Pluto is an alien world in almost all respects, but in one way it resembles Earth: it has a large, close companion. In 1978, James Christy at the US Naval Observatory in Washington DC realised that a bump on some images of Pluto wasn't a defect in observations, as had been assumed, but a giant moon. This was the largest moon to be discovered since 1846, when Neptune's moon Triton was found a couple of weeks after its planet. As Pluto is god of the underworld, Christy named it Charon, after the ferryman who carries souls to the underworld across the river Styx.
Rather than Pluto's chemical tutti-frutti, Charon's surface is mainly water ice. But it also has a dash of ammonia, which produces a distinctive dip in its infrared spectrum. Since the 1970s ...
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