Moons of Pluto

by Fraser Cain on April 24, 2008

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When Pluto was first discovered back in 1930, astronomers thought it was just a single, solitary planet orbiting the Sun. Almost 50 years later, astronomers discovered that it actually had a very large moon. And then in 2005, astronomers working with the Hubble Space Telescope announced that they had found two more moons of Pluto, officially named Nix and Hydra. Are there more, waiting to be found? How many moons does Pluto have?

Astronomers now know that Pluto has three natural satellites. The first and largest of the Pluto moons is Charon, first identified back in 1978 by astronomer James Christy. He made the discovery while examining a photograph of Pluto and noticed that it had a bulge on one side. Christy and his colleagues thought this bulge came from a defect in the alignment of the telescope, but then they noticed that only Pluto was elongated, and not the background stars. They realized they were looking at a moon for Pluto.

Pluto’s moon Charon is named after the boatman in Greek mythology who guides the dead across the River Styx. This works well, considering Pluto is the roman god of the underworld (no, not the Disney Dog).

Charon is large and massive, compared to its parent dwarf planet Pluto. While Pluto measures 2,306 km across, Charon is 1,205 km.across.

One of the remarkable things about Pluto and Charon is that they’re actually a binary system. The two objects orbit a common center of gravity which is outside Pluto itself. For comparison, the Earth and Moon’s center of gravity is inside the Earth.

Back in 2005, astronomers working with the Hubble Space Telescope discovered two additional Pluto moons; they named them Nix and Hydra (originally S/2005 P1 and S/2005 P2). Nix measures 46 km across, and Hydra is 61 km. The Hubble research suggested a n upper limit for moon sizes orbiting Pluto. It appears that Pluto has already reached this limit with Nix and Hydra, and anything larger would be clearly visible.

The discovery of these moons has given hope to the theory that Pluto has a ring system, created with micrometeorites impact with the surface of the dwarf planet. Another possibility is that Charon produces ice geysers, similar to Saturn’s moon Enceladus.

More on this will be discovered when NASA’s New Horizons spacecraft finally arrives at Pluto in 2015. At its closest point, New Horizons will get within 10,000 km of the dwarf planet’s surface, and capture images at an unprecedented level of quality.

We’ll finally know what Pluto really looks like. And we’ll get a chance to see Pluto’s other moons at the same time.

Go here if you’d like a picture of Pluto.

Source: NASA

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

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