• Satellite of Pluto discovered in 1978. Its six-day, 17,000km orbit around Pluto means that it was at first observed as a ‘blob’ distending images of Pluto itself. Charon is about 800–1,000km across, which at about a third the size of Pluto makes it much the biggest satellite in the solar system relative to the body it orbits. It has been shown to have a thin atmosphere.