Two of the most reliable changes in the sky are the daily rising of the sun in the east and setting of the sun in the west. But if you lived on a couple of Pluto's moons you wouldn't know when the day would begin, or even what direction the sun would rise. That's because, unlike Earth's moon, at least two of Pluto's small moons, Hydra and Nix, are tumbling chaotically through space. Why? Because they orbit inside a dynamically shifting gravitational field caused by the system's two central bodies, Pluto and Charon, that are whirling around each other. The moons are also football shaped, and this contributes to the chaotic rotation.

By contrast, Earth's moon keeps the same face toward us, because the gravitational forces between Earth and the moon cause the moon to dynamically settle into a condition called tidal lock, where it keeps one hemisphere facing Earth. Almost all of the solar system's major moons also behave similarly. But the Pluto moons essentially orbit a "double planet." And this makes life complicated. Over the past several years, the Hubble Space Telescope has discovered four tiny satellites orbiting Pluto and Charon. Researchers were puzzled by unpredictable changes in the sunlight reflected off the two brighter moons. They finally concluded that at least two of the moons must be tumbling unpredictably.