The real Pandora, and two mooning brothers

By Phil Plait | May 6, 2010 11:45 am

Cassini continues making loop-de-loops around Saturn, returning tens of thousands of way cool pictures. Like this one:

cassini_epimetheus_pandora

From 1.3 million kilometers (800,000 miles) away — 3 times as far as the Moon is from the Earth — Cassini spied this pretty scene. It shows, obviously, Saturn’s rings to the right. The very thin ring extending to the left is the F-ring; it’s very faint and wasn’t even discovered until 1979, when Pioneer 11 passed the planet.

The two moons are Pandora (the flying saucer-shaped one) on the left, and Epimetheus on the right. Usually, in pictures like this, perspective is a problem; one moon is much farther away than the other, so your sense of scale gets a bit bollixed. But in this case, both moons are about the same distance from Cassini! Pandora is about 114 x 84 x 63 km (68 x 50 x 38 miles) in size, and Epimetheus is a bit heftier at about 144 x 108 x 98 km (86 x 64 x 58 miles). In this shot, the rings are in the background relative to the moons, and Pandora is just a hair closer to Cassini than Epimetheus.

I was surprised that they appeared so close together, so I did some checking. Pandora orbits Saturn at a distance of about 141,700 km (85,000 miles), and Epimetheus orbits at 151,400 km (91,000 miles). So really, they never get any closer than 10,000 km (6000 miles) to each other. Since they do look pretty cozy in this image, there really is a little perspective going on, since Epimetheus must be a few thousand kilometers farther away. That’s only a trifle compared to the more than 1 million km distance Cassini was from the pair when it took this shot, though. What this means is that if you compare the sizes of the two moons you get a good idea of their relative diameters, but their positions relative to Saturn are a little messed up due to perspective. Got it?

Once I got the orbital distance of the moons, I was curious how long it takes them to orbit Saturn. Turns out, it only takes about 15 hours for Pandora and about 17 hours for Epimetheus! So both moons are screaming around the planet at speeds of roughly 60,000 kph (36,000 mph), far faster than even low Earth satellites move. That’s because Saturn is a lot more massive than the Earth, and has far more gravity. It yanks much harder on those moons, whipping them around at greater speeds.

There’s more, too. Pandora is one of the shepherd moons of the F-ring, helping it maintain its shape. It shares an orbit (more or less) with the moon Prometheus. As it happens, Epimetheus shares an orbit (more or less) with the moon Janus.

Now follow along here: in mythology, Epimetheus and Prometheus were very close brothers. Their names means hindsight and foresight, respectively. Prometheus gave us fire and civilization, and had his liver pecked out by birds every night for his sins against the gods. Epimetheus was supposed to give mankind positive traits, but was a bit of an absent-minded goofball, and he ran out of raw materials before he got to us. For this, the gods gave him the "gift" of Pandora, whom he married.

Well, that’s not fair! Being smart and clever and helpful gets your organs ripped out of you, and being an idiot with no eye for the future gets you rewarded*.

Of course, in reality, Epimetheus’s wife shares an orbit with his brother. That’s gotta hurt.

Sigh. I think I prefer the confusion of the actual Prometheus and Epimetheus to the confusion of their mythical namesakes. Reality may not always be fair, but at least (to borrow a phrase from George Hrab) it’s fair in its unfairness.


* Feel free to extrapolate this myth to science and politics, if you wish.

Related posts:

Watch Saturn’s shadow dancing

Cassini eavesdrops on orbit-swapping moons

Saturnian eclipse

CATEGORIZED UNDER: Astronomy, Pretty pictures

Comments (22)

Links to this Post

  1. joechemistry | October 25, 2011
  1. Agustina Iansilevich

    Actually, Pandora was a punishment for Prometheus and humanity for the stolen fire. Prometheus, been as awesome as he was says “bro, thnks but no LOL” so the impulsive Epimethius as impulsive as it was says “free hot chick, zink!” and go all the way marrying her… well we all know how that ended (sorry for the random chatter, currently reading Aeschylus’ Prometheus Bound and kind hype about it)

  2. IVAN3MAN AT LARGE

    Phil Plait:

    Well, that’s not fair! Being smart and clever and helpful gets your organs ripped out of you, and being an idiot with no eye for the future gets you rewarded.

    Ferengi Rule of Acquisition #15: Acting stupid is often smart.

  3. Well, tricky topic here, Phil, but the surface gravity of Saturn is only about 90% the surface gravity of Earth due to its very low density. Of course, that is at a substantially larger distance from the center of mass than you would get if you were on Earth! You are correct that the force of gravity at the orbital distance of the Moons is much greater than it would be at the same orbital distance on Earth, but most people think about the surface gravity of a planet when you say one has more gravity than another. Just a subtle distinction for you.

  4. DrFlimmer

    Well, well. Obviously human beings did and do not learn from history. They do the same mistakes ever and ever again. ;) (Referring to the passage already quoted by Ivan3man)

    Nice picture btw…

  5. Very fascinating. Lovely graphic.

  6. hale_bopp (#3): Oh, I know. I didn’t want to go into great detail because I was already starting to ramble. :)

  7. Rob

    When I saw the picture I thought it was the Discovery (from 2001 book) orbiting Saturn only the engine and the big sphere visible. :)

  8. XPT

    And still this doesn’t explain why Pandora Boxx didn’t win Rupaul’s Drag Race. HA! ;)

  9. Steve in Dublin

    Phil, any idea where we could get higher resolution versions of this image? I blew it up to 1680 x 1050, but it loses a lot in the translation. Would make a mind-blowing desktop background otherwise.

    I couldn’t find anything useful in this regard on the linked site, only the low res image :-

  10. So much more interesting than the Pandora of James Cameron’s Greatest Motion Picture Ever Made™.

  11. Messier Tidy Upper

    Great photo. :-)

    @ 10. kuhnigget Says:

    So much more interesting than the Pandora of James Cameron’s Greatest Motion Picture Ever Made™.

    Yes, but there’s no jungle or sexy aliens with a sentient planet on this Pandora just a giant lump of ice with some interesting orbital properties. Oh & craters I presume. ;-)

    @6. Phil Plait Says:

    hale_bopp (#3): Oh, I know. I didn’t want to go into great detail because I was already starting to ramble.

    Hey if you can’t ramble on your own blog .. then where *can* you! What else is it for? ;-)

    I, for one, don’t mind if you go into more detail and ramble on a bit.
    FSM, I do that myself often enough. ;-)

  12. Pi-needles

    @3. hale_bopp Says:

    Well, tricky topic here, Phil, but the surface gravity of Saturn is only about 90% the surface gravity of Earth due to its very low density.

    Interesting factoid there – thanks. But can a planet without a proper surface being gas and then high pressure fluid most of the way down really be said to have “surface” gravity at all? ;-)

    Cloudtop gravity mayhaps?

  13. jcm

    I guess this is a good time to get familar with Roman mythology.

  14. Gkopel

    jcm Says: I guess this is a good time to get familar with Roman mythology.
    —————————————————————————————————————

    It’s Greek mythology actualy. Don’t you see the self sarcasm in the myths? :)

  15. Just me

    Kind of related question, and something I’ve been wondering for a long time: how is the rotational period for the gas giants determined? I mean, they all have high winds, storms and such, but I assume that the rotational period is not determined by the cloud movements, right? Any thoughts?

  16. Messier Tidy Upper

    @ ^ Just me :

    but I assume that the rotational period is not determined by the cloud movements, right? Any thoughts?

    I’m not sure & I could well be wrong – please correct me if I am – but I think cloud patterns *are* actually used along with perhaps magnetic fields to judge the gas giants rotation periods.

    Its a good question and pretty hard to work out given these planets are gas and fluid rather than solid bodies.

    I vaguely recall reading somewhere that Saturn’s rotation period has actually changed between the time Voyager II last flew past the planet in 1980-83 ish (?) to when Cassini arrived in 2004 ~ish (?) – although maybe it was just a case of a more precise measurement being made. I’ll have to research this further myself now. :-)

    You might try asking this question on the BAUT forum too :

    http://www.bautforum.com/forumdisplay.php/8-Space-Astronomy-Questions-and-Answers

    if you haven’t done so already.

    Otherwise, if anybody can explain or elaborate further on this it’d be great.

  17. Messier Tidy Upper

    Okay I’ve just asked that question there for you – let’s hope we get some good answers – see :

    http://www.bautforum.com/showthread.php/103799-Measuring-the-days-of-gas-giants?p=1728440#post1728440

    EDIT TO ADD :

    Yes – Wikipedia check confirms that Cassini began orbiting Saturn in 2004.

    Also Voyager II fly past Saturn in 1981 & Voyager I fly by it in 1980.

  18. Andrew Barton

    Surface gravity: I looked up Saturn’s on the web the other day and the first seven sites, all reasonably good educational or academic sources, gave me seven different values ranging from 1.12 to 0.87 times that of Earth. They were all using different definitions of ‘surface’. One was ‘the level at which Saturn’s atmospheric pressure equals that of Earth’ as though that meant anything.

  19. The only thing that would make this more awesome, is if these similar sized moons were orbiting Uranus… I think we all know why… They would be “The Mythical and Epic Twin Moons Of Uranus”.

  20. alboraq

    There’s a heck of a lot going on in that, pic, Phil.

    If you look carefully, there’s four, maybe five glitches in the middle of the three outermost rings – but are they all caused by the two moons, especially the one closest to the viewer just before the three rings appear to merge?

    If you look at the moons themselves, both of them seem to have highly visible craters on their upper bodies, but Epimetheus, (and Pandora, to a degree) appears to have a highly excavated – impact created? – ‘under’ region, adjacent to which is what almost appears to be a mini-me moon, (although viewed in inverted colours it could be a pyramid-esque mountain, but surely that’d be almost as odd?).

  21. Pi-needles

    @19. MichaelL:

    That was cheek-y! ;-)

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