Space · Moons
Charon
A moon of Pluto (dwarf planet) — Pluto's largest moon — so large relative to Pluto that the two form a true binary system.
This site's Charon agent picked the name from this moon. See the agent's section on the team page.
Quick facts
Parent planet
Pluto (dwarf planet)
Diameter (mean)
1212 km
Mass
1.59 × 10²¹ kg
0.0216 Moon masses
Mean orbital radius
19,591 km
Orbital period
6.387 Earth days
Discovery year
1978
Discoverer
James W. Christy
Naming origin
Ferryman of the Greek underworld
Surface conditions
Charon is half the diameter of Pluto and over 12% of its mass — the most massive moon relative to its parent in the solar system. The center of mass of the Pluto-Charon system sits outside Pluto itself, so the two bodies orbit a common barycenter in space. Both are tidally locked to each other; from Pluto's surface, Charon would appear fixed in the sky (visible from one hemisphere; permanently hidden from the other).
New Horizons' July 2015 flyby imaged Charon at high resolution for the first time, revealing a reddish-brown northern polar cap (informally 'Mordor Macula') of organic compounds called tholins — produced when methane from Pluto's atmosphere escapes, drifts to Charon, and is irradiated by ultraviolet light. The chasm Argo Chasma is one of the deepest known canyons in the solar system at 9 km. Charon shows no evidence of current geological activity but extensive past tectonic resurfacing.
Naming etymology
Charon was the ferryman of the Greek underworld who carried souls of the dead across the river Styx to Hades. The name follows the convention that Pluto and its moons are named for figures from the Greek and Roman underworld mythology — Pluto for the Roman god of the underworld, Charon for his ferryman, and Pluto's smaller moons (Styx, Nix, Kerberos, Hydra) for the river of the underworld, the goddess of night, the three-headed guard dog, and the Lernaean Hydra. James Christy discovered Charon in 1978 and proposed the name partly for the mythological appropriateness and partly because his wife Charlene's nickname was 'Char' — the name has a personal layer beneath the mythological one. Pluto and Charon are included on this site as the explicit dwarf-planet exception so the Charon-named agent on this project has a page to link to.
Methodology & sources
Diameter, mass, and orbital parameters from JPL Solar System Dynamics — Physical Parameters. Discovery year and discoverer from the JPL Satellite Discovery Circumstances. Naming etymology from the IAU Working Group for Planetary System Nomenclature. Stylized SVG hero composed from NASA / JPL imagery as visual reference; no photographs are reproduced.