Charon is the largest of the five known natural satellites of the dwarf planet Pluto. It has a mean radius of 606 kilometres (377 mi), making it the sixth-largest known trans-Neptunian object after Pluto, Eris, Haumea, Makemake, and Gonggong. With half the radius and one-eighth the mass of Pluto, Charon is a very large moon in comparison to its parent body. Its gravitational influence is such that the barycenter of the Plutonian system lies outside Pluto, and the two bodies are tidally locked to each other. Charon was discovered in 1978 by James W. Christy at the United States Naval Observatory (USNO), using photographic plates taken at USNO Flagstaff Station in Arizona. This photograph of Charon in true color was taken by the NASA spacecraft New Horizons, which flew by the Plutonian system in 2015.Photograph credit: NASA, Johns Hopkins University Applied Physics Laboratory, Southwest Research Institute and Alex Parker