Naming Pluto’s Moons

Image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute

Before selecting names for Pluto’s two newly discovered moons, astronomers invited the public to cast their votes. The name “Vulcan,” associated with the Roman god of fire, was the clear favorite—likely boosted by its connection to Mr. Spock’s home planet in Star Trek. However, more cautious decisions were made. So, which names did the International Astronomical Union ultimately decide on, and what are their origins? This seems like the perfect opportunity to test your knowledge of the mythology behind Pluto, the “icy, distant dwarf planet,” and its five moons.

6. Styx

underworld of Greek mythologyAn 18th-century engraving depicting the underworld of Greek mythology, showing (foreground) Charon, the ferryman, in his boat; (bottom left) Cerberus, the three-headed dog guarding the entrance; and (centre left) Hades, ruler of the underworld, and his wife, Persephone. The body of water is the River Styx. Image: © Photos.com/Thinkstock

As noted earlier in this list, Styx—the name given to Pluto’s most recently identified moon and likely the final one found using the Hubble Space Telescope—is most famously recognized as one of the rivers in the Underworld. According to Hesiod, Styx was the eldest daughter of Oceanus, the primordial Titan, and Tethys, his sister and wife. In Homeric tradition, the gods swore their most unbreakable oaths by the waters of the Styx. Achilles’ mother made him nearly invincible by immersing him in the Styx, leaving only his heel vulnerable—the spot where she held him.

5. Kerberos

Pluto and Persephone enthronedPluto and Persephone enthroned, with Cerberus at their feet, from *Les Echecs amoureux,* a 15th-century manuscript produced for Louise de Savoy, the mother of King Francis I of France. Image: © PHOTOS.com/Getty Images Plus

Kerberos is an alternate spelling of the better-known Cerberus (the more common spelling had already been assigned to an asteroid). Most would agree that either name is far more vivid than the moon’s original designation, “P4,” given after its discovery in 2011. Kerberos is the mythical hound that guards the gates of Hades. According to Hesiod, the creature has 50 heads, though most other accounts describe it as having three. Ancient Greek pottery often portrays Kerberos with just two heads. The final task of Hercules (Heracles) in his 12 labors was to capture Kerberos and bring him from the Underworld to Mycenae. A famous Caeretan hydria (vase) housed in the Louvre shows the dog with three heads, adorned with multiple coiling serpents.

4. Hydra

Heracles battling the Lernaean HydraHeracles (Hercules) battling the Lernaean Hydra; at the southern entrance to the Hofburg (Imperial Palace) in Vienna.
Image: © v0v/Fotolia

In Greek mythology, Hydra was a monstrous creature said to have nine heads, though the exact number varies depending on the source. Alongside Kerberos (Cerberus), Hydra was believed to guard the gates of the Underworld. The Roman poet Statius described Hydra as the “companion of the savage beast Kerberos.” Greek myths also refer to another Hydra, the Lernaean Hydra, whose defeat became the second of Hercules’ (Heracles’) twelve labors. This task was made especially difficult because whenever one of the Hydra’s heads was severed, two would grow back in its place. However, once Hercules discovered how to prevent this regeneration, the Hydra was ultimately defeated. Still, Hydra may have had the last laugh, as Sophocles suggests in Trachinian Women.

3. Nix

Here’s a quick example of the importance of alternate spellings: In Greek mythology, Nyx represents the female embodiment of night and is a powerful cosmic figure, so formidable that even Zeus held her in awe. According to the poet Hesiod, Nyx was born from Chaos and gave birth to Sleep, Death, the Fates, Nemesis, and Old Age—entities certainly worthy of fear. She was also the mother of Charon, the ferryman of the Underworld. However, when her name was proposed for one of Pluto’s moons, the spelling “Nyx” had already been used for an asteroid. Undeterred, astronomers adopted a slightly different version of the name—and just like that, the problem was solved!

2. Charon

Gustave Doré; DanteCharon, illustration by Gustave Doré for an 1861 edition of Dante’s *Inferno* (*The Divine Comedy*).
Image: © Photos.com/Getty Images

Charon, the namesake of Pluto’s largest moon, served as the ferryman for Pluto, also known as Hades. His role was to transport the souls of the dead—provided they were properly buried—across the Rivers Styx and Acheron. In exchange for his service, he accepted a coin that the ancient Greeks traditionally placed in the mouth of the deceased. Charon frequently appeared in early art and literature, becoming a well-known figure. Writers like Aristophanes in Frogs and Virgil in The Aeneid (Book VI) referenced him, as did Lucian. His character was later featured in The Invention of Love, a play by Tom Stoppard that explores the life of 20th-century poet A.E. Housman.

1. Pluto

Gian Lorenzo Bernini: *Pluto and ProserpinaPluto and Proserpina*, marble sculpture by Gian Lorenzo Bernini, 1621–22; in the Borghese Gallery, Rome.
Image: Anderson—Alinari/Art Resource, New York

Yes, while Pluto is also famously the name of Mickey Mouse’s dog, the dwarf planet takes its name from the Roman god of the Underworld, known in Greek mythology as Hades. The connection between outer space and the Underworld is an intriguing, if somewhat ironic, association. After the fall of Cronus, the king of the Titans, the universe was divided among his sons by drawing lots. Poseidon claimed the Sea, Zeus took the Sky (or Heavens), and Hades was given dominion over the Underworld. Hades became the ruler of the dead and eventually came to symbolize death itself. One of the most famous myths about him recounts the abduction of Persephone, whom he tricked into eating a pomegranate—a fruit associated with the dead—ensuring her periodic return to his dark realm.