Cassini gives Dione a close shave

Bad Astronomy
The entire universe in blog form
Dec. 13 2011 6:00 AM

Cassini gives Dione a close shave

The Cassini spacecraft has been touring Saturn and its moons for 7 years now, and yet still manages to send back images that are simply astonishing. Just yesterday, the probe swung past the icy moon Dione at a distance of just 99 kilometers (62 miles) over the surface! Compare that to the moon's diameter of over 1100 km (670 miles) and you get an impression of how close that was.

The purpose of the pass was to get infrared spectra of the moon, so only a few visible light images were taken. But oh, what pictures they were! Check this out:

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death From the Skies!  

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Wow! Dione dominates the view, its cratered surface of ice looking like a golf ball that had a dimpling machine accident. You can see Saturn's rings on the left, nearly edge-on, and two more moons as well: the gray lumpy potato of Epimetheus, only about 130 km (70 miles) across, and Prometheus, also about 136 km along its long axis. My first guess is that Prometheus is farther away than Epimetheus in this shot, since it looks smaller (I wondered for a second if it's possible we're seeing it rotated a bit so it's pole-on, but it's a very elongated rock; so we're definitely seeing it mostly from the side here).

After seeing that picture, I excitedly grabbed the next one, and got confused for a moment:

Now, wait a sec. There's Dione, the rings, and Epimetheus. See how before, Epimetheus was mostly above the rings, but now it's mostly lower? That means Cassini moved up a little bit from the plane of the rings, so the little moon looks like it moved down. So then why did Prometheus move up?

Because it didn't. That's not Prometheus, it's Pandora! A different moon, though they're related: they are shepherd moons, which means they have very similar orbits, and occasionally swap places! It's weird, but I've explained it before. Anyway, Pandora and Prometheus are almost exactly the same size, and both are elongated like an Idaho spud. So I'm not too surprised I was confused for a moment when I saw the second picture. When you look a little more closely you can see the shapes are different, though.

More pictures were returned from the pass (including a couple showing Mimas peeking out from behind Dione), so you should take a look. They're pretty dramatic.

Image credit: NASA/JPL-Caltech/Space Science Institute



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