Upper right: The first image of Charon (as a small bump on Pluto), taken 40 years ago. Main image: Charon as seen from NASA’s New Horizons space craft.

Pluto’s moon Charon (technically a binary partner) was first discovered 40 years ago, on June 22, 1978, by U.S. Naval Observatory astronomer James Christy.  Like Pluto, it was known only as a blurry smudge until the New Horizons flyby in 2015.  Charon was then revealed to be an intriguing world of its own, with mountains, canyons, and signs of the one-time existence of an underground ocean.  Even more striking is the spray of reddish material located at the north pole, thought to be colored hydrocarbon deposits created by chemical reactions of outgassed methane catalyzed by UV light from the sun.