Charon is fully half the diameter of Pluto itself - so large in proportion that the pair are best described as a double world. New Horizons revealed a gray, canyon-torn surface capped by a dark red north pole, informally named Mordor Macula.
Relative to its parent, it is the largest moon in the Solar System - half of Pluto’s diameter.
Its dark red pole is painted by methane that escapes Pluto, freezes onto Charon, and is reddened by radiation.
James Christy spotted it in 1978 as a bump on blurry photographic plates - and named it partly after his wife Charlene.
A true binary
Every planet and moon really orbit a shared balance point, the barycentre - normally buried deep inside the bigger body. Charon is so massive relative to Pluto that their barycentre sits in open space between them, so both worlds visibly circle it every 6.4 days. They are also mutually tidally locked: each permanently shows the same face to the other, hanging motionless in the other’s sky.
Physical characteristics
Diameter
1,212 km
Mass
1.590e+21 kg
Surface gravity
0.29 m/s²
Density
1.7 g/cm³
Tidally locked
Yes
Mean temperature
-220 °C
Atmosphere and composition
Atmosphere
None detected - New Horizons set very strict upper limits.
Composition
A water-ice crust over a rocky interior; ammonia compounds on the surface hint at ancient cryovolcanism.
Discovery
Discovered by
James Christy
Year
1978
How
photographic plates at the US Naval Observatory
Missions
New HorizonsNASA · Flyby probe · 2006 · active
New Horizons gave humanity its first close look at Pluto in July 2015, revealing nitrogen-ice glaciers and towering water-ice mountains. It then flew past the Kuiper belt object Arrokoth in 2019 - the most distant world ever explored - and continues outward on an extended Kuiper belt mission.