Two alien worlds, superposed

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Sept. 22 2010 7:00 AM

Two alien worlds, superposed

Every time I think Cassini can't possibly send back a more dramatic image from Saturn, it takes my preconceived notions and crushes them to dust.

cassini_titan_rhea

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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Oh. My.

That shows the moons Titan (in the background) and Rhea, the two largest of the gazillions of moons circling the ringed planet. Titan has an atmosphere, which is why it looks fuzzy.

This picture is crazy amazing! Rhea looks like it's about to smash into Titan -- if you're curious, this is what my nightmares look like -- but in fact they're very far apart. Rhea was about 1.1 million kilometers (680,000 miles) away from Cassini when this was taken, and Titan was more than twice that. So they were actually separated by over a million kilometers, about three times the distance of the Moon from the Earth! Cassini used the narrow-angle camera (essentially a big telescope) to get this shot, so it looks foreshortened -- Titan is actually more than three times wider than Rhea, but it looks smaller than that here because it's twice as far away. The two moons are in no danger of collision.

What a stunning shot! And I love how these two worlds are so different. Icy Rhea is pitted and cratered, but Titan's thick atmosphere smudges out all details in visible light. Together, they're an excellent example of diversity in the Saturn system, and a reminder of just why we sent Cassini there in the first place.

Tip o' the spacesuit visor to Carolyn Porco. Image credit: NASA/JPL/Space Science Institute