Pluto's baby twin moons, formerly known as S/2005 P 1 and S/2005 P 2, have been christened Nix and Hydra. The objects, discovered last year by the Hubble Space Telescope, received their names from the International Astronomical Union (IAU). A formal announcement will be issued this Friday, 23 June.
The names were proposed this spring by the discovery team, who first identified the moons in May of last year. "We had a giant list of possible names to consider," says team member Andrew Steffl of the Southwest Research Institute in Boulder, Colorado. "It was a fun thing to do."
In mythology, Pluto ruled the underworld. Nyx was the goddess of night and the mother of Charon, the boatsman who takes souls across the River Styx and into Pluto's grasp. Pluto's large satellite, discovered in 1978, is called Charon. Because an asteroid with the name Nyx already exists, the IAU decided to use a slightly different spelling for the inner one of the two small Plutonian moons, to avoid confusion. Hydra was the mythological nine-headed serpent that guarded the underworld. A large but inconspicuous constellation in the spring sky also bears this name.
Pluto-philes read even more significance into the two names. The first letters, N and H, also refer to NASA's New Horizons spacecraft, which was launched in January and is now on its way to an encounter with the Pluto system in the summer of 2015. And according to the team, nine-headed Hydra is a fitting companion for the ninth planet. This summer, the IAU will decide on the much-discussed planetary status of Pluto. The naming of Hydra "could possibly" help convince the IAU to preserve Pluto's planethood, says Steffl.
James Christy, the now-retired discoverer of Charon, says he would have preferred Persephone--Pluto's involuntary spouse--as the name for one of the satellites, but again, that name has already been used for an asteroid. Back in 1978, Christy chose Charon to honor his wife Charlene. "Names are very important," says space physicist Fran Bagenal of the University of Colorado. "They make it easier for people to relate to cosmic objects." If galaxies were known as Tom, Dick, and Harry instead of some arcane catalog number, she says, more people would probably take up an interest in astronomy.