At Saturn, Two Distorted Moons Pass in the Night

Bad Astronomy
The entire universe in blog form
Aug. 18 2013 8:00 AM

Warped Moons Flirt with Each Other

I am endlessly fascinated by the endless combination of moons and rings the Cassini spacecraft sees as it orbits Saturn. Because the moons all orbit Saturn in the same plane, the geometry of the situation winds up having them appear to pass close to each other when Cassini is also in that plane. It’s like watching cars race around the track from the side; you can see cars pass each other even if they don’t physically touch.

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!  

And because that Saturnian plane also contains the rings, you get tremendous lineups, like this recent one showing the moons Mimas and Pandora passing while the rings do a cameo in the corner of the frame:

Saturn, Mimas, Pandora
Mimas, the Death Star moon, looms over tiny Pandora while Saturn's rings dominate the lower left corner. Click to encronosenate.

Photo by NASA/JPL-Caltech/Space Science Institute

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Mimas is about 400 kilometers (250 miles) across, and Pandora is football shaped and about 100 km (60 miles) across its long axis. Mimas was 730,000 km from Cassini when this picture was taken, and Pandora about 690,000 km. That means they were pretty far apart in this shot, and just appear close due to perspective. But that 40,000 km difference is small compared to their mutual distance to Cassini, so the difference in size you see here is pretty much real; Mimas really is four times bigger than Pandora.

Or is it? Note that Pandora is not seen exactly sideways here. The long axis is angled a bit away from us, making the moon seem smaller than it otherwise would (hold up a football or some other elongated object and rotate it a little to see this for yourself). So even though Mimas is about four times the width of Pandora, it looks bigger than that, because Pandora looks smaller than it really is.

And while Pandora is hardly round, you can see in this picture that Mimas isn’t a perfect sphere either. It’s actually egg-shaped! It’s about 10 percent out-of-round, which is noticeable to the eye. Mimas is warped because the immense gravity of Saturn stretches the moon via tides.

This all shows you that there’s more to these images than you can immediately see; it’s far more than just a pretty picture. Invisible forces act on these bodies, distorting them, while perspective distorts our view even further. But if you have a bit of experience, and look for the subtleties, then the truth behind them can be found.

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