Contest to name Pluto's two moons opens Pluto is the Roman equivalent of the Greek's Hades, lord of the underworld, and its three bigger moons have related mythological names: Charon, the ferryman of Hades; Nix for the night goddess; and the multi-headed monster Hydra. The two unnamed moons - no more than 20 miles across - will also take their names from the underworld myth. At the moment they go by the bland titles of P4 and P5. Online voting will last two weeks, ending on Feb. 25. Twelve choices are available at the website http://www.plutorocks.com and more suggestions are welcome, but they need to come from Greek or Roman mythology and deal with the underworld. Among the choices suggested so far are: Hercules, the hero who slew Hydra; Obol, the coin put in the mouths of the dead as payment to Charon; Cerebrus, the three-headed dog guarding the gates of the underworld; Orpheus, the musician and poet who used his talents to get his wife, Eurydice, out of the underworld only to lose her by looking back: Eurydice; and Styx, the river to the underworld. As of Monday afternoon, Styx was leading. The vote tally is updated hourly. "The Greeks were great storytellers, and they have given us a colourful cast of characters to work with," said Mark Showalter, senior research scientist at SETI Institute's Carl Sagan Center in California. He and other astronomers who discovered the two mini-moons using the Hubble Space Telescope will make the winning selections. NASA's New Horizons spacecraft is en route to Pluto, arriving in 2015 on the first robotic flyby ever of the planetoid. The winning moon names will need final approval by the International Astronomical Union.