Hyperion in crescent.
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(NASA/JPL-Caltech/SSI. Minor editing by N. Drake)
Hyperion in crescent.

So Long, Hyperion, and Thanks for all the Pits

Saturn’s moon Hyperion looks more like a frosted honeycomb or cosmic kitchen sponge than just another cratered old sphere. In fact, the 360-kilometer long moon is one of the more bizarre-looking objects in the solar system. On May 31, NASA’s Cassini spacecraft took its last good look at the strange little world.

From 34,000 kilometers away, Cassini snapped its final closeups of oblong Hyperion. Earlier in its tour of the Saturnian system, the spacecraft had revealed for the first time the true extent of Hyperion’s weirdness, which had only been hinted at in Voyager 2 observations. Those Cassini images, shot in 2005, have since become iconic views of a moon that really is unlike anything else in the solar system.

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A stunning false-color image of Hyperion shot in 2005.  (NASA/JPL/SSI)

This time around, there was a chance scientists could take a look at Hyperion’s other side.

But it was not to be. Unlike most other moons in the solar system that spin like a top around a fixed axis, Hyperion tumbles. It rotates chaotically, meaning that up, down, left, and right are more or less random, a non-pattern produced in part by gravitational interactions with Saturn’s giant moon Titan. So, as Cassini approached Hyperion over this past weekend, there was no predicting which part of the moon it would see.

Turns out, it captured the same enigmatic, familiar face from a decade ago.

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Hyperion in 2015, as seen by Cassini. (NASA/JPL/Caltech)

Over that decade, we’ve learned that Hyperion’s unusual appearance can be blamed on its extremely low density. It’s roughly equal parts space and substance, and about half the density of water. So, rather than excavating material from the moon, impactors simply press themselves into and compress the moon’s surface, creating those odd-looking pits. And because Hyperion’s gravity is so weak, any material that’s blown off during impact just keeps flying into space instead of falling back to the surface and obscuring the craters’ original shapes.

That’s not all. Tucked into the bottoms of Hyperion’s pits are dark compounds that are likely a frigid hydrocarbon slurry. These compounds are actually vaguely reddish — which is why Hyperion appears slightly ruddy in natural-color images.

There won’t be any more close-up shots of Hyperion from Cassini. The spacecraft’s journey through the Saturn system is scheduled to end in 2017, and farflung Hyperion isn’t close enough for another flyby. But until the spacecraft plunges into Saturn, it will continue gathering data about Saturn’s wonderfully diverse and mysterious moons, and then begin diving through the space between the planet and its rings.

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Hyperion’s strangely pitted surface, seen in 2015. (NASA/JPL/Caltech)