Want to name Pluto's two tiniest moons? Then you'll need to dig deep into mythology.

Astronomers announced a contest Monday to name the two itty-bitty moons of Pluto discovered over the past two years. 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 24 to 32 kilometres across — need similarly shady references. Right now, they go by the bland titles of P4 and P5.

Online voting will last two weeks, ending Feb. 25. Twelve choices are available at the website http://www.plutorocks.com.

Among the choices: 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 colorful cast of characters to work with," said Mark Showalter, senior research scientist at SETI Institute's Carl Sagan Center in Mountain View, Calif.

He and other astronomers who discovered the two mini-moons using the Hubble Space Telescope will make the winning selections.

Write-in name suggestions are welcomed, but they need to come from Greek or Roman mythology and deal with the underworld.

The name for the planetoid, or dwarf planet near the outer fringes of the solar system came from a little English girl. Pluto's discoverer, Clyde Tombaugh, liked that the first two letters were the same as the initials of late American astronomer Percival Lowell. Tombaugh discovered Pluto in 1930 using the Lowell Observatory in Flagstaff, Ariz.

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.

Hopefully, there won't be any conflicts like when the name Nix was picked. The night goddess actually is spelled Nyx, but an asteroid already had the moniker so the proper spelling for the moon had to be nixed.