Moonlets in Saturn's Rings

Future space historians will recall two great epochs of discovery concerning Saturn's rings.

In the first, circa 1980, Voyager 1 discovered that the planet's stunning ring system is not a single smooth sheet but rather thousands of interconnected ribbons, at once beautiful and strange in their collective organization. In the second, circa 2005-10, the Cassini orbiter found the rings to be even more beautiful — and stranger — than we'd previously imagined.

Take, for example, mysterious twists of icy rubble dubbed "propellers" because of their distinctive shape. Cassini's cameras have spotted thousands of them in three narrow zones within the A ring, the outermost of the planet's main bands. Predicted to exist even before the craft's arrival, they're apparently caused by tiny moonlets embedded within the ring — too small to be resolved in images, but massive enough to disturb the flow of particles in their vicinity.…

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