Moons · Space Codex
Charon
Half Pluto's width, Charon is the largest moon relative to its parent.
Pluto's half-sized partner
At a glance
Key points
The pair are locked face to face, and Pluto paints Charon's north pole red with escaping gas.
- Largest moon relative to its parent
- Pluto and Charon always face each other
- Pluto's escaping methane paints Charon's pole red
A double world
Charon is so massive relative to Pluto that the two orbit a point in empty space between them. Each has spun down until its day equals the 6.4-day orbit, so from Pluto, Charon hangs motionless in the sky.
Surface
Charon is mostly grey water ice. A belt of canyons and scarps, some longer and deeper than the Grand Canyon, crosses its middle; to the south lies a smoother plain that may have been flooded by icy lava. The rusty north polar cap forms as methane escaping from Pluto freezes onto Charon's cold, dark winter pole and is cooked by ultraviolet light into reddish tholins.
What we still don't know
Whether Charon once had an ocean whose freezing split the crust, and how much of its red cap comes from Pluto rather than from Charon itself.
Missions
- 2006–
Pluto flyby 14 Jul 2015; Arrokoth 1 Jan 2019; now studying the outer heliosphere.
First-hand · our 3D model
How we render it
Charon is a sphere wrapped in a map built from the New Horizons global mosaic, with a tint added for the rusty north-polar cap, Mordor Macula. New Horizons did not image Charon's southern hemisphere, so that part of the map is procedural fill, not data.
It is drawn at about 48% of Pluto's radius, slightly smaller than its true 51%, because the size rule that enlarges small moons gently compresses large ones. It orbits almost exactly in Pluto's equatorial plane with its 6.39-day period. The two are locked face to face: Pluto's spin is matched to Charon's orbit, so Charon hangs over the same hemisphere of Pluto while keeping one face towards it.
Moon sizes and orbits are compressed so a whole system fits on screen, but never reordered: each moon is drawn at 0.72 × (true size ratio)^0.6 of its planet, capped so no large moon is drawn bigger than it really is, and orbit distances are log-compressed into a band that clears the planet's rings. Orbital periods keep their true ratios, every moon is tidally locked, and on lighter devices only the two largest moons of each planet are drawn.
Related entries
- PlutoDwarf planet · heart of the Kuiper belt
- New HorizonsPluto and the Kuiper belt
Sources
- JPL Solar System Dynamics: satellite physical parameters (opens in a new tab)
- JPL Solar System Dynamics: satellite mean orbital elements (opens in a new tab)
- NASA Science: Charon (opens in a new tab)
- NASA NSSDC Pluto Fact Sheet (opens in a new tab)
- Grundy et al. 2016, Nature (WashU summary) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.