As well as Charon, Pluto has two other satellites, both very tiny. Pluto is thus the largest member of a small family. In this chapter the emphasis is on the family as a whole.

The two tiny satellites were discovered in May 2005 by H A Weaver and colleagues, with the Hubble Space Telescope during studies of Pluto in relation to NASA's New Horizons spacecraft, now on its way to Pluto (Chapter 8). The discovery team proposed the Greek names Nyx and Hydra, but as Nyx already appeared in the names of two asteroids, the spelling was changed from the Greek Nyx to the Egyptian Nix. In mythology Nix (Nyx) is the goddess of darkness and light, very appropriate for a satellite orbiting the god of the underworld. Nix is also the mother of Charon – rather a small mother for such a large child! Hydra is a nine-headed serpent, a suitable name for the ninth planet Pluto. In June 2006 the International Astronomical Union approved these names.

Figure 4.1 shows an HST image of Pluto and all three satellites obtained on 15 February 2006. Pluto and Charon are grossly overexposed so that the very faint Nix and Hydra can be imaged. Nix and Hydra are the two dots, Hydra being the further from Pluto.