Dione and Mimas have a mutual event

By Phil Plait | January 30, 2012 7:00 am

As Cassini weaves its way around the multiple moons of Saturn, it’s not really a coincidence when one gets in the way of another. As a matter of fact, it’s a guarantee. These are called mutual events, and when Cassini dove past Dione, it saw this terrific view of Mimas peeking out from behind it:

Nifty, huh? [Click to encronosenate.]

Dione is nearly 3 times larger than Mimas (1100 versus 400 km wide), but Mimas was also more than 6 times farther away, making Dione loom nearly 20 times larger in this shot. I like how you can’t really see the unlit side of Dione, but Mimas marks it pretty well, sliced in half by the edge of the larger moon.

Funny, too: I was thinking to myself that if Cassini was in position to catch this shot, then it should have also caught Mimas when it was on the other side of Dione, the lit part. Well, seek and ye shall find: I searched the Cassini raw image archive and found it! I put a small version of it here; click to embiggen. You can just barely see a small segment of Saturn’s rings in the lower left corner, too.

Neat! I like it when stuff makes sense. While this alignment is rare to see from Earth — we’re a lot farther away, and the geometry has to be precise — we do see moons transiting across their parent planets, and, far less often moons in front of moons. But what’s rare to us is common to Cassini, with its front row seat to this amazing system of worlds.

Related posts:

A marvelous night for a (Saturn) moon dance
Dione and Rhea, sitting in a tree
The more distant moon
Midnight on a ringed world

CATEGORIZED UNDER: Astronomy, Pretty pictures
MORE ABOUT: Cassini, Dione, Mimas, Saturn

Comments (18)

  1. It would be cool if there was a graphic that showed where all the moons are relative to one another and relative to Cassini… like a top down view of the Saturn system. That way we could see what kind of movement Cassini and these moons made to make both of these images possible.

  2. Chris

    And you can tell it’s cold too by how much Mimas is poking up.

    (I can’t be the only one who thinks that picture looks like a giant …..)

  3. What an amazing probe Cassini is! The pictures it sends beack just continue to astonish and amaze me to no end. I wish there was a way to keep that probe going for years and years to come.

    @Chris (#2), I didn’t think of that until you mentioned it. Now that’s all I see! ūüėÄ

  4. Mike

    ooooh how titillating. lol

  5. There’s an ample supply of double entendres in this amazing system of worlds, as we peek out from our front row seats at this pair of moons. A very mutual event indeed.

  6. VinceRN

    Nice, the most subtle pareidolia post yet.

  7. JC

    Is Dione and Mimas actually having a mutual event or is one of them faking?

  8. Chris

    What a lovely pair :-)

  9. Hampus

    Is Cassini using its thrusters to navigate or is it flying a planned path, using the gravity of Saturn and it’s moons?

  10. Greg

    I’m sorry…..”embiggen”? Seriously? Is this Gizmodo?

  11. vince charles

    9. Hampus:

    Both. Cassini is using gravity assists from Titan, the only moon with palpable gravity on these scales. But since there’s only one such moon, unlike at Jupiter, Cassini must occasionally use thrusters, much more than Galileo did. Relying on Titan alone would make the mission really inflexible.

  12. mfumbesi
  13. CR

    In that first pic, I can’t help but notice the three ‘claw marks’ connecting the large-ish crater (the one near the terminator, with the central peak) to the two smaller side-by-side ones. Interesting.

  14. @ Greg: New around here, huh? :)

  15. Hey, if Volkswagen gets farfegnugen you can have “embIggen”. It’s only fair.

  16. Hampus

    @ 11, Vince: Thanks!

  17. Matt B.

    @0: “Dione is nearly 3 times larger than Mimas (1100 versus 400 km wide)”

    Actually, that’s only almost 2 times larger, but it is almost 3 times as large. “Larger” means “more large”; “more” means “in addition to what’s already there”. So “3 times larger” means “add 300% to the size”, which means “multiply the size by 4”.

    When you get into factors of millions, thousands, hundreds, or even dozens, the choice of phrase doesn’t matter, because I’ll take the difference to be within the margin of approximation. But when the factor is less than a dozen, it really makes a difference and can screw up order-of-magnitude estimates. Please be careful.


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