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Pluto's moons in chaotic state

Irene Klotz, Spaceflight Correspondent
Jun 4, 2015, 6:44 UTC

Sen—NASA’s New Horizons spacecraft, which is heading toward a July flyby of the dwarf planet Pluto, will find a far weirder world than scientists imagined when the probe began its journey more than nine years ago.

For example, Pluto’s four small outer moons—all discovered after intricate analysis of Hubble Space Telescope images—are chaotic, the result of constantly battling shifting gravitational forces of Pluto and its primary companion Charon, according to a study published this week in the journal Nature.

Pluto and Charon orbit each other, the only binary pair in the Solar System.

As a result of orbiting a binary system—and an uneven one at that, as Charon’s diameter is just over half as big as Pluto’s—the outer moons wobble, flip and turn in unpredictable ways.

“This is called chaos,” lead researcher Mark Showalter, with the SETI Institute in Mountain View, Calif.,  told reporters on a conference call. “It’s a phenomenon that comes up in certain physical systems where they become fundamentally unpredictable.

The analysis was done on the moons Nix and Hydra, but scientists believe the results are applicable to Kerberos and Styx as well.

“You can never really know enough about the orientation of Nix at any given time to really predict what its going to do very far into the future,” Showalter said.

“If you have real estate on the north pole, you might suddenly discover one day that you’re on the south pole of Nix instead,” he added. “The Sun might rise in the west and set in the east. The Sun might rise in the west and set in the north, for that matter.”

Complicating an already complex orbital ballet are the moons’ non-spherical shapes, more like American footballs or rugby balls than round.

Scientists believe the moons formed after a collision between Pluto and a similarly sized body early in the Solar System’s history.

More details are expected when New Horizons zooms between Pluto and its outer moons on July 14. The spacecraft is expected to come as close as about 7,750 miles to the dwarf planet and about 17,900 miles to Charon.

The study also found that Kerberos is surprisingly dark, a sharp contrast to its highly reflective sibling moons. Why Kerberos is so black is another mystery that New Horizons may help solve.

“We have already learned that Pluto hosts a rich and complex dynamical environment, seemingly out of proportion to its diminutive size,” lead researchers Showalter and Douglas Hamilton, with the University of Maryland, write in the Nature paper.


Illustration showing the scale and comparative brightness of Pluto’s small moons. The surface craters are for illustration only and do not represent real imaging data. Image credits: NASA/ESA/A. Feild (STScI)