June 3, 2015: 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 READ: Junior version of this article Learn about this story in the Star Witness, a science newspaper available on our sister site, Amazing Space. 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
Please join this week's Hubble Hangout at 3pm EDT on Thurs., June 4, to learn about these exciting Hubble findings in anticipation of the NASA New Horizons spacecraft flyby of Pluto in July 2015. To join, visit http://hbbl.us/y6E .