The moons of Pluto

Usually, someone who discovers a celestial body gets the naming rights. But there are rules regarding astronomical names, and the International Astronomical Union (IAU) has the final say about what's acceptable. Here's how Pluto and its moons got their names.

Pluto
Clyde Tombaugh discovered Pluto in 1930 at Lowell Observatory in Arizona. The observatory assumed that Tombaugh's discovery was the Planet X that Percival Lowell had been searching for beyond Neptune. They even chose March 13 to announce the discovery. This was the anniversary of William Herschel's discovery of the planet Uranus and also Percival Lowell's birthday. Sadly, Lowell himself had not lived to see the discovery.

With the right to name the new planet, the observatory asked for ideas. There were lots of suggestions, including Lowell's widow's wish to call it Percival. A more appropriate name came from an 11-year-old English schoolgirl who'd been learning about the Solar System. Venetia Burney thought that this dark world so far from the Sun should be called Pluto after the Roman god of the Underworld. The astronomers liked it, especially since it came with the bonus of the first two letters of the name being Percival Lowell's initials.

By the way, in case you're wondering, the Disney dog Pluto came to public attention sometime after the planet was named.

Charon
In 1978 – nearly half a century after the discovery of Pluto – James Christy discovered that Pluto had a moon. He wanted to name it after his wife Charlene, but was obviously not going to get that past the IAU. Colleagues suggested some names relevant to Pluto, and he was lucky that one of them was Charon.

Charon was the mythological ferryman who rowed the souls of the dead across the river Styx to Pluto's realm. English dictionaries commonly gave the pronunciation of Charon as KARE.on. But Christy would say it as SHAR.on, his wife's nickname being pronounced shar. (Many astronomers say it that way too.) The name was accepted. The IAU wasn't going to police the pronunciation.

Nix and Hydra
Some 27 years after Charon's discovery, the Pluto Companion Search Team was looking for small Plutonian moons in preparation for NASA's New Horizons mission. Using the Hubble Space Telescope, they did find two of them in the images. They wanted to name the inner moon Nyx and the outer moon Hydra.

Nyx is the goddess of the night and the mother of Charon. Hydra was a giant serpent with nine heads. If one head got cut off, two grew in its place. The Hydra guarded an entrance to the Underworld until the hero Hercules managed to destroy it.

There are many names associated with the classical Underworld, but the team chose this pair for a specific, yet unstated, reason. So it was a problem when the IAU accepted Hydra, but rejected Nyx because that was already the name of an asteroid. Undaunted, the team proposed using the alternative spelling Nix, and that was accepted.

The pair of names, Nix and Hydra, has the initials NH. These are also the initials of New Horizons that was being launched to Pluto in January 2006. Pluto was the only planet in the Solar System that hadn't been visited by a space mission. However, in August 2006 the IAU changed Pluto's classification from planet to dwarf planet. The mission was no longer to the last unvisited planet, but had become the first mission to a dwarf planet.

Kerberos and Styx
As New Horizons made its way ever closer to its rendezvous with Pluto, NASA was concerned about rings or tiny moons that might be a potential hazard to the spacecraft. With a team led by Mark Showalter, it was back to the Hubble Space Telescope. This team had access to a new wide field camera that astronauts had installed during the 2009 servicing mission.

And they found a tiny moon in 2011 and a second one in 2012. Instead of naming them, they opened up the naming to a public vote. Nearly half a million votes were cast, and the clear winners were Vulcan topping the poll and Cerberus in second place. Mark Showalter sent these names to the IAU.

The name Vulcan was proposed for the smallest moon – it orbits between Charon and Nix. The IAU rejected this name for two reasons. First of all, they didn't consider the god of fire and metalworking relevant to Pluto. In addition, they noted that the name was already associated with the region between Mercury and the Sun.

And Cerberus wasn't approved either, even though it was the three-headed dog that guarded the gates to the Underworld. But there was an asteroid named Cerberus.

Showalter tried to make the case for Vulcan, but the IAU wasn't budging on it. Fortunately, the Cerberus problem was amenable to the Nix solution. Although Cerberus was the creature's Roman name, its Greek name was Kerberos, and that was acceptable. It orbits between Nix and Hydra.

Styx was third place in the voting, and this name went to the smallest moonlet instead of Vulcan. Styx is not only the name of the river, but also the goddess of that river.