Charon, a moon of Pluto
Charon from the New Horizons spacecraft, image credit: https://photojournal.jpl.nasa.gov/catalog/PIA19968
12,176 miles
753 miles
Charon is the largest and closest of Pluto’s five moons, and the earliest to be found. It was the first discovered on 22nd June 1978 by American astronomer James Christy at the United States Naval Observatory in Flagstaff, Arizona. With a diameter of 1,212 kilometres (753 miles), it is roughly half the size of Pluto itself. This remarkable size ratio makes Charon one of the largest moons in the Solar System relative to its parent world. Charon orbits Pluto at an average distance of just 19,596 kilometres (12,176 miles), meaning the two bodies sit unusually close together.
Because Pluto and Charon are so similar in size and orbit so closely, the two worlds do not revolve in the usual "planet-centre" manner. Instead, they orbit around a shared centre of gravity, known as the barycentre, which lies outside both bodies. This unique arrangement has led many astronomers to consider Pluto and Charon to be a double dwarf planet system rather than a typical planet-moon pair.
Both Pluto and Charon keep the same face towards each other at all times - a state known as mutual tidal locking. To a viewer on Pluto (imagining you could stand on its icy surface), Charon would appear fixed in the same place in the sky, never rising or setting.
Charon’s surface is dominated by ice, including water ice that forms vast plains and rugged cliffs. One of its most striking features is a dark reddish region near the moon's north pole known as Mordor Macula. This reddish colouring is thought to come from tholins, which are organic molecules formed when radiation breaks apart methane escaping from Pluto’s thin atmosphere.
When the New Horizons probe flew past Pluto and its moons in July 2015, it transformed our understanding of Charon. New Horizons revealed deep canyons, evidence of ancient tectonic activity, and regions that may once have held subsurface oceans. Compared to Pluto’s complex and varied landscape, Charon appears more stable and ancient, preserving a record of the early Kuiper Belt.
Star Trekkin' Across the Universe
So, you've sent a spacecraft to a distant moon, and you've spotted some interesting features on its surface? What do you do next? You've guessed it - you give them names! And, to give names to Charon's newly discovered places of interest, astronomers took inspiration from fantasy and science fiction books, films and television shows. The dark northern region, Mordor Macula, is named after the land of Mordor in The Lord of the Rings. There is also Vulcan Planum from Star Trek, TARDIS Chasma from Doctor Who, Kubrick Mons, a tribute to the director of 2001: A Space Odyssey, and Nostromo Chasma from Alien. Even craters have similar origins. If you're a fan of Star Wars, there are craters called Organa, Skywalker, and Vader, whereas Trekkies have Kirk, Spock, Sulu and Uhura craters to satisfy their inner geek.
Charon is named after Charon, the ferryman of the dead in Greek mythology. According to legend, Charon transported souls across the River Styx into the Underworld, a fitting theme given Pluto’s association with Hades, the ruler of that realm. The name was suggested by James Christy, the astronomer who discovered the moon in 1978. Christy noted that the moon’s name also resembled the nickname of his wife, Charlene (“Char”). The name was officially adopted by the International Astronomical Union later that year.
