On approach to the Pluto system in July 2015, NASA’s New Horizons spacecraft captured images of the dwarf planet and its biggest moon, Charon, rotating over the course of a full day. A day on the dwarf planet Pluto. Image credit: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute. Pluto and the largest of its five known moons, Charon, orbit each other every 6.4 days. These objects are gravitationally locked...