Dr. Tongue's 3D House of Prometheus

By Phil Plait | February 19, 2010 4:30 pm

If you’ve ever wondered what it would be like to hang out near the Cassini Saturn spacecraft and get the same view it does, then put on your red/green glasses and check out this anaglyph of the moon Prometheus:


Mmmm, threedeealicious. Click to enjovianate.

Prometheus is a bit weird. OK, it’s a lot weird. It’s an irregularly-shaped elongated spud of a moon, measuring about 119 x 87 x 61 km (71 x 52 x 37 miles) in size. The long axis always points toward Saturn due to tides; basically the change in Saturn’s gravity from the front end of Prometheus to the back end acts like a stretching force on the moon, keeping it aligned. The tip on the right always points toward Saturn, and the long side we see in the image is the leading half of the moon, always facing ahead into the direction it orbits. Think of it as facing into the wind if that helps any.

Prometheus is a shepherd satellite, meaning its orbit gets it near Saturn’s F ring, where it helps keep the ring particles in place. It does this along with Pandora, another smallish moon. Prometheus orbits Saturn inside the F ring. When it gets close to the ring, it gives a little bit of its orbital energy to any ring particles that are on the inside edge of the ring, which boosts them to a slightly higher, slower orbit. Pandora does the opposite; it orbits outside the ring, and it steals energy from ring particles on the outside edge, which drops them into a slightly lower, faster orbit. Together, the two moons shepherd the F ring particles, corralling them and keeping the ring narrow. The animation shows the effects of Prometheus on the inner edge of the ring.

Just so you know, I think this is one of the coolest things ever. Shepherding moons were theoretically discussed for a long time, but we didn’t have any evidence of them until Voyager swept past Saturn a few decades ago, and now Cassini has the chance to study them in detail. It’s such a weird thing, and there it is playing out in the solar system for us to examine! It’s a good reminder that Nature is sneaky, and a lot more clever than we are. I’m glad we’re clever enough to catch up with it, too.

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (22)

  1. Queen B

    i love the word enjovianate

  2. It’s a good reminder that Nature is sneaky, and a lot more clever than we are.

    Pffft! I had the same idea. That biatch nature stole it from me.

  3. So why is it shaped like a cigar or tongue or whatever? Why isn’t it round? Or maybe, why is our moon round and it isn’t?

    I am just today aware that a moon can be other than round. Although, there is no reason why I think that, except of course being entire human/earth centered, I assumed that all things in the universe will be in someway like earth…

    Once again, proof that I am not as open minded as I thought I was.



    So why is it shaped like a cigar or tongue or whatever? Why isn’t it round? Or maybe, why is our moon round and it isn’t?

    The reason that Prometheus is not round is due to it having insufficient mass and, therefore, gravity to overcome its own rigidity and achieve hydrostatic equilibrium — a spheroid.

  5. Brian

    “Enjovianate”? But shouldn’t that be “enkronosize”?

  6. Mike

    “… get the same view it does, the put on your red/green glasses …”

    Should that be then? 😀

  7. Messier Tidy Upper

    Nice photo, interesting moon, good write up. Thanks. :-)

    It strikes me as a bit ironic that a moon named after the mythical Greek giver of fire is so cold.

    If I recall right I saw another image of Promethues via Cassini not too long ago that made it look sorta like a fried egg!

    Edited to add – Yes & its here (wish I knew how to embed images etc ..) :


    I emailed it to the BA but he never posted it – not sure if he’s getting my emails or not. :-(

  8. Bob Portnell

    1) I agree with Mike: enkronosize.

    2) “Nature is … cleverer than we are.” Tsk. Indugling in anthropomorphism is the first step down the slippery slope to supernaturalism. (Unless it’s a giant squirrel costume, in which case it’s … no, no. Too easy.)

  9. The Other David M.

    If you think sheparding moons are cool, what about Epimetheus and Janus?

  10. jcm

    Man! I need to get some (a pair of?) 3D spectacles.

  11. Murdats

    only anaglyph :( no parallel/cross eyed images for freeviewing or even the 2 seperate images so people can 3d-ify them as need be

  12. Very cool and definitely interesting. Its little things like these (sheparding moons) that make you really notice the complexity and near-perfectness of nature — and why we study it. Just astounding.

  13. KAE

    I’ll echo what Murdats (#12) said.
    I had a pair of paper red/green “glasses” that came in an issue of Sky&Telescope many moons ago. They became so scratched-up that they became unusable.
    Anyone have a link to a place that sells real 3D glasses?

  14. JP

    I will also add my +1 to Murdats (#12). Can we please stop posting these as 1950s red/green anaglyphs that require special 3D glasses and start posting them as parallel / cross-eyed images that only require the ability to cross your eyes? Click my name above for an example.

  15. bigjohn756

    There are several websites that will give you one pair of glasses. Otherwise, you can purchase them for a couple of bucks. Just google 3d glasses and take your choice.

    BTW, the concept of shepherding moons cameth from the Good Shepherd, and, thusly, He created them along with the rest of the universe. Now you know.

  16. If you look at it without the glasses (which I don’t have (who does?)) it looks like there is writing on it.

  17. JP (15): When people make anaglyphs like that, I will post them. Until then, “we” will continue to post what’s available.

  18. JP

    Maybe someday if budgets allow it NASA and JPL will have the “technology” to be able to display the images side by side – until then I will use the blue/red glasses of my imagination. In the meantime you can try out my weak attempt: http://www.pbase.com/jpercia/image/122127438

  19. John Paradox

    I have to say, the photo looks like Stargate SG-1’s Prometheus, still in the bubble wrap….

  20. Richard Smith

    Whooo! Skelly stuff, keeds! A-wooo!

  21. And here, I thought I was the only one who remembered the “Dr. Tongue’s 3-D House of …” series from Monster Chiller Horror Theater.


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