Charon: Pluto’s Moon at the Edge of the Solar System

Before being reclassified as a dwarf planet in 2006, Pluto was considered the ninth and final planet of the Solar System. It was discovered in 1930 but it wasn’t until 1978 when astronomers discovered a second object. Named Charon, the moon was nearly half the diameter of Pluto and defied all expectations.

While working at the United States Naval Observatory in Flagstaff, Arizona, astronomer James Christy was sitting in the office of his supervisor Robert Harrington when he asked if there was anything he could do. Harrington considered him for a moment before reaching into his desk, pulling out a file and handing it to Christy.

The file was filled with images of Pluto from the Lowell Observatory, the place where the planet was discovered in 1930. The Lowell Observatory was only about 10 kilometres from where Christy and Harrington worked too.

Christy was tasked with refining Pluto’s orbit and he got to work studying the photographic plates under a microscope.

On June 22, 1978, a strange bulge on one side of the small planet caught his eye. He looked at other photos taken at different times and noticed the bump moved position as Pluto rotated.

Going to Harrington, Christy had a hard time convincing the more experienced astronomer of his finding. The two went away and carried out their own separate calculations, Christy did the measurements and Harrington worked out the statistics. They regrouped and found their figures matched up; there was definitely something there. Pluto and the object orbited each other every 6.39 days and were separated by about 20,000 kilometres.

Christy carried out more research that included using the naval observatory’s 61-inch optical telescope. The two men’s observations were confirmed: they had found a moon of Pluto. They announced the news to the world.

As the discoverer, Christy had the honour of giving the moon a name and he wanted to call it after his wife, Charlene. She’s known as “Char” for short and, thinking of protons and electrons, he settled on “Charon”. He began to stress the International Astronomical Union (IAU), an organisation who have the final say in celestial object naming conventions, wouldn’t accept it.

Christy went to the dictionary, looked up the word and the strangest thing happened. Charon turned out to be the River Styx boatman who ferries dead souls to Hades in Greek mythology. Considering Pluto is the Greek god of the underworld, the name was perfect. In 1986, the IAU officially designated the moon Charon.

Since becoming operational in 1990, the Hubble Space Telescope has made a few studies of the Pluto system. In 1994, it took the first images showing Pluto and Charon as separate bodies. Thanks to Hubble, scientists identified another two tiny moons (Nix and Hydra) in 2005, a fourth (Kerberos) in 2011 and a fifth (Styx) in 2012.

On July 14, 2015, the New Horizons spacecraft flew past Pluto and its moons, coming within 29,000 kilometres of Charon. The mission provided astronomers with a treasure trove of photos, information and scientific measurements.

Charon turned out to be more unusual than originally expected. Instead of being heavily-cratered, the moon was covered in cracks and large canyons and they spanned close to 16,000 kilometres across its surface. Charon had ice cliffs 6 kilometres high and showed a violent geological past.

Though mostly grey, Charon’s north pole has a reddish hue. Because of its close proximity, the moon’s gravity attracts methane gas from Pluto’s atmosphere. The gas then freezes on the surface. Over time radiation from the Sun turns the methane into heavier organic compounds that give the region its colour.

For almost 40 years, Charon was just a black, distorted dot in a photo for Christy. High resolution images taken by the New Horizons probe transformed the moon into a unique world for him and many others. Scientists are still working through the data collected and Charon still has many secrets.

Photo Credit: NASA (


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