Charon

Charon is the largest of Pluto's five known moons (as of August 2026) and, relative to its parent body, the largest satellite in the solar system. NASA gives its diameter as about 1,214 km, roughly half Pluto's 2,377 km, and its mass is about 12% of Pluto's. That ratio is so large that the pair's common centre of mass, the barycentre, lies in the space between them rather than inside Pluto, which is why the system is often described as a double dwarf planet.

Charon orbits about 19,640 km from Pluto once every 6.4 Earth days. Pluto rotates on its axis in the same period, so the two bodies are mutually tidally locked: each keeps the same face turned permanently towards the other.

Charon was discovered on 22 June 1978 by James Christy, working with Robert Harrington at the US Naval Observatory station in Flagstaff, Arizona, when he noticed that photographic images of Pluto appeared elongated and that the bulge moved around the dwarf planet. The name comes from the ferryman of Greek myth who carried souls across the river Acheron; Christy also chose it because its first four letters matched his wife Charlene's name.

Only one spacecraft has visited. NASA's New Horizons flew past Pluto on 14 July 2015 and passed Charon at a distance of about 28,800 km. It revealed a grey, water-ice world with a surface temperature of about minus 220 °C. A belt of huge fractures runs around the equator, including Serenity Chasma, at least 1,000 km long, and Argo Chasma, which may be as deep as 9 km. This tectonic belt suggests Charon once had a subsurface ocean that froze and expanded, cracking the crust.

The most striking feature is the dark reddish region around the north pole, nicknamed Mordor Macula by the mission team. The leading explanation is that methane and nitrogen escaping Pluto's thin atmosphere are cold-trapped at Charon's winter pole, where ultraviolet light converts them into reddish organic compounds called tholins.

A common error is to describe Charon as a captured object. Most researchers instead favour a giant impact early in the solar system's history, similar to the leading model for the origin of Earth's Moon.