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Discovered orbiting about 1.5 million km from Saturn in 1848, Hyperion is one of the largest irregular moons in the Solar System. Spanning a distance of 360 km at its widest, but only 205 km at its narrowest, the moon suggested a violent past even before its close study by Cassini. Voyager 2 captured images of the moon from about 700,000 km that showed hints of a large crater. Ground-based observers tracked Hyperion over many nights to reveal that its rotation is chaotic: the moon isn’t tidally locked to Saturn, but it doesn’t have a regular rotation period either. Its irregular shape, elliptical orbit, and 3:4 orbital resonance with Titan are the causes of this unusual state. The New Horizons spacecraft discovered that two moons of Pluto — Nix and Hydra — are also in chaotic rotation.

During a flyby of Hyperion in 2005 that approached to within 500 km, Cassini captured the view shown here. A giant crater that is almost as wide as the moon dominates the sponge-like surface. The impactor causing this feature was sufficiently big to break Hyperion up, and maybe it did, as Hyperion’s density is only 0.54 g cm–3, about half that of the water ice that comprises most of the moon. This low density implies the moon’s interior possesses void spaces; perhaps it is even a rubble pile.

Credit: NASA/JPL/Space Science Institute.

Many small, dust-filled craters dot Hyperion’s surface, and some of them have low-albedo floors. This dark material may be similar to the 'Phoebe dust' produced by Saturn’s giant Phoebe ring that coats the dark side of Iapetus. This material, which is rich in organic molecules and carbon, is warmer than the water ice and 'drills down' into the craters to form deep pits similar to the Earth’s suncups. Hyperion is a strange world, but it has a bit of the familiar.

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Correspondence to Bonnie J. Buratti.

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Buratti, B. Hyperion. Nat Astron 1, 574 (2017).

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