Just two days after Cassini visited Saturn鈥檚 moon Tethys, it has flown past Hyperion, one of the smaller and odder moons of the ringed planet.
Hyperion is potato-shaped: 360 kilometres long, but only about 250 kilometres across. Its rotation is chaotic, tumbling unpredictably under the influence of Saturn鈥檚 and Titan鈥檚 gravity. And it is exceptionally dark for a Saturnian moon, reflecting only 30% of the light that falls on it, with a distinctly red tint.
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The biggest question for Cassini to answer is why Hyperion is so misshapen when other asteroids and moons of about this size are much more spherical. One theory is that it is merely a fragment of a larger moon that was shattered in a violent impact.
Cassini鈥檚 pictures certainly show a tortured world, riven by craters and girdled by a giant cliff face tens of kilometres high.
The new images were gathered early on Monday morning, from as close as 500 kilometres to Hyperion鈥檚 surface. During the flyby, Cassini鈥檚 radar also measured slight changes in the speed of the spacecraft. This will give an idea of the strength of Hyperion鈥檚 gravity, and therefore its mass. Mission scientists hope to discover whether the moon is solid rock or a loosely packed 鈥渞ubble pile鈥�.
The Cassini team should also be able to map the chemical make-up of Hyperion鈥檚 surface to discover whether it is dusted with dark material drifting in from Saturn鈥檚 sooty outer moon, Phoebe.