Peter Lobner
On 28 May 2015, NASA presented surprising information, derived from observations by the Hubble Space Telescope, on the behaviors of Pluto and it’s five known moons: Charon, Hydra, Nix, Kerberos, and Styx. Details of the study were reported in the paper, “Resonant Interactions and Chaotic Rotation of Pluto’s Small Moons,” by Douglas Hamilton and Mark Showalter, in the 3 June 2015 issue of Nature.
An artist’s conception of the relative sizes and shapes of Pluto’s known moons is shown in the following figure:
Source: NASA
The largest moon, Charon, and Pluto form a binary system that orbits a point between the two, as shown in the following figure, in which Pluto’s orbit is shown in red and Charon’s orbit is shown in green.
Source: NASA
As described in my 14 March 2015 post, this binary system behavior also was observed from the New Horizons spacecraft, which is approaching Pluto for a flyby on 14 July 2015.
The Pluto-Charon binary system creates an irregular, rotating, dumbbell-shaped gravitational field that acts on the other moons orbiting the binary pair, resulting in chaotic (unpredictable in the long-term) orbits of the outer moons. The behaviors of Hydra and Nix are further complicated by their non-spherical shapes and tumbling orbital flight. Nonetheless, it appears that the orbits of Hydra, Nix and Styx are synchronized with each other in a 3-body resonance.
You can read more details on the 28 May NASA briefing at the following link: