Charon

The giant moon that dances with Pluto

Charon is Pluto's moon, but it is so big (half of Pluto!) that the two circle each other — like dancing hand in hand. They always face each other: standing on Pluto, you'd see Charon never move in the sky. Charon's north pole is reddish — 'painted' by gas that drifted over from Pluto.

Charon (radius 606 km) was discovered by James Christy in 1978; at 12% of Pluto's mass it is the largest moon relative to its parent in the Solar System. The centre of mass lies 960 km above Pluto's surface — a binary system; both are tidally locked (6.387 days). The surface is water ice, darker than Pluto's, with craters and Serenity Chasma, a 1,000 km canyon (4 times deeper than the Grand Canyon). Mordor Macula is a red patch at the north pole: methane escaping from Pluto froze on Charon's cold pole and turned into tholins (reddish organics) under ultraviolet light. Named after the ferryman of the river of the dead in Greek myth. New Horizons photographed it from 29,000 km in 2015.

Fun facts

• Charon and Pluto orbit a shared point — which lies outside Pluto.
• The red patch on Charon's north pole is named 'Mordor' — from 'The Lord of the Rings'.
• Charon's canyon is 1,000 km long — 4 times deeper than the Grand Canyon.