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SYFY WIRE Bad Astronomy

Last Call for Hyperion

By Phil Plait

My favorite moon of Saturn — really, one of my favorite objects in the whole solar system — is Hyperion. It orbits Saturn about 1.5 million kilometers out, farther than the Cassini spacecraft usually goes, so it’s only been visited a handful of times in the past decade.

On May 31, 2015, Cassini flew past the moon at a distance of about 34,000 km, and took a series of amazing pictures of it. The one at the top of this post was taken when it was almost directly between Cassini and the Sun, so it shows a thin crescent. Hyperion’s not even close to being round, so the crescent is a bit gnarled.

Even there, you can see it’s heavily cratered. But when you see it in its full glory, well, you’ll get an idea of why I like this little moon so much:

That color portrait was put together from individual images by Jason Major. In the Saturn system, where everything is bizarre, Hyperion is the bizarriest.

You can see how potato-shaped the moon is. It’s about 360 km long, and dominated by that huge, flat crater. It’s covered in smaller craters, making it look porous like a sponge… and that’s not far from being true. Its average density is about half that of water, which is lower density even than ice! That means the bulk structure of the moon must be full of holes, like a pile of rubble.

It looks for all the world(s) like a Styrofoam blob that someone has shot with a BB gun.

Many of the craters have a very dark material pooling in their floors, too. Those aren’t shadows; they’re real. They’re hydrocarbons, molecules made up of carbon and hydrogen. It’s possible that was material sputtered away from another moon by micrometeorite impacts, which then fell on Hyperion.

There are so many weird things about Hyperion… and this is our last chance to investigate them, at least for a long, long time. The Cassini mission is drawing near its end, and no more flybys of this odd little world are planned. That’s sad, but we’ll be seeing a lot more of these final farewells as Cassini nears the end, and visits each of the moons one last time.

But their images will live on in the Cassini raw image archive. I suggest poking around in there, as it’s fascinating; it’s full of amazing (unprocessed) shots from Saturn, more than a billion kilometers away. Astronomers will be using it for decades, I’d wager, even if and when we send more probes to that ringed bauble.

And there’s poetry to that, from a mission that has brought us so much. Even after the spacecraft itself plunges into Saturn, the data it sent back will live on, and help us understand the world that Cassini will then be an eternal part of.

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