Saturn’s main rings are sufficiently bright that you need only a small telescope to see them. But just outside their orbit lies the F ring, a ribbon so faint and narrow that it took a visiting spacecraft to spot it. Only 100 kilometers wide, the F ring stays slender thanks to two “shepherd moons” (shown in the inset), the gravitational pulls of which maintain the structure of the ring. As scientists report online today in Nature Geoscience, computer simulations suggest that this gravitational pas de trois arose after these two moons hit and partially destroyed each other, spewing material that their gravity sculpted into a narrow ribbon. If right, this scenario has likely occurred elsewhere in the solar system and even across the galaxy: Two shepherd moons tend one of the rings of Uranus, and similar collision-spawned rings may encircle many of the giant worlds orbiting other stars.