This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
After a ten year journey, NASA's New Horizons mission is still 420 million kilometers from the Pluto system - but that's close enough to begin to see the orbital dance of an icy world and its major moon.
This far out from the Sun it's easier for planetary objects to hold onto satellites, so even little Pluto at 0.2% the mass of the Earth has no less than 5 known moons: Charon at about 12% Pluto's mass, and a diminutive underworld crowd of Styx, Nix, Kerberos and Hydra orbiting outside the dominant Pluto-Charon pair.
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And a pair they are, with more in common with a binary planet than the kind of planet-moon system we're familiar with.
The Earth and Luna orbit their common center of mass, which is situated about 4,700 kilometers from Earth's center (but still within its bulk) - causing us to effectively 'wobble' around as we orbit the Sun (a characteristic being applied to search for exomoons in distant systems). By contrast, Charon tugs its system's center of mass out to nearly 1000 kilometers above Pluto's surface, so both planet and largest moon waltz around a point in space every 6.4 Earth days.
The New Horizons Long Range Reconnaissance Imager snapped a series of images in late July that show this motion very clearly.
The viewpoint is somewhat tilted - we're not looking directly 'down' onto the system, so the orbits appear elliptical rather than their true circular shape.
Over the coming months we can expect better and better images, culminating in what promises to be a spectacular flyby within 14,000 kilometers of Pluto on July 14th 2015 (for comparison, Charon is orbiting about 17,500 kilometers from the system center of mass). But it won't last long, moving at 13.8 kilometers per second relative encounter speed, New Horizons will be a temporary blip in Pluto's skies.
The science produced by this fleeting encounter should be more than worth the decade-long wait. We've never seen the details of the surfaces of these ancient trans-Neptunian objects. Doing so will reveal a vast amount about their origins and history. For example, some theories suggest that in the distant past Charon's tidal effects on Pluto could have encouraged the existence of a subsurface ocean of water - a couple of hundred kilometers down. It's even possible such a place still exists, insulated by Pluto's crust of frozen nitrogen and methane. Clues to the presence of an abyssal realm may be present on the surface, in the form of geophysical features and compositions that New Horizons will be able to glimpse.
The Pluto-Charon waltz is just the opening act...
