Moon Charon: Largest natural satellite of Pluto

It is half as wide as Pluto and is much like the dwarf planet itself. It was discovered many years after Pluto. They are sometimes considered a binary system, as Charon does not orbit Pluto, and Pluto does not orbit Charon. They orbit a barycentre in space.

The moon is named after Charon in Greek mythology, a boatman who would carry the souls of dead people across the river Acheron to the underworld.

 

Charon was found by astronomer James Christy on June 22, 1978, when he was looking at highly blown up picture of Pluto on a photographic plate that taken a couple of months before. Christy noticed that a slight bulge was able to be seen every now and then. This finding was made public on July 7, 1978. Later, the bulge was confirmed on plates dating back to April 29, 1965.

Later observations of Pluto were able to figure out that the bulge was due to a smaller body. The orbit of the bulge fit in with Pluto's rotation period, which was already known from Pluto's light curve.

Any final doubts were erased when Pluto and Charon entered a five-year period of mutual eclipses between 1985 and 1990. This happens when the Pluto-Charon orbital plane is edge-on as seen from Earth, which only happens at twice in Pluto's 248-year orbital period. It was very lucky that one of these intervals happened to occur so soon after Charon's discovery.

 

Pictures showing Pluto and Charon as separate disks were taken for the first time by the Hubble Space Telescope in the 1990s. Later, after a new technology called adaptive optics was discovered, it was possible to see Pluto and Charon as separate disks using ground-based telescopes.