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Pluto's Tiny Moons Are Spinning Like a Bunch of Hooligans

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Mayhem in the outer solar system! Pluto’s four baby moons—Nix, Styx, Hydra and Kerberos—are spinning like mad, surely a sign that the once-ninth planet is gearing up for a full-out assault on the denizens (us!) of the sunlit realm.

Okay, maybe my imagination is getting carried away. But seriously, those little moons got some spin. Take Hydra. Lumpy, lopsided, 27-mile long Hydra rotates an unprecedented 89 times for every pass around Pluto. You know how many times our moon rotates as it orbits the Earth? Once, like a reasonable moon! New Horizons co-investigator Mark Showalter notes that “if Hydra were spinning much faster, material would fly off its surface due to the centrifugal force.”


You’d be better off living on the Wipeout at your local amusement park than trying to colonize Hydra. But what about Nix, our favorite pink jellybean moon? Surely a cosmic body with such sophisticated taste orbits its local dwarf world in a decorous fashion? WRONG. Nix is tilted on its axis and spinning backward, the little hellion!

Not an ounce of common sense to be found in Styx or Kerberos, either—these moons are also twirling and wobbling like turbocharged tops.

The small moons of Pluto behave like spinning tops. Pluto is shown at center with, in order from closest to farthest orbit, its moons Charon, Styx, Nix, Kerberos and Hydra. Image Credit: NASA/JHUAPL/SwRI/M. Showalter


Most moons in the solar system circle their planets obediently, one side always facing the host world in slow, synchronous rotation. I don’t know what’s wrong with these Plutonian moons, and apparently, neither does the New Horizons science team. At least, they haven’t ironed out the details. They suspect that Charon—Pluto’s large, battered, tidally-locked companion—is somehow behind it all. As Pluto and Charon dance around a common center of gravity (a barycenter), their motion produces a time-varying, asymmetric gravity field. In a recent paper, Showalter predicted that this wobbly gravity might cause the small moons to tumble chaotically, like dice in a pop-o-matic.

Seems he was right. Still, the situation is much stranger than anyone imagined. “We expected chaos, but this is pandemonium,” Showalter noted.

Clearly, we’ve still got a lot to learn about Pluto’s frosty little moon nuggets. In the meanwhile, if anybody wants to claim “Charon and the Hellraisers” as your new band name, that’s still up for grabs.


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Top image : NASA/JHUAPL/SwRI