Pluto's Tiny Twin Moons Named

June 22, 2006 — Born long ago but undiscovered until last year, two tiny moons of Pluto this week received their names, Nix and Hydra.

Like big sister Chandra and Pluto itself, the moons were named after characters from Greek and Roman mythology.

"I was having insomnia one night so I went and did a web search for all things related to Pluto and Charon," said Andrew Steffl, with the Southwest Research Institute in Boulder, Colo., one of three astronomers credited with discovering the moons.

In the myths, Pluto is the lord of the underworld and Charon ferries the boat that brings souls across the River Styx to Pluto.

After collaborating with colleagues, Steffl and his co-discoverers came up with a list of proposed names for the International Astronomical Union (IAU), the governing body for all things astronomical.

At the top were Nix — originally spelled Nyx after the goddess of the night and mother of Charon, and Hydra, a mythological nine-headed beast that guards the entrance to Pluto's realm.

IAU changed the spelling to Nix to avoid confusion with an asteroid named Nyx.

Other than that they are small, cold and far away, not much is known about Nix and Hydra, which orbit their mother planet at about 49,000 kilometers (30,447 miles) and 65,000 kilometers (40,389 miles) away, respectively.

Astronomers suspect that because the moons' rotations are synchronized with Charon's, they may all have had a common birth.

Like Earth's moon, Charon and her sisters may have been formed when an asteroid or comet smashed into the planet during an early stage of its formation.

Nix and Hydra would have moved out as Charon distanced itself from Pluto, Steffl said.

Another theory is that the newly-named moons were captured by Pluto's gravity from the Kuiper Belt region, the frozen backyard of the solar system located beyond Neptune.

The moons were discovered last year in pictures taken by the Hubble Space Telescope.

Steffl spent two days studying the Hubble pictures, in hopes of finding additional moons. "It was really a 'what-if' scenario," he said.

He created a four-frame movie from the Hubble data and placed random artificial moon renderings in the images to keep his hunting eye sharp during the long hours. When he spotted the real moons, he didn't even know it at first.

"I put the artificial moons in, but I didn't know where they were so when I found the real moons, I assumed they were part of the program," Steffl said.

"Then, when I checked and realized what they were I just about fell off my chair. It was really interesting because no one believed me," he added.

Unbeknownst to Steffl, astronomer Max Mutchler at the Space Telescope Science Institute in Baltimore, Md., and Hal Weaver, with the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., had reached the same conclusion.

It is possible that Pluto's brood is even larger. A second study with Hubble is planned for next year, and a robotic science satellite is on its way to Pluto for close-up studies in 2015.

In addition to their mythological etymologies, the names Nix and Hydra also are grounded in reality. The moons' initials, N and H, are a nod to New Horizons, the name of the Pluto probe.