Figure: The Pluto system: i.e., Pluto and its moons

Image 1 Caption: The Pluto system with ex-planet Pluto (now a dwarf planet) near the center of mass (AKA the barycenter) and 3 of the 5 currently known moons of Pluto.

Features:

1. The moons of Pluto:
   1. Charon: Named for the mythological Charon the ferryman to the Greek underworld which was the land of the dead in Greek mythology and which was ruled by Hades.
   2. Nix (AKA P2): Named for the mythological Nyx, the primordial goddess of night in Greek mythology with another spelling.
   3. Hydra (AKA P1): Named for the mythological Hydra, a 9-headed chthonic serpent-thing in Greek mythology.
   4. Kerberos (AKA P4): The Greek form for Cerberus, the hellhound who guards the door of the Greek underworld.
   5. Styx (AKA P5): Named for the River Styx that that separates the land of living from the Greek underworld. The mythological Charon ferries the dead across this river.

   The New Horizons spacecraft (2006--) did a flyby of the Pluto system with closest approach on 2015 Jul14. It discovered NO new Plutonian moons, and so it is likely that the 5 currently known ones are all there are.

2. The specifications of the Pluto system are given in the table below.

    _______________________________________________________________________________________
    The Pluto System
    _______________________________________________________________________________________
    Astro-Body    Discovery Year   Radius     Mass        Orbital Radius   Orbital Period
                                    (km)    (10**18 kg)         (km)             (days)
    _______________________________________________________________________________________
    Pluto             1930          1153       13050            2040            6.3872
                                  70 % Moon  18 % Moon
    Charon            1978           602        1520           17530            6.3872
                                  35 % Moon   2 % Moon    15.20 Pluto radii
    Nix               2005            44           1           48708             24.9
    Hydra             2005            36            .391       64749             38
    Kerberos          2011            20 approx    ?           59000             32.1
    Styx              2012          10--25         ?           42000±2000      20.2±0.1
    _______________________________________________________________________________________ 

3. Pluto and Charon are mutually tidally locked as illustrated in Image 2 below.

   

4. Image 2 Caption: An artist's conception of of the Pluto-Charon system seen from above their mutual orbital plane. The Charon orbit is the large blue circle and the Pluto orbit is the small red circle. Note, Charon is massive enough that the center of mass of Pluto system (consituted by Pluto and all its moons) is physically outside of Pluto, thus Pluto and Charon (overwhelmingly the largest moon) can be considered to be a binary system." The center of mass is the center of the Pluto orbit.

   The image is at least approximately to-scale: Pluto and Charon are about 8 Pluto radii apart (see Wikipedia: Moons of Pluto: List).

   Pluto and Charon are mutually tidally locked to each other. They consitute a sort of rotating dumbbell.

5. Because Pluto and Charon are mutually tidally locked, their orbital periods and rotation periods are all exactly the same on average with the average value being 6.3872 days.

6. Tidal locking means, among other things, on one side of Pluto/Charon you always see Charon/Pluto sitting in the sky at approximately the same orientation relative to the surface of Pluto/Charon.

   The same orientation relative to the ground means having the same horizontal coordinates: i.e., same altitude and azimuth.

7. On the other side of Pluto/Charon, you never see Charon/Pluto.

8. The rotation periods of the other moons are unknown, but they may be tidally locked to Pluto, and so have rotation periods equal to their orbital periods---on the other hand, they might have some strange relation between rotation period and the other periodicities of the system because of the multiple sources of gravity.

9. See the NASA Pluto video: NASA: A Close Look At Pluto Released Friday, February 26, 2016 | 0:36: Authoritative: Explore Pluto's frozen peaks, icy plains and other landforms in this flyover animation created from images taken by the New Horizons spacecraft (2006--). Good for the classroom.