June of this year marks the 40th anniversary of the discovery of Pluto’s largest moon, Charon. At the time of Charon’s discovery, scientists weren’t even looking for it.

Pluto was discovered in 1930, and since then, scientists have been working hard to understand everything they could about the tiny celestial object 4.6 billion miles away. Improving technology  meant that over the years we could always learn something new about Pluto and constantly refine what we knew about things like its size and distance from the Sun, and even monitor things like its changing brightness. In particular, scientists wanted to better understand and refine Pluto’s strange orbit.

A "sideways" view of our solar system from the side, showing Pluto's orbit around the Sun. Credit: NASA JHU/APL

In 1978, James Christy was working on learning more about Pluto’s orbit and noticed that there was a weird blob sticking off the side of Pluto.

Christy noticed this blob would reappear in the same place every 6.39 days, making it clear to Christy and his team that this wasn’t some error with the equipment. Using archived images of Pluto, Christy and his colleagues continued to work on making sure they were right in their interpretations of their observations. On July 7, 1978, it was announced that they had confirmed the discovery of Charon.


In 1978, astronomer James Christy noticed something unusual in this image of Pluto. It would lead Christy and his team to the discovery of Charon, one of Pluto's moons. Credit: NASA

The bodies in our solar system are named after mythological characters. In the case of Charon, the name comes from one of the ferryman who carried souls across the river Acheron. Pluto is named after a Roman god of the underworld (also known as Hades in Greek mythology) and Pluto’s underworld is surrounded by five mythical rivers, Acheron being one of them. But why choose this specific ferryman on this specific mythical river? James Christy’s wife’s name is Charlene, which isn’t so different from Charon.  

In the 40 years since Charon’s discovery, scientists have continued to explore Pluto and its moons. The flyby of the Pluto system by the New Horizons spacecraft in July 2015 revealed to us that Charon was no longer something we should envision as a blob hanging off the side of Pluto: Charon is its own spectacular world.


New Horizons was about 6 million kilometers (3.7 million miles) from Pluto and Charon when it snapped this portrait late on July 8, 2015.

Charon also isn’t just Pluto’s moon, it is one of Pluto’s five moons. (Five!) As New Horizons approached the Pluto system, scientists worked hard to ensure that the spacecraft could safely fly through the Pluto system, collect data, and be healthy enough after the encounter to spend the next year sending that data back to Earth. The whole system of Pluto, and its moons Charon, Styx, Nix, Hydra, and Kerberos with all their wonderful complexity and strangeness, makes the whole accidental discovery of Charon even more inspiring.

Related Topics Space Solar System Dwarf planets
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