New Horizons’ First Look at Pluto’s Big Moon

Pluto and Charon as seen by the approaching New Horizons spacecraft
Pluto and Charon as seen by the approaching New Horizons spacecraft

The two bright clusters of pixels in the image above might not seem like much of a big deal, but they are… those two blocky blobs are the dwarf planet Pluto and its largest moon, Charon, as seen by the rapidly-approaching New Horizons spacecraft, destined for its ultimate close encounter in July 2015!

This represents a major milestone on the spacecraft’s 9½-year journey to conduct humanity’s first close-up reconnaissance of the Pluto system and the Kuiper Belt and, in a sense, begins the mission’s long-range study of the Pluto system.

And that’s a very big deal indeed. Read on, fellow space fans…

The largest of Pluto’s five known moons, Charon (pronounced kair-un) orbits about 12,000 miles (more than 19,000 kilometers) away from Pluto. As seen from New Horizons, that’s only about 0.01 degrees of spatial separation — that’s a hundred times smaller than the width of your little finger held at arm’s length!



Read more here. 

“We’re excited to have our first pixel on Charon, but two years from now, near closest approach, we’ll have almost a million pixels on Charon –and I expect we’ll be about a million times happier too!”

–  Alan Stern, New Horizons Principal Investigator, SwRI

At just about 750 miles ( 1207 km) wide, Charon is almost half the size of Pluto. In fact it’s so big in comparison that Pluto and Charon are often referred to as a double-dwarf planet system. The distance between them is a mere 12,200 miles (19,640 km)… compare that to the ~240,000 miles that separate our Moon from us!

If Charon were orbiting Earth at the same distance as it does Pluto, it would be as far away as GPS satellites (and much closer than geostationary weather and comm sats.)

Artist's impression of Pluto's surface with Charon in its permanent spot in the sky. (NASA)
Artist’s impression of Pluto’s surface with Charon in its permanent spot in the sky. (NASA)

Charon’s orbit around Pluto takes 6.4 Earth days, and one Pluto rotation (a Pluto day) takes 6.4 Earth days, meaning Charon neither rises nor sets but rather hovers over the same spot on Pluto’s surface. The same side of Charon always faces Pluto in an effect called tidal locking. Compared with most of the planets and moons, the Pluto-Charon system is tipped on its side, like Uranus. Also like Uranus (and Venus) Pluto’s rotation is retrograde: it rotates backwards, from east to west.

Pluto has a total of 5 known moons: Charon, Hydra, Nix, and the recently-named Styx and Kerberos.

Top image: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute



  1. Very cool!
    I have heard Neil Desgrasse Tyson speak about planets that are to close to one another, and how it alters their orbit around their star. Is this a factor with Charon and Pluto since Charon is so large?


    1. J. P. Major says:

      Pluto/Charon’s orbit around the Sun is stable, but they are so large compared to each other that they orbit a central point (barycenter) outside of Pluto’s own radius. It’s almost like who’s the moon of whom!


  2. Ah, tidal locking! 🙂 Awesome!


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