Charon, in Greek mythology, was the old and wizened ferryman who would transport the spirits of the dead across the Styx and Acheron, the rivers of the underworld. His fare per spirit was one obolos, the smallest silver coin available at the time. Charon's employer was a god named Pluto, the ruler of this underworld, son of Saturn, and brother to Jupiter and Neptune. Both Pluto and Charon were more or less forgotten, until they appeared three billion miles away on this date, February 18th, in 1930.
At first, only Pluto could be seen. It was a new planet, and it was given its name because of its very great distance from the sun, but it was also obviously apt in the context of the god's immediate family. But this new Pluto held a few surprises.
First, it was a very tiny body compared to its solar system siblings; it was only 1,850 miles in diameter and smaller than our earthly Moon. Then in 1978, when better telescopes became available, it could be seen that Pluto had a moon that was nearly half as big as Pluto itself; what had looked like a single planet entity was in fact a kind of celestial pas de deux.
This moon was christened Charon. Like most moons in the solar system, ours included, Charon keeps the same face always turned in the direction of its parent planet. But the Pluto system is unique in that the planet itself keeps the same face turned towards its moon. If Pluto had inhabitants, those living on one side would always see Charon hanging in their sky, about six time larger than our Moon appears from Earth; those living on the other side would never see their moon.
Pluto's path was found to be more elliptical than that of any other planet in the solar system, and to be such that it takes nearly 250 of our years to complete an orbit around the sun. And with an estimated temperature of nearly 240 degrees below zero, it is probably the coldest spot in the solar system.
Pluto has a very thin membrane of nitrogen and methane for an atmosphere, so that its barometric pressure is about a millionth of that provided by our air. But when the planet is at it farthest from the sun, the low temperature causes the atmosphere to freeze on to the surface, becoming a solid, frozen amalgam of water, ammonia and methane. Pluto becomes an airless world, perhaps not too unlike the mythological kingdom of its eponym.