair.

Which planet does Charon orbit?

Pluto.

Charon's chasms and its cryovolcanic plain

Around Charon's equator New Horizons found a belt of chasms 1,600 km long and up to 9 km deep — four times the depth of the Grand Canyon. They are read as the crust splitting when an interior ocean froze and expanded, meaning Charon once had a subsurface sea that later froze solid. The broad southern plain Vulcan Planum is the cryovolcanic terrain where water–ammonia ice welled out at the time.

The reddish patch over Charon's north pole, nicknamed Mordor, is explained as methane escaping from Pluto, freezing onto Charon's cold pole and turning red under ultraviolet light. Each winter the pole sinks into darkness for over a century and collects methane; when spring returns only the hardened organic residue remains. Nowhere else in the solar system does one body's atmosphere stain its neighbour's surface.

The moon in depth

Charon was discovered in 1978 by James Christy of the US Naval Observatory, who noticed a faint bulge on photographic plates of Pluto. At 1,212 km across it spans more than half of Pluto's 2,377 km, and carries 12% of Pluto's mass — enough that the pair's shared center of mass lies outside Pluto, which is why astronomers sometimes call the system a double dwarf planet.

Pluto and Charon are mutually tidally locked: from one hemisphere of Pluto, Charon hangs motionless in the sky, while from the other it never appears. When New Horizons flew past in 2015, Charon turned out to be an active world of vast canyon belts and smooth plains. Its dark red northern cap, nicknamed Mordor Macula, is methane that escaped Pluto's atmosphere and froze onto Charon's frigid pole.

Track Pluto in the orbit view to watch Charon complete a lap at 19,600 km every 6.39 days. The system also holds four small moons — Nix, Hydra, Kerberos and Styx — that circle the Pluto–Charon pair together.

Related

Figures are representative approximations. Simulated positions use J2000 mean elements.