On June 23rd the cast of underworld characters in the outer solar system will expand by two, as the International Astronomical Union (IAU) officially names Pluto's two smallest moons Nix and Hydra.
The moons, which were first imaged in May 2005 and confirmed in March 2006, were originally called S/2005 P1 and S/2005 P2. Their new names will reflect Pluto's netherworld theme. In Greek mythology, Nix (or "Nyx”) was the goddess of night and the mother of Charon, the ferryman who lent his name to Pluto’s largest moon, discovered in 1978. Hydra was the nine-headed monster that guarded Lake Lerna, one of the entrances to the underworld.
“There was quite a bit of discussion on the names,” says Brian Marsden, director of the Minor Planet Center and a member of the Working Group for Planetary System Nomenclature, the IAU committee that names satellites of planets. “Somebody suggested the name ‘Typhon.’ With Charon, the C and T give the initials of Clyde Tombaugh [who discovered Pluto].”
In Greek mythology, Typhon was the god of fiery hot winds — hence the word typhoon. The committee also considered one of Typhon’s sons: a hundred-headed dragon named Ladon.
“If we had a name beginning with L, then with Pluto, we would have had P L for Percival Lowell,” says Marsden. “I thought that was rather nice.”
The committee eventually settled on Nix and Hydra, the names proposed by the team that discovered the moons. The team had originally proposed “Nyx,” the more common spelling of the Greek goddess, but this name had already been given to a near-Earth asteroid.
Although the satellites’ initials do not stand for Clyde Tombaugh or Percival Lowell, the N and H could represent New Horizons, the NASA mission to Pluto that launched on January 19th. The $650-million spacecraft is now halfway to Jupiter, which it will encounter on February 28th of next year, using the gas giant’s gravity to slingshot itself towards a July 14, 2015, rendezvous with Pluto, Charon, Nix, and Hydra.
The IAU’s announcement comes three months before another naming decision. The IAU plans to publish an official definition of “planet” in early September, ruling on whether to call Pluto a planet or a Kuiper Belt Object (KBO). Pluto’s status as a planet has been fragile since the discoveries of KBOs Quaoar, Sedna, and 2003UB313 (informally named “Xena”), which is slightly larger than Pluto. The IAU will discuss the issue at its August meeting in Prague.