Dione and Rhea, sitting in a tree

By Phil Plait | September 24, 2010 9:23 am

I know, just a couple of days ago I was saying that I was surprised that Cassini could still, um, surprise me. But then it sends back a completely crazy picture like this!

cassini_figure8moons

HA! What an awesome shot! But what’s going on here?

OK, let me explain this gently. When two moons are in love, they…

No, wait. So, moons are really gigantic single-celled organisms, and when they reproduce, they fission…

No, wait. OK, seriously: the top moon there is Dione, and the bottom one is Rhea. As Cassini flew by them, Dione was closer (a little more than 1.1 million km or about 690,000 miles), and Rhea farther away (1.6 million km or 1 million miles). The angle of Cassini’s trajectory was just right such that Dione passed right in front of Rhea, and it snapped this image just as it happened.

From Cassini’s viewpoint, the two appeared to momentarily connect. It just so happens the two have almost the same albedo (reflectance), making them look very similar in this picture, so where they overlap it looks like they’re connected. Making this even more convincing is the big crater at the bottom of Dione, somehow fooling your eye into getting confused as to which moon it belongs to! So it’s really easy for your brain to merge the two spherical moons into one figure-8 moon.

cassini_dione_rhea1This isn’t the first time Cassini has caught the two moons in a rendezvous; in 2005 it captured this cool image of the pair as well from a distance of a little over a million kilometers from both. All the moons of Saturn orbit the planet in the same plane, directly above its equator, so it’s common to be able to see more than one in a single image… and sometimes even eclipsing each other.

Some of the most dramatic images returned from Cassini show more than one moon in the frame, reminding us that not only are each of these objects worlds in their own right, but also that Saturn is a really, really strange place.

Tip o’ the RTG to Carolyn Porco. Image credit: NASA/JPL/Space Science Institute


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MORE ABOUT: Cassini, Dione, Rhea, Saturn

Comments (33)

  1. Nige

    Cool picture, its amazing how they look the same size and the lighting is more or less spot on, well done universe!

  2. @LaureeAshcom

    i am trying to enjoy the science but mainly i see a moon in a whole other sense… you know, a bare moon hanging over the side of a pier or …..

    sorry

  3. That’s a great photo. Does Cassini take photos at intervals, or was this planned?

    The two moons seem to be similar in their level of meteorite strikes as well. Dione might be a little less scarred.

  4. M

    “Making this even more convincing is the big crater at the bottom of Dione, somehow fooling your eye into getting confused as to which moon it belongs to! ”

    I read that and thought to myself that it wasn’t confusing at all – I clearly identified that the crater was at the top of the lower planet.

    Then I read the sentence again. Then I looked at the picture more carefully…

  5. Messier Tidy Upper

    Awesome image – & natural optical illusion. Love it! :-)

    I wonder if they planned to get this shot juu-ust so ..?

    So many possible inappropriate comments spring to mind! ;-)
    Like .. no,
    And .. er , no
    Or maybe .. umm, no better not say that one either! ;-)

    So, moons are really gigantic single-celled organisms, and when they reproduce, they fission…

    I’m thinking, (possibly mistakenly but anyhow), that this could actually be true.

    Not the single celled organisms part so much – although I’m sure there’s an SF story in that somewhere – but if an impact shatters a moon couldn’t it create two or even many more moons just as asteroids produce “asteroid familiies” by impacts chipping off the old block?

  6. kevbo

    So as Cassini orbits, is NASA tweaking her trajectory to get these ‘artistic’ shots, or is this serendipity?

    (no, not Serenity)

  7. CW

    I don’t suppose there’d be a time in the future when Dione & Rhea may collide? Maybe if Deeprak Chopra meditates hard enough?

  8. Ron

    Hey Phil, how about an article talking about keeping Cassini in proper orbit with all these bodies in (relatively) close proximity? I know their orbits must be stable else they’d have been ejected or spiraled in long ago, but does Cassini stay out of their respective Hill spheres?

  9. alfaniner

    This is one pic that is probably best NOT rotated 90 degrees for modesty’s sake.

  10. Brian Schlosser

    That crater totally had me fooled… I was thinking there was some kind of compression error, or imaging artifact at work. Once you clarified that the crater is on the BOTTOM of the top moon, it clicked…

    Amazing work, Cassini…

    And yeah, add me to the chorus: ARE these planned? I know some are, but are some of these just random chance? Or just the best looking shots out of hundreds of dull (artistically!) images?

  11. Imrryr

    *shakes fist at moons*

    Get a room, you two!

  12. Grand Lunar

    I’d swear it’s like two moons stack on top of each other!

    If humans ever visit Saturn in person, they’ll be in for quite the visual feast.
    Cassini is giving us the appetizers.

  13. Duane

    Are shots like this planned ahead of time by JPL, is it simply luck, or does Cassini have a great eye for photos all by itself???

  14. Keith (the first one)

    I wonder what it would be like to stand on one of them and watch the other go by. Can anyone tell us how big one would appear in the “sky” while standing on the other?

  15. Calli Arcale

    Shots are very much planned ahead of time. Cassini has to be told where to aim, whether or not to slew, when to start recording and when to stop. So they had a pretty good idea something like this would be captured. But knowing two moons will appear in the same frame is nothing like seeing the result come down from the spacecraft.

    That big crater on Dione is *wild*, the way it makes itself look like part of Rhea. How the mind fools us, with it’s awesome edge-detecting capabilities! The arc of the crater mirrors the limb of Rhea so well that the brain fills in the rest. Crazy, wild, and wonderful.

  16. TRL

    Every picture Cassini takes is planned way in advance. It’s completely possible to predict when interesting conjunctions or framings will be visible, given the trajectory and other constraints, and set up the commands to take advantage of them. The trajectory of Cassini, itself, is also planned years ahead to allow for major encounters with the satellites and other objectives. Indeed, it’s now planned that the mission will end with Cassini plunging into Saturn, taking high-resolution images of the rings on the way in.

  17. Tribeca Mike

    Best deep focus photography since Citizen Kane.

  18. After years of orbiting Saturn, the Cassini spacecraft decides to “moon” its operators on earth…

  19. NickM

    Got to steal this pic. I’ve written songs inspired by both these moons, as well as one about little Phoebe!

  20. OtherRob

    I would’ve sworn that Rhea was the closer one. Took me a good bit of staring at the picture to finally see it correctly. Great shot. :)

  21. Bubba

    That is just completely insane.
    I can’t tell from the way you wrote the article, but does Cassini take these pictures by itself, which would imply intelligent robots, or do the scientists predict this and tell the camera when to shoot?

  22. Jim Craig

    To me, it looks like a sideways Master Card logo.

  23. Brian Too

    Does anyone else see a vertical line of craters, right on the edge of night? I’m sure it’s an optical illusion or perceptual artifact. Still, it looks like a line of craters spanning 2 moons. All similar sizes too!

  24. I’m not normally that impressed by the B&W Cassini images. But this one I like. Beats the Malls Balls any day.

  25. I look forward to Hoagland’s conspiracy theory that this photo shows to be the truth that NASA is concealing.

  26. Messier Tidy Upper

    @25. Adrian Morgan Says:

    I’m not normally that impressed by the B&W Cassini images. But this one I like. Beats the Malls Balls any day.

    Aha! I just knew it would be that familiar Adelaide landmark! ;-)

    Are you from / in South Australia too, Adrian? :-)

    @23. Jon Hanford : Thanks. I knew there were binary asteroids too but couldn’t recall that name. I think trojan asteroid Hektor is a similar case too. :-)

    @26. IMForeman Says:

    I look forward to Hoagland’s conspiracy theory that this photo shows to be the truth that NASA is concealing.

    Meh, I don’t! ;-)

    @21. Bubba Says:

    … does Cassini take these pictures by itself, which would imply intelligent robots, or do the scientists predict this and tell the camera when to shoot?

    I’m pretty sure the spacecraft is instructed on when to click its cameras from Earth. If I recall right Cassini has an imaging team that is responsible for deciding what photos are taken when.

    EDIT : Yep, the Cassini Imaging Team Leader is Carolyn Porco, there’s a great website (which I’ll link separately below for moderation) that has details and more :

    CICLOPS or Cassini Imaging Central Laboratory for Operattions.

  27. Messier Tidy Upper

    Here’s the link :

    http://www.ciclops.org/index.php

    to the CICLOPS website as promised above.

    Also here’s a link to the Hektor asteroid wiki-page :

    http://en.wikipedia.org/wiki/624_Hektor

    & it turns out to be the largest Jovian trojan and another contact binary asteroid as are several others.

    Then there are examples further out again in the Trans-Neptunean realm such as 1998 WW31 :

    http://en.wikipedia.org/wiki/1998_WW31

    which was the first binary object found in the Edgeworth-Kuiper Cometary belt region.

  28. ColonelFazackerley

    @Mike 17
    I am surprised one moon is not sharper than the other, given that they are at different distances.

  29. WOW what an amazing pic XD lovin the twin moon pic 2. plz post many more xxx

  30. Nigel Depledge

    First off : Wow!

    Another superb image from the Saturnian system. Once again, a hearty “well done” to the Cassini team.

    @28 – Why?

    Surely it makes sense to have designed the cameras on the Cassini probe to give us the best chance of taking sharply-focussed pics? Even the best autofocus system sometimes needs a helping hand. I wonder what the aperture of the camera that took this shot is.

  31. Tibs

    Hello, police? I think I’ve just been mooned.

  32. Ok, this is FREAKY …

    Just Today, Completely without Reading this Article, I’d Decided to Name a Pair of Hamster Sisters, Rhea and Dione!

    Now, I’d been Planning to use those Names, for QUITE Some Time …

    But Seeing them Listed here, was Downright CREEPY, to Say The Least!

    :-o

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