Pluto

We do not know much about Pluto because it is so far away. At its furthest, it travels more than 7000 milion kilometres from the Sun. Even in powerful telescopes, it looks only like a faint star. So far, no space probes have visited the planet. All we know is that Pluto is a deep-frozen ball of rock and ice. It probably has a covering of ‘snow’, made up of frozen methane gas.

Because it appears fixed in Pluto’s sky, Charon can only be seen from one side of the planet. From that side, Charon would appear huge, much bigger than the Moon does on Earth. This is because Charon circles very close to Pluto, only about 20000 kilometres away. Our Moon circles 20 times further from us.

Pluto’s only moon, Charon is unique in the Solar System, as it is half as big across as Pluto itself. No other than moon is as big compared with its planet. Also, it circles Pluto in the same time it takes Pluto to spin round once. This makes Charon appear fixed in Pluto’s sky.