Kleopatra and her kids

By Phil Plait | March 26, 2011 7:00 am

One of my favorite asteroids is Kleopatra: a big, 217 km (135 mile) long main-belt rock that’s a wee bit weird. This image may give you a hint as to why:

It’s shaped like a cartoon dog bone! It circles the Sun out past Mars, tumbling end-over-end, and its origins have always been something of a mystery. However, new observations and analysis reveal quite a bit about how this asteroid got its unusual shape. I won’t spoil it, but instead simply point you to Emily Lakdawalla’s excellent summary of Kleopatra on The Planetary Society blog. It’s a tale of collisions, spin, and eventual reconciliation, as many good stories are.

One thing I didn’t know is that Kleo has two moons: Alexhelios and Cleoselene. They orbit the asteroid in the plane of the its rotation, and may be cast-offs from the formation of Kleo itself. Read Emily’s article for the whole scoop.

Man, the solar system is a cool place. And there’s still so much left to see!

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (47)

  1. Cindy

    While I appreciate the new posts, Phil, you’re on vacation in a nice warm place. I hope this post was prepared ahead of time and you’re not taking time away from your relaxation with your family.

    Next time have guest bloggers. Maybe Adam Savage or Wil Wheaton could step in. Or have a contest for your followers (except not on Twitter).

  2. Joel

    Maybe Pluto is chasing after it?

    I shouldn’t joke. Astronomy is a Sirius business.

  3. Hephaestus

    Stanley Kubrick was right on target. There are tapir femurs in space.

  4. Dennis

    “…metallic, it must consist of almost 50% empty space…”

    That’s no asteroid!

  5. DrFlimmer

    I was going to say something about a dog-asteroid, but not did #3 Joel already do it, he also made a much better joke! 😀

  6. JJA

    It’s just the Satellite of Love. You should really just relax.

  7. Messier Tidy Upper

    Is this your tribute to the late Elizabeth Taylor BA? 😉

    @ 3. Joel : LOL. 😀

  8. I Canis get that doggie bone shape description out of head, thanks Phil.

  9. Kaeli

    Even in space, there’s no love for Caesarion. Poor kid :)

  10. Bobito

    You are all so humerus.

  11. Robert S-R

    Huh, a real, bone-ifide asteroid!

  12. Wzrd1

    The orbital dynamics of those two moons must be murderous to compute!

  13. db26

    It is a bone! They haven’t been spotted yet, but don’t discount space dinosaurs. Their fossils are floating around all over the universe.

  14. Kleopatra’s shape reminds me of the asteroid Kali in Arthur C. Clarke’s novel “The Hammer of God”: http://en.wikipedia.org/wiki/The_Hammer_of_God

  15. Diederick

    I’m living in a cool place. Excellent.

  16. Gendels Dad

    Holy Cthulu’s shinbone! That’s pretty cool!

  17. Michael Swanson

    “Man, the solar system is a cool place. And there’s still so much left to see!” – Phil

    I’m always astounded when I recall my understanding of science as a child, informed mainly buy seriously out-of-date books in my house and the common knowledge of family members (which was commonly very wrong) compared with the tip of the iceberg that I have easy access to today. When I was a kid, the solar system was very cool, but I was sure there wasn’t that much more to find. Nine planets, a bunch “boring” asteroids, and some comets. A hundred years of rocketing around the solar system, and we would have ferreted out all of its secrets! It’s marvelous to know how wrong I was, how much is out there, much of it undiscovered, and to know that humankind will be busy with the solar system, not to mention the rest of the universe and all other sciences, for a very, very, very long time.

    Although I’m getting tired of physics and quantum physics in particular. They keep discovering new things that are more weird and incomprehensible than the last thing they discovered. I’m stopping at quarks. I’m done. I’m behind the times, and I’m staying there! No leptons, muons, or anything up, down, half-spun or strange! Heck, I might just go back to the four elements and the ether. It’s easier on my tired old brain. :)

  18. Melusine

    Wow, dog-bones galore! ((;

    There is still so much to see. Off-topic: I made a video this morning (only my second try) using Saturn and its moons’ images to a favorite Frederic Chopin nocturne piano piece, and it’s incredible that we’ve discovered SO FAR 62 of its moons. We’re pretty moonless in comparison. The universe is like music…there’s just more and more to create and find. (Thanks, BA)

    If anyone likes classical music and/or Saturn, here’s my video: http://www.youtube.com/watch?v=Lk_u1bo_7fI

  19. Keith Bowden

    Now Phil, this is a family site… :)

  20. Andrew W

    “doggie bone shape”

    People, have some respect, it’s Kleopatra! We use the term “hour glass”!

  21. Pete Jackson

    Your picture of the ‘bone’ in a number of orientations immediately brought to mind the bone tossed in the air in the opening sequences of “2001 A Space Odyssey”, that morphed into the Space Station!

  22. 3. Joel Says: “Maybe Pluto is chasing after it? I shouldn’t joke. Astronomy is a Sirius business.”

    Joel, as JJA @7 alluded to, we know you’re just trying to keep your sanity with the help of your robot friends, but these were pretty good!

    – Jack

  23. Messier Tidy Upper

    I find it amazing that objects as small as asteroids with so little gravity can hold onto moons – even moonlets like this.

    One of the comments – by BobMarsian #6 – on Emily Lakdawalla’s blog page linked here (hope its not breaching netiquette to mention it in this – my apologies if so) noted that there are no less than *five* (!) main belt asteroids with two moons :

    1) 216 Kleopatra
    2) 87 Sylvia (the only other “2-mooner asteroid” I was aware of previously)
    3) 45 Eugenia
    4) 3749 Balam &
    5) 93 Minerva

    On researching online further, I find here :


    that :

    As of October 2009, there are 180 minor planets known to have moons

    Wow. 😮 8)

    That’s a lot more than I would’ve thought – although mind you its also counting the moons of ice dwarf type planets like Pluto, Eris and Haumea which I’d call planetary moons rather than asteroidal ones. Still, impressive.

    I wonder what the smallest object to boast a natural satellite is and how big it and its moon are?

    Does anyone know and care to enlighten us, please?

  24. Messier Tidy Upper

    Looks like we can add 1994 CC :


    to that list as well – and a sixth “double mooner” asteroid. :-)

    See :


    for the wiki-list of them. (Via scrolling down from the earlier link above.)



    For another source listing asteroid moonlets.

    Still trying to work out which one is the smallest moon-holder. :-)

    (Sadly, many of the smaller wikipedia entries seem to have been deleted. :-( )

    Maybe 2003 SS-84 as that’s looking the best contender so far? :-)

  25. DrBB

    @20 Melusine, re: Saturn’s many moons, “We’re pretty moonless in comparison.”

    So why is that, actually? Even Mars, a much smaller planet, has two (albeit itty-bitty ones) and as Messier Tidy Upper points out over on Emily Lakdawalla’s thread there are 180 minor planets known to have moons. As I write this I realize that we inner planets all get short shrift moon-wise, the other guys inward of us having none, and unlike all those gas-giant moons (and Mars’s?), ours is most likely the result of an apocalyptic collision rather than capture. Leading me to guess/conclude it’s because we’re too far from the asteroid belt… though it’s kind of an unsatisfying explanation, since there seems to be a decent number of earth orbit-crossing asteroids and over the millenia you’d think we might have captured a few. Anyone know what the actual science is on this?

  26. Messier Tidy Upper

    @ 28. DrBB : Capturing an asteroid is pretty hard for a planet with relatively low gravity such as Earth or so I gather.

    Recall that spaceprobes entering orbit around other planets have to burn their rockets and slow down significantly in order for this to happen and asteroids and comets, of course, lack rockets (& motive) to do so. So something else has to slow them down.

    I also understand that closeness to the Sun reduces the chances of worlds like Venus and Mercury as the Solar gravity field tends to “out-compete” them and make it harder for them to capture moons into orbit – although I could be mistaken on this.

  27. Gunnar


    I am sure that you know more about celestial mechanics than I do, but it is my understanding (and I am sure yours too) that any object approaching earth (unless it was already orbiting the earth), and not on a collision course with it, would certainly be moving faster than escape velocity at its closest approach to earth, so it would be extremely unlikely to be gravitationally captured by the earth unless something happened to slow it down (like perhaps being slowed by passing close enough to graze our atmosphere, or by an unusually strong gust of solar wind due to an incredibly huge solar flare, or actually colliding with some other body). This would be true even without the Solar gravity field “out-competing” earth’s gravity field.

    But then, you already covered that in your second paragraph, so never mind! 😉

  28. Gary Ansorge

    SO, if you were standing on one end of the bone, what angular acceleration would you experience from its rotation?

    Gary 7

  29. DrBB

    @28 Thanks for the sensible explanation. So I guess our diminutive outer neighbor, Mars, just got lucky? From the animation on the Wikipedia page it looks like it’s greater proximity to Jupiter assisted–that Jupiter slowed these bodies down without capturing them itself, so that they fell inward and were snagged by Mars’s gravity well.


  30. Gary Ansorge

    From the animation, Kleopatra appears to rotate 15 times for every one revolution of its moon, Alexhelios, which would give it a rotation rate of about 4 hours per rotation. With a density of 3.6 gms/cc, the implication is that it is a metallic, loosely aggregated rubble pile. I would expect it to disintegrate from its rotational rate.

    Naw, it’s a generation ship and it’s two satellites are atmospheric entry vehicles. OMG, it’s the Battle Star Galactica,,,

    Gary 7

  31. Gary Ansorge

    I’ll note it’s actual length; “Its spatial extent is about 271 x 65 km”

    That’s 271 km, not 217.(Oops! Dyslexia.)

    Gary 7
    PS We really need to send a probe to this.

  32. Bryan Feir

    @30 Messier Tidier Upper:

    I have a vague recollection of an old Isaac Asimov essay where one of the things he did was calculate differential gravitational ratios for planetary moons. I believe one of the results was that Earth’s Moon was actually unique in the solar system: the only moon for which the Sun’s gravitational pull was greater than the planet’s. So your comment about the Sun’s gravity out-competing the planetary gravity may have some truth in that. Granted, we’ve discovered a LOT of other moons since that essay was written…

    (It’s worth noting that for this calculation, the mass of the moon itself is completely irrelevant: the only relevant values are solar mass, planetary mass, mean solar distance, and mean planetary distance.)

  33. Messier Tidy Upper

    @ ^ Bryan Feir : That could be in Asimov’s ‘The Tragedy of the Moon’ or ‘Asimov on Astronomy’ essay collections – or another of his many ones. Isaac actually wrote more science non-fiction than fiction. :-)

    Let’s see now [checks bookshelf .. flips through Asimov collection] Aha! 😀

    The essay you refer to there is Chapter 9 ‘Just Mooning Around’ in Asimov on Astronomy ( Coronet, 1974.) where Asimov comes up with a “tug-of-war’ figure for various planets holding the moons against the pull of our Sun including this pertinant quote :

    “Thus you see, where each of the outer planets has a range of two million miles or more within which true satellites could form, the situation is far more restricted for the inner planets. Mars and Venus have a permissable range of but 10,000 miles. Erath does a little better, with 20,000 miles. Mercury is the most interesting case. The maximum distance at which it can expect to form a natural satellite against the overwhelming competition of the nearby Sun is well within the Roche limit. It follows from that, if myreasoning is correct, that Mercury cannot have a true satellite and that anything more than a possible spattering of gravel is not to be expected.

    [Italics original.]

    – Page 136, Isaac Asimov, ‘Asimov on Astronomy’1974.

    @33. DrBB : No worries! My pleasure. :-)

  34. Melusine

    @ DrBB #28

    A short explanation of why Jupiter and Saturn have so many moons:

    I can see why the asteroid belt location and size of Saturn and Jupiter would factor in the reason, but as far as asteroids that come closer to us, I don’t know why we haven’t captured any. I read that Saturn’s moon Phoebe is thought to be a captured asteroid. Maybe that’s a question for Phil…why hasn’t Earth captured an asteroid, a small one at least.

    At least we have one. Poor Mercury and Venus are lonely. (;

  35. Bryan Feir

    @37 Messier Tidier Upper:
    Thanks for the research: most of my Asimov’s stuff is with my father, who was the one who actually bought it. And yes, I’m quite familiar with Asimov’s non-fiction output; in fact, I recall hearing about a somewhat joking ‘Asimov-Clarke treaty’ in which Asimov agreed that Clarke was a more thorough fiction writer in return for Clarke agreeing that Asimov did better non-fiction.

    @38 Melusine:
    Well, if you don’t count things like Cruithne (which doesn’t actually orbit the Earth, but its orbit around the Sun is pretty much synchronized with the Earth’s), I suspect the main reason Earth doesn’t have any captured asteroids that we’ve noticed is the competition from the Moon. A small body would have to be in a fairly precise orbit to dance around the Earth-Moon system without being thrown back out again by the perturbations in its orbit.

    (after poking around Wikipedia) Hunh, I hadn’t realized that folks up here in Toronto were involved in mapping Cruithne’s orbit; I think I’ve even met Paul Wiegert back when I was working at CRESTech. You learn something new every day.

  36. Gunnar

    I still think that an important reason why the earth hasn’t captured any asteroids (unless you count Cruithne) is the extreme unlikelihood that any object approaching the earth from deep space and not on a collision course with it would have a relative velocity less than earth’s escape velocity at its closest approach to the earth, unless some other object or phenomenon interfered with it to slow it down below escape velocity. There are obviously a few satellites in the solar system that were later captured by their primary rather than formed from material left over from the initial cloud out which the primary formed (such as Neptune’s moon, Triton), but it is my understanding that these are comparatively rare flukes caused or aided by additional influences other than just the gravitational dynamics of the primary and the captured satellite alone.

  37. Gunnar

    @#36, Brian Feir

    I remember reading that article by Asimov too, and was quite surprised and intrigued by his pointing out the fact that the Moon’s trajectory relative to the Sun is always concave towards the Sun even when the Moon is between the earth and the Sun. Our planet’s gravitational influence on the Moon is only enough to slightly flatten that trajectory with respect to the Sun during that portion of its orbit. I suppose it could be argued that the Earth/Moon system is really a double planet with the Sun as the actual primary for both of them.

    I have read a couple or three science fiction stories that featured a double planet system with both planets nearly equal in size to each other and their rotational periods gravitaionally locked to their mutual orbital period, and both of them life-bearing with sentient beings and civilizations. Imagine how it would be to grow up in such a place!

  38. Messier Tidy Upper

    @ ^ Gunnar : Asimov also noted the “double planet” aspect of Earth & Moon in that essay referred to earlier. (#37)

    The closest thing that our solar system has to a double planet is Pluto – Charon has ½ Pluto’s size & 1/10th of its mass.

    But if you reject Pluto as a dwarf planet (which btw. I don’t!) then Earth is the next best thing as our Moon has ¼ Earth’s size & 1/80th of the Earth’s mass, a ratio much closer than any other the other planet: moon combos. :-)

    Double planets are likely to exist in other planetary systems orbiting other stars and there are several good SF novels featuring them notably Robert Forward’s Rocheworld series. :-)

    Sadly, the IAU anti-Pluto ruling and definition would seem to make them impossible rendering both worlds – regardless of size and mass – into “dwarf planets” since they hadn’t cleared each other from their orbit. Further evidence of how silly and wrong the IAU got it.

  39. CB

    Sadly, the IAU anti-Pluto ruling and definition would seem to make them impossible rendering both worlds – regardless of size and mass – into “dwarf planets” since they hadn’t cleared each other from their orbit. Further evidence of how silly and wrong the IAU got it.

    More like further evidence of how wrong your characterization of the IAU decision is.

    Gravitational capture is a valid way to clear an object from the orbit. That’s why you don’t count direct satellites against an object when calculating the Planetary Discriminant. Mutual satellites as in a binary, having “captured” each other, would not have their partners count against them.

    But keep up with the crusade trying to save Pluto with inaccurate smears against the IAU definition. Whatever you say, it won’t change that there’s 6 orders of magnitude between Pluto and any planet in terms of orbit-clearing. Anyone can recognize this. And in fifty years, our children and their children will think it’s silly to classify Pluto as a planet when it’s clearly just a belt object. You’re on the wrong side of history of this one.

  40. DrBB

    @38. Melusine: “as far as asteroids that come closer to us, I don’t know why we haven’t captured any.”

    Someone in today’s (3/31) Mercury pix thread points out that it’s actually a lot trickier to place an object in orbit around a body further down the gravity well than one further out. As you move in you gain huge amounts of velocity, so a fly-by is easier than getting the planet–Mercury in this case–to capture you even when that’s what you’re *trying* to do. So I expect the same applies here, since the asteroid belt is way farther out from us. Just a thought.

    Thanks for the link–I’ll check it out.

  41. Gunnar

    @#42, MTU,

    Yes, you’re right. Asimov did indeed note the “double planet” aspect of the Earth/Moon system. I should have clarified that that was how I got the idea in the first place.

    I may not yet have read Robert Forward’s Rocheworld series, though it seems to ring a bell in my mind. I’ll have to look that up. Even if it turns out that I have already read it, I will surely enjoy reading it again. Thanks for the reference!

  42. I know, let’s call the next space vehicle Fido. ”Here Fido!”


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