June 22 – Discovery of Pluto’s Moon, Charon (1978)

Pluto‘s biggest moon, Charon, was discovered on June 22nd, 1978 by American astronomer James W Christy at the United States Naval Observatory.

Until recently, Charon, like Pluto, was mostly a collection of blurry smudges on photographic plates, but thanks to the 2015 flyby of NASA’s New Horizons mission, they both suddenly got a lot closer.


True colour image of Charon (Image credit: NASA New Horizons).

At just over 1,000 km wide, Charon is quite small.  But because Pluto is also rather diminutive, Charon has a much greater effect on its host than the average satellite, particularly regarding the location of their barycentre, or the point in space about which their two-body system orbits.   Whereas the effect of our own Moon is only enough to move the barycentre slightly away from the centre of the Earth and produce a small “wobble” because the relative masses of the two bodies are so different, Charon is so big relative to Pluto that they orbit a point well outside the surface of Pluto.  This has led some to argue that the Pluto-Charon system should be considered as a binary system, rather than a dwarf planet/moon arrangement.


Charon up close (image credit: NASA New Horizons).

Charon’s orbit of Pluto is almost completely circular, with an eccentricity of 0.0002 (our Moon’s is a distinctly wonky 0.05, and Pluto’s eccentricity with regard to the Sun is a positively inebriated 0.25).  So the distance of Charon from Pluto can therefore be said to be the same all the time if you round to the nearest kilometre, 17,536. On Pluto there would be no media frenzy for the next “Super Charon”).

Pluto and Charon from just under 4 million miles (6m km). Image:NASA New Horizons

Charon is named after the ferryman who the Greeks needed to pay to take their dead to the underworld (ruled by Hades, or Pluto to the Romans).  Charon was one of the old “primordial” gods of Greek mythology. His father, Erebus, was the personification of darkness, which probably gave him a head start for landing a job in the underworld.

Caron and some guests, as depicted by Ukrainian artist Alexander Dmitrievich Litovchenko