Pluto's New Moons Named Nix, Hydra

Richard A. Lovett
for National Geographic News
June 23, 2006
Our solar system has two new named bodies: Nix and Hydra.

The names were approved last week by the International Astronomical Union, the organization charged with keeping tabs of astronomical nomenclature.

Previously called S/2005 P 2 and S/2005 P 1, the two moons were discovered a year ago in orbit around Pluto.

The discovery of the new bodies was announced on October 31.

"They're the 'Halloween moons' of Pluto," said Alan Stern, a planetary astronomer with the Southwest Research Institute in Boulder, Colorado, whose team made the find.

Stern's team picked the names Nix and Hydra—both figures in Greek mythology—to reflect the underworld theme used in naming objects in the dim reaches of the outer solar system.

But the scientists also wanted them to be special.

The nine members of the discovery team brainstormed about 30 names, Stern said. "Then we wondered what made sense as a pair."

They settled on Nix and Hydra because their initials could stand for New Horizons, the space probe now heading for Pluto, the solar system's last unexplored planet.

(See a virtual solar system.)

The initials seemed particularly appropriate because the astronomers discovered the moons while searching for exploration targets for the New Horizons mission using the Hubble Space Telescope.

There is also a tradition of coding initials into the names of celestial objects, Stern noted.

The p and the l in Pluto's name stand for Perceval Lowell, the astronomer who long ago spearheaded the search for Pluto.

Nix and Hydra

Nyx is the Greek goddess of darkness and the mother of the underworld ferryman Charon, which is the name of Pluto's largest moon.

Hydra was a nine-headed monster that guarded the underworld. Its nine heads make it a particularly good companion for the ninth planet, Stern says.

Originally, the astronomers proposed "Nyx" as the name for the first moon, but the IAU rejected that spelling because it had already been used to name a small asteroid.

Still, Stern is very pleased with the official name.

"[The IAU] maintained everything we wanted, except they had to change a vowel," he said.

"My joke is that the IAU nixed 'Nyx,'" Stern said.

Much about the small moons remains a mystery. Nobody is sure how large the moons are. They could be tiny and bright or larger and dimmer.

What is known is that they are considerably smaller than Charon and travel well outside its orbit.

"They're somewhere between 30 and 100 miles [48 and 160 kilometers] in diameter," Stern said.

As for New Horizons, "[it's] halfway to Jupiter," Stern said.

The probe is due to arrive at Pluto on July 14, 2015.

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