Potato Shepherds

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There are lots of potato-shaped objects in the solar system - small moons and asteroids with too little mass to be rounded out by gravity, or to hold onto the barest scrap of an atmosphere, or to experience the weakest quiver of geological activity. But the Saturnian system, with its myriad tenuous rings, is a place where a small, misshapen lump of rock can do beautiful things. Pandora (above) and its close companion Prometheus (the speck next to Saturn's disc, below), being fine examples.

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Although their irregular bodies barely stretch past a hundred kilometres at their widest points, they have enough pull to affect the small particles that make up one of Saturn's outer rings, known as the F ring. Below, Prometheus can be seen drawing out a thread of dust from the F-ring.

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Prometheus and Pandora (and other moons in a similar situation) are known as shepherd moons. The F-ring is extremely fine and narrow compared to the rest of Saturn's rings, and it seems that Prometheus and Pandora, which orbit close on either side of it, are keeping the ring 'on track', so to speak: preventing it from dispersing or widening.

But Prometheus and Pandora may well not be the whole story. Close-ups (of a stretch a mere kilometre across) of the F-ring show strange and elegant structures like this:

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It seems that these small perturbances are created by even greater misfits than Prometheus and Pandora, tiny moonlets too small to see directly, but which we can detect by the ripples they leave in the F ring. It's odd to think that of the whole cast of characters, it is these invisible rocks that are the closest to everyday human scales. And yet, from a typical Cassini-eye view (below), Prometheus and Pandora, both much more massive than Mount Everest, are tiny motes either side of the F ring, seemingly dwarfed by their cousin Janus (not really that much larger), to the right.

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