Charon

Moon· orbits Pluto

Charon is Pluto's largest moon — about half Pluto's size, so the two orbit a point in empty space as a 'double dwarf planet'. New Horizons revealed deep canyons and a dark reddish polar cap ('Mordor Macula').

Open in 3D viewSee Charon in the live, interactive 3D map

Key facts

Mass

1.59 × 10²¹ kg

0.0216 Moon masses

Radius

606 km

35% of our Moon's radius

Surface gravity

0.288 m/s²

3% of Earth's gravity

Day length

6.4 days

Orbit around Pluto

6.39 Earth days

Science

Physical

Mass: 1.59 × 10²¹ kg · 0.0216 Moon masses; JPL / IAU mean radii; spacecraft-tracked GM where measured
Radius: 606 km · 35% of our Moon's radius; JPL / IAU mean radii; spacecraft-tracked GM where measured
Density: 1,701 kg/m³ · 1.7× water; Computed from mass and radius
Surface gravity: 0.288 m/s² · 3% of Earth's gravity; Computed from mass and radius
Escape velocity: 591 m/s · 5% of Earth's escape velocity; Computed from mass and radius

Orbit around Pluto

Distance from Pluto: 19,600 km; From JPL orbital elements
Orbital period: 6.4 days; From JPL orbital elements
Eccentricity: 0; From JPL orbital elements
Inclination to the J2000 equator: 0.00 °; From JPL orbital elements

Rotation

Rotation period: 6.4 days
Tidal locking: Tidally locked to its parent

Temperature

Mean: 53 K (-220 °C)

Composition

Bulk composition: A water-ice crust (with ammonia-bearing ices) over a rocky interior, and a dark, reddish, tholin-stained north pole.

Radiation & temperature

Insolation: 0.000788 S⊕ · 1/1,269 of the sunlight Earth receives; Computed from the distance to the Sun (inverse-square; Earth's sunlight = 1), before any atmosphere.
Geometric albedo: 0.38; Geometric (V-band) albedo (New Horizons); Bond albedo lower.
Equilibrium temperature: 41 K (-232 °C); Computed from albedo + distance from the Sun (airless black body); uses the geometric albedo as an estimate
Measured mean temperature: 53 K (-220 °C)
Greenhouse warming: +12 K; Measured temperature minus the airless equilibrium — the greenhouse effect

Discovery & history

Discovered: June 22, 1978
By: James W. Christy
Method: Photographic plates
Name: Charon, the ferryman of the dead in Greek myth — and a nod to discoverer James Christy's wife, Charlene.

James Christy spotted Charon in 1978 as a bump that kept reappearing on photographs of Pluto. Fully half Pluto's size, it is so large relative to its world that the two orbit a point in empty space between them — a genuine double system. New Horizons revealed its great reddish polar cap, nicknamed Mordor, in 2015.