Three of Pluto’s moons have gravitationally locked orbits

Nature : As NASA’s New Horizons spacecraft approaches Pluto, researchers are trying to determine as much as possible about the dwarf planet so that the spacecraft can focus on specific questions. Currently, New Horizons is looking for more moons around Pluto, which already has five known satellites. A team led by Mark Showalter of the SETI Institute in Mountain View, California, has examined images collected by the Hubble Space Telescope and discovered that three of the moons—Nix, Styx, and Hydra—have stable, gravitationally locked orbits whose periods are integer multiples (or fractions) of each other. Showalter’s team has also determined that the moon Kerberos is darker than Nix and Hydra, which suggests that it could be made of a different material, and that Nix and Hydra rotate chaotically on their axes, similar to Saturn’s moon Hyperion.