This image series, taken by NASA’s New Horizons spacecraft in late January 2015, reveals the dwarf planet Pluto and its largest moon, Charon, orbiting their common center of mass. For the geeks among us (including yours truly), this is seriously cool. Technically speaking, a smaller body doesn’t orbit around a stolid larger body; they both orbit their common center of mass, called the barycenter. That holds even for the solar system: instead of the planets twirling around a stationary Sun, Sun and planets orbits a common center of mass. Currently that center of mass is inside the Sun, but it moves around as the planets move and should be outside the Sun in a couple of years.

Charon is about half Pluto's width and somewhere around one-eighth to one-tenth its mass, and with a center-to-center orbit of roughly 20,000 km, the center of mass for this system is actually above Pluto’s surface. And you can totally tell when you watch this time-lapse series! New Horizons’ Long Range Reconnaissance Imager (LORRI) took the images from January 25th to 31st as part of the effort to better pinpoint Pluto and Charon’s locations for the spacecraft flyby this July.

(And if you’re wondering where Pluto’s other four moons are, they’re too faint to be resolved at the spacecraft's distance — about 120 million miles, or 200 million kilometers.)

You can read more about it in the press release from Johns Hopkins University Applied Physics Laboratory.

Pluto and Charon

NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute




Image of Mark-Bunch


February 13, 2015 at 5:39 pm

Isaac Asimov once noted that the Earth-Moon system was the only planet/satellite group in the solar system whose center of mass lay outside the main planet -- but that must have been written before Charon's discovery.

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Image of Justin S

Justin S

February 14, 2015 at 12:26 pm

The Earth/moon barycenter is 1700 km below the Earth's surface.

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Image of Jim-Baughman


February 13, 2015 at 8:30 pm

I think Charon's existence was first suspected from a curious recurring bump on Naval Observatory photos of Pluto in 1979.

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February 13, 2015 at 11:41 pm

This is really disturbing!! You can not orbit your own center of mass!!!!! That is physically impossible!! Now have serious doubts of the so called scientists that make statements on here!!!!!!!!!!!!!!!

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Image of Jim DeCamp

Jim DeCamp

February 16, 2015 at 2:23 pm

I recall reading that Charon and Pluto are the only known mutually tidally locked objects in the solar system. The earth's moon is tidally locked to earth, and therefore always presents the same side to the earth. The earth is not locked to the Moon, or else it would always appear to be at the same place in sky, and remain invisible from half the places on the planet. Certainly no other planet is tidally locked to its moon, and I know of no binary asteroids which are.

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