Christening Pluto’s Moons

Eliot Young, Southwest Research Institute; NASA’s Planetary Astronomy Program

Before choosing names for the two most recently discovered moons of Pluto, astronomers asked the public to vote. Vulcan, the name of a Roman god of fire, won hands down, probably because it was also the name of the home planet of Star Trek’s Mr. Spock. But cooler heads prevailed. So what names did the International Astronomical Union choose? And where did those names come from? We thought this would be a good time to challenge your memory of the mythology behind the names of that "icy, distant dwarf planet" and its five moons.

6Styx

As mentioned elsewhere in this list, Styx—the name of Pluto’s most recently discovered moon and almost certainly the last moon to be discovered by means of the Hubble Space Telescope—is best-known as one of the rivers of the Underworld. Hesiod identified Styx as the oldest daughter of Oceanus (the oldest Titan) and Tethys (his sister and consort). In Homer the gods swear by the water of the Styx as their most binding oath. By dipping Achilles into the Styx, his mother rendered him invulnerable—except for the heel by which she held him.

5Kerberos

Kerberos is another spelling of the more-familiar Cerberus (that spelling also was already assigned to an asteroid). We would probably agree that either one is much more colourful than "P4," the moon’s provisional name after its discovery in 2011. Kerberos is the dog who guards the entrance to Hades. Hesiod says he has 50 heads, but most sources give him three. Ancient Greek pottery often depicts him with only two heads. The last of the 12 labours of Hercules (Heracles) was to take Kerberos from the Underworld to Mycenae. On a noted Caeretan hydria (vase) in the Louvre, the dog is shown with three heads and enhanced by several coiled serpents.

4Hydra

In Greek legend, Hydra was a monster with nine (or so) heads—the number varies among sources. With Kerberos (Cerberus), the Hydra was said to guard the entrance to the Underworld. The Roman poet Statius calls Hydra "the comrade of the cruel beast Kerberos (Cerberus)." There is mention in Greek mythology of a second Hydra, the Lernaian Hydra, whose killing was the second labour of Hercules (Heracles). It was a task complicated by the fact that each time one head was cut off, two grew in its place. Well, you probably can guess that once Heracles figured out the solution to that problem, Hydra lost. But the Hydra had the last word, as it were, if you believe Sophocles (see Trachinian Women).

3Nix

Here is a little lesson in the value of variant names: in Greek mythology Nyx is the female personification of night and a great cosmological figure, awesome even to Zeus himself. According to the Greek poet Hesiod, she was the daughter of Chaos and the mother of Sleep, Death, the Fates, Nemesis, and Old Age. Surely to be feared. She was also the mother of Charon. Alas, when her name was presented as a possibility for the moon’s name, Nyx (so spelled) had already been assigned to an asteroid. Never daunted, astronomers simply used a variant spelling and voilà!

2Charon

Charon—the name of Pluto’s largest moon—is a ferryman in the service of Pluto, or Hades. It was Charon’s duty to guide the souls of the deceased (when properly buried) over the Rivers Styx and Acheron. As payment for this act, he received the coin that ancient Greeks placed in the mouth of the deceased. He is a popular figure in early art and literature. Aristophanes (Frogs) and Virgil (Aeneid, Book VI) wrote about him, as did Lucian. His character is an integral part of The Invention of Love, playwright Tom Stoppard’s portrait of the 20th-century poet A.E. Housman.

1Pluto

Yes, Pluto is the name of Mickey Mouse’s dog. However, the recently reclassified dwarf planet is named for the Roman god of the Underworld, better known to the Greeks as Hades. Hmm, Underworld and outer space. A somewhat ironic name, no? After Cronus, king of the Titans, died, his kingdom was divided by lots. The Sea became the realm of Poseidon, the Sky (or Heavens) of Zeus, and the Underworld fell to Hades. He became the god of the dead, and soon death personified. He is said to have abducted Persephone and to have given her a pomegranate, the forbidden fruit of the dead, to cause her to return to his realm at regular intervals.

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