A couple of days ago, the still-active Cassini spacecraft orbiting around Saturn performed a flyby of the small Moon Hyperion, taking high resolution photos of the surface, and showing the sponge-like surface of the moon in a different light. Hyperion is a moderate 250 kilometers wide, yet it doesn’t tug very hard on Cassini.  Performing measurements of its surface gravity, Cassini has shown that the moon is mostly empty space, similar to a pumice stone on Earth.  The odd shapes of the craters are thought to be a result of this.  When an impactor hits Hyperion, it blows the material...