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 off into space, instead of creating a circular shock wave that spreads out surface material. This gives the craters on Hyperion a distinctive look, like deep chunks have been removed.
The largest of the non-circular irregular moons of Saturn, Hyperion is thought to be the result of the breakup of a larger moon long ago. The smaller material from the breakup would pepper the resulting moons, giving Hyperion its heavily cratered surface. The recent flyby is the first time Cassini has passed the small moon since 2005. It is wonderful to see that Cassini is still going strong and sending back good science data from the Saturnian system after more than a decade of operations.