As far as relative planet-to-satellite size ratios go, the Pluto-Charon system stands out as the largest body compared to parent planet in the Solar System. It has a diameter of 1186 km compared to the 2320 km diameter of Pluto. The Earth-Moon system is next, with the Moon possessing 16% of the Earth's mass. The table below provides some information on Charon, including orbital characteristics (Illingworth, 1994).

Satellite Year of
Radius (Km)
Eccentricity Orbital
Period (days)
Charon 1978 1 186 20 000 < 0.001 6.39 98.80


As no spacecraft has ever visited the Pluto-Charon system, we know very little about it. Since Voyager 2's encounter with Neptune's largest satellite Triton, planetary geologists have thought it highly likely that both Pluto and Charon would be very similar to Triton in composition.