The Moons of Pluto: Chaotically Unpredictable

As we are approaching the New Horizons bypass of Pluto just over a month from now, there is a lot of focus on the Plutonian system, from its strange Moons to its enigmatic surface.  As we wait for the first ever high resolution images of the surface of Pluto, we can look to Hubble data to give us our fix.  The best image of Pluto taken up to today, by Hubble, is blurry and at best can lead us to speculation about what we are seeing.

The best image of Pluto, set to be replaced by better images a month from now. Credit: NASA/ESA

But Hubble, as always, produces valuable science, and has given new insights into the strange and surpringly-numerous moons of Pluto.  Aside from the largest massive moon of Charon, Pluto has four small rocky orbiters, arranged from largest to smallest they are: Hydra, Nix, Kerberos, and Styx.  It is very likely that Pluto has many more small rocky orbiters, as its system is thought to have resulted from a collision of two large bodies long ago. The gravity of the Pluto-Charon duo causes some strange orbital effects on the other moons.  They orbit in strange resonances and exhibit chaotic behavior, which means that no matter how many measurements we take, we cannot predict their rotational state in the future.  As an example, see this video:

The moon Nix flips and spins, giving weird perspectives to any creature that might want to live there.  to the observer on the surface, the Sun would randomly rise from a different direction each day.  All this weirdness is attributed to the location of the center of gravity of the Pluto-Charon system, which lies in the space between the two objects, a little bit closer to Pluto, since Charon is half as large as Pluto.

The center of mass of any two bodies is always located somewhere between the individual centers of mass of the two objects.  If two bodies had the exact same mass, they would orbit a point in space directly between them.  But if one is much larger, like the Earth-Sun system, the center of mass is within the more massive object, so the Earth appears to orbit the Sun, but the Sun has a tiny wobble.  In other star systems, we can detect planets by looking for this wobble.

Once New Horizons starts snapping close up pictures of Pluto and its moons, it will likely find lots of smaller rocks scattered about the Pluto-Charon system, leftover from the ancient collision that led to its creation.  Pluto has a ton of unanswered questions, but this summer we will get all the data we need to find the answers.

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