New names for Pluto's moons


In March, astronomers Hal Weaver and Alan Stern surprised the world with the announcement that the Hubble Space Telescope had spotted two new moons orbiting the planet Pluto.

Rewind 28 years. It was an even greater surprise back in 1978 when astronomer James Christy announced the discovery of the first moon of Pluto, now named Charon. I don't think anyone really expected that a planet as small as Pluto could hold onto a moon of its own. After all, Pluto is 1/500th of the mass of the Earth, considerably smaller than our own moon. So, when Christy found a moon nearly half the size of Pluto circling in a 6.4-day orbit, it came as quite a shock.

Fast-forward 28 years. Upon discovery, the two new moons were temporarily designated as satellites 2005/P1 and 2005/P2. Last week, the International Astronomical Union, or IAU, approved and announced the official names given the two newcomers to Pluto's family. Welcome Nix and Hydra to the list of 160 moons that orbit planets in our solar system. Why Nix and Hydra? Nix was the goddess of the night and the mother of Charon in Greek mythology, and Hydra was a nine-headed monster that guarded the entrance to the underworld, where Pluto was king. Charon was the ferry boatman who carried the souls of the dead to the underworld. Could it be a coincidence that the initials of the two new names, N and H, are the same as the initials of NASA's New Horizons spacecraft that, at this very moment, is speeding toward Pluto for a July 2015 rendezvous? Probably not. Astronomers love to play little word games like that.

When New Horizons passes through the Pluto system nine years from now, taking close-up images of Pluto, Charon, Nix and Hydra, astronomers hope to confirm their hypothesis that all three moons are the fragments remaining after a massive collision shattered Pluto into pieces. Charon is fairly large --36 miles in diameter. Hydra is 93 miles in diameter, and Nix is 62 miles in diameter. But because of their great distance, they are near the limit of what Hubble can see. There might be other smaller fragments waiting to be discovered. Rumor has it that the IAU will make another announcement in September: the official definition of the word "planet" and whether Pluto will be demoted from planetary status. Stay tuned.

Jimmy Westlake teaches astronomy and physics at Colorado Mountain College's Alpine Campus. He is an avid astronomer whose photographs and articles have been published on the Web sites of, NASA's "Astronomy Picture of the Day,",,, and in Sky & Telescope, Astronomy, Night Sky, Discover, and WeatherWise magazines.


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