asteroids, some have questioned whether it should be called a planet at all. Pluto is also unique in that its moon, Charon (named after the figure in Greek mythology who ferried dead souls across the river Styx), is half the size of the planet itself. Charon is by far the largest moon relative to its planet and the only in the solar system that causes significant swings in its planet’s motion. Charon was discovered in 1978, when it appeared as a bulge in images of the planet, much as it does in this ground-based photo- graph. With the Hubble Space Telescope, however, which is not subject to the distortions caused by the Earth’s atmosphere, the two objects are easily differentiated. 
    Pluto’s orbit is highly eccentric.