Cassini gives Dione a close shave

By Phil Plait | December 13, 2011 6:00 am

The Cassini spacecraft has been touring Saturn and its moons for 7 years now, and yet still manages to send back images that are simply astonishing. Just yesterday, the probe swung past the icy moon Dione at a distance of just 99 kilometers (62 miles) over the surface! Compare that to the moon’s diameter of over 1100 km (670 miles) and you get an impression of how close that was.

The purpose of the pass was to get infrared spectra of the moon, so only a few visible light images were taken. But oh, what pictures they were! Check this out:

Wow! Dione dominates the view, its cratered surface of ice looking like a golf ball that had a dimpling machine accident. You can see Saturn’s rings on the left, nearly edge-on, and two more moons as well: the gray lumpy potato of Epimetheus, only about 130 km (70 miles) across, and Prometheus, also about 136 km along its long axis. My first guess is that Prometheus is farther away than Epimetheus in this shot, since it looks smaller (I wondered for a second if it’s possible we’re seeing it rotated a bit so it’s pole-on, but it’s a very elongated rock; so we’re definitely seeing it mostly from the side here).

After seeing that picture, I excitedly grabbed the next one, and got confused for a moment:

Now, wait a sec. There’s Dione, the rings, and Epimetheus. See how before, Epimetheus was mostly above the rings, but now it’s mostly lower? That means Cassini moved up a little bit from the plane of the rings, so the little moon looks like it moved down. So then why did Prometheus move up?

Because it didn’t. That’s not Prometheus, it’s Pandora! A different moon, though they’re related: they are shepherd moons, which means they have very similar orbits, and occasionally swap places! It’s weird, but I’ve explained it before. Anyway, Pandora and Prometheus are almost exactly the same size, and both are elongated like an Idaho spud. So I’m not too surprised I was confused for a moment when I saw the second picture. When you look a little more closely you can see the shapes are different, though.

More pictures were returned from the pass (including a couple showing Mimas peeking out from behind Dione), so you should take a look. They’re pretty dramatic.

Image credit: NASA/JPL-Caltech/Space Science Institute

Related posts:

The real Pandora, and two mooning brothers
Dr. Tongue’s 3D House of Prometheus
Cassini eavesdrops on orbit-swapping moons
The bringer of fire, hiding in the rings

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (12)

  1. Carey

    Potatooooooooes iinnnnn Spaaaaaaaaaaaaace!

  2. Nicholas

    This is amazing; seeing celestial bodies this small yet this far away from Earth is amazing!

    Please NASA start sending landers to the Moons that could possibly support life in liquid oceans.

  3. Trebuchet

    My first thought when I saw the first picture was “Hey, a moon with moonlets of its own!” That was before reading the post, of course. Is such a thing even possible, or does gravitation from the planet prevent it from being stable?

  4. zatytom

    What’s the white dot just up and left from Prometheus in the first picture? In the zoomed in version it looks very bright indeed – an artifact?

  5. dcsohl

    For a moon such as Dione to have a satellite, that satellite would have to orbit entirely within the Hill sphere. The radius of the Hill sphere is roughly a*(1-e)*cubert(m/3M) where a would be the semi-major axis of Dione, e would be Dione’s orbital eccentricity, m is Dione’s mass and M is Saturn’s mass.

    Plugging in the numbers, we see that Dione’s Hill sphere has a radius of about 3250 km, which is pretty tiny especially since Dione itself has a radius of about 560 km.

    Especially since the Hill sphere is only an approximation, and other orbital factors (such as other moons) can confound the dynamics. True stability can only be achieved at 1/2 or even 1/3 of the Hill sphere radius, bringing us down to the 1100-1600 km range.

    So: yes, it’s possible, although more improbable things are known to have happened.

    There are, for example, asteroids known to have moonlets. Asteroid (66391) 1999KW4 has a moonlet, despite the main asteroid having a Hill sphere only 22 km in radius!

    So there’s hope for satellites such as Dione. But it is very very VERY improbable!

  6. Monkey Hybrid

    @zatytom – It’s also visible in the second photo, slightly higher above Saturn’s ring than in the first, but very faint. If I was to have a guess, I’d go for Helene or Polydeuces.

  7. zatytom (5) & Monkey Hybrid (7): I think it’s a cosmic ray or a bad pixel or something like that. These images are raw and haven’t been processed yet to remove junk like that.

  8. Great images. Thankyou Cassini & BA. :-)

    Except, umm, isn’t Dione a feminine name and doesn’t that then make it it bit rude to be talking of shaving her? What is she the bearded lady? 😉

    .. the gray lumpy potato of Epimetheus ..

    Mmm .. potato! (Imagines covering Epimethus in grated cheese, maybe some carrot or pineapple and sour cream.) But I’m pretty sure that’s rock and dirty ice there instead. 😉

    @4. Trebuchet : December 13th, 2011 at 8:07 am

    My first thought when I saw the first picture was “Hey, a moon with moonlets of its own!” That was before reading the post, of course. Is such a thing even possible, or does gravitation from the planet prevent it from being stable?

    Well, #6 dcsohl has answered that very well already but I’ll just remind y’all that Saturnian moon Rhea possibly has not moonlets but rings of its own!

    I also wonder about whether Enceladus could capture at just that right height some of the material escaping from its “tiger stripe” geysers and have that coalesce into a moonlet orbiting it or not? Mind you, if something like that has happened for Enceladus we haven’t found it yet that I’ve heard so maybe not. Too fine a source material and thus likely to disperse rather than acrete perhaps?

    Then there’s the issue of co-orbital moons which aren’t quite moons of each other but are in an interesting orbital relationship with each other. It also occurs to me that we could also put an “artificial moon” – an unnatural satellite – into orbit around one of the moons.

    Also asteroid 87 Sylvia has no less than three moons – but then it isn’t orbiting a planet.

  9. Wouter

    @phil Question; the black lines on the right of image ‘Dione ‘Rev 158′ Raw Preview #1’ are these bar codes, or a bit of a connection error between Cassini and Earth?

  10. Wow!! If Phil wasn’t telling us about ’em, I’d think these were computer generated or something.
    What I’m wondering is whether Cassini still has enough propellant to do a lot of maneuvering, or if NASA has just done a really good job of picking out an orbit that makes close approaches like these? Sounds like they’re doing some barnstorming 😉

  11. @#4 Trebuchet: My first thought when I saw the first picture was “Hey, a moon with moonlets of its own!” That was before reading the post, of course. Is such a thing even possible, or does gravitation from the planet prevent it from being stable?

    From what I understand, it’s certainly possible, hypothetically, for a planet to have a moon that is both large enough and far away enough for there to be an orbit around the moon that is stable on astronomical* timescales. However, it doesn’t look like that situation exists in our solar system.
    The Earth’s own moon is probably the closest candidate for this sort of thing, being fairly far away and also quite large compared to most other moons and their parent bodies. Since we haven’t seen any Moon moons, that tells us we’re not likely to in our solar system.
    For more info, Google “Hill sphere.” I’m not exactly sure what the Hill radius of the moon is, but I know that for smaller satellites (eg, a weather satellites) the Hill sphere itself is much smaller than the actual object (meaning it’s impossible for anything to be in a stable orbit of the satellites).

    *I almost put astrological, which would have pissed many people off no end, I’m sure :) But if we were talking about the Earth, I’d have said “geological” timescales. Is there such a thing as “geonomy”? Serious question :)


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