Warped Moons Flirt with Each Other
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Sunday, Aug. 18, 2013, at 8:00 AM
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.
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:
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
Photo by NASA/JPL-Caltech/Space Science Institute
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.
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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