First near-Earth triple asteroid found

By Phil Plait | February 14, 2008 11:30 am

Another cool bit of astronomy news: astronomers have discovered the first triple asteroid known to get near the Earth. Asteroid 2001 SN263 was thought to be a typical single body that has an orbit that sometimes brings it near us. This made it a target for mapping using the giant 300 meter Arecibo radio telescope in Puerto Rico. They can actually get three-dimensional maps of asteroid using the dish. By sending pulses of what is essentially radar at an asteroid and timing how long it takes the pulses to reach the asteroid and return to Earth, a rough map of the rock can be made — a pulse that takes slightly longer to get there and back means you’ve bounced it off a part of the asteroid that is slightly farther away. Think of the astronomers as bats, the telescope as the bat’s sonar, and the asteroid as a mosquito, and you’ll get the idea.

When astronomers took a shot at SN263, they got a big surprise: they got three separate returns, indicating that a big rock is being orbited by two smaller ones. The main mass is roughly spherical and 2 kilometers across. The smaller moons are about half that size. The observations were made when SN263 was about 11 million kilometers from Earth.

Other triplet asteroids are known (like 87 Sylvia), but this is the first one known that gets near Earth. This is a pretty interesting find. The orbits of the moonlets will allow the densities of the objects to be determined, which can yield insight into the formation mechanism. One possible formation explanation is a slow collision with another asteroid causing the main body to fracture. Or did the system form all at once? The densities of the three bodies can support or rule out different mechanisms (for example, a very low density main mass is hard to fracture).

Plus, the stability of the system needs to be understood; if the orbits are unstable, that means it formed this way recently (well, astronomically recently). Did this object form in the asteroid belt and get sent inward by some gravitational encounter? Or did it form in its current orbit?

All of these questions are interesting scientifically, but they have real-world implications as well. The more we know about near-Earth asteroids, the better! If you disagree, I suggest going to your local natural history museum and checking out the dinosaur bones. Look around. See how all the dinosaurs on display are dead, and no live ones are around?

Yeah. They didn’t know anything about asteroids. But we do, and we need to learn more.


Comments (26)

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  1. Astronomy Pictures - Images of moon | February 14, 2008
  1. Blinky

    Ii was always told those things chirping in the trees were dinosaurs!

    Nevertheless, cool info about the asteroid.

  2. Yojimbo

    OMG! We’re gonna get hit THREE TIMES! Its the end of the world as we know it….

    or not.

  3. Pat

    I’m wondering – has anybody developed high fidelity models of various impact scenarios? I mean, ones that might have worldwide import, apart from extrapolating on old nuclear blast data?

    I’m talking ones that would shake the crust: what would that much deformation do? Would it propagate in a kind of earthen tidal wave? Would it ring the earth like a bell? Did we get any idea from the comet that struck Jupiter, even though it didn’t have a solid surface?

  4. Michael H

    The image looks like a visual image highlighted from above. The raw data is from radar reflections so the image must be computer generated right? So why is the image not illuminated from the front? Can anyone out there explain how this is done it in simple terms?

  5. Jarrod Henry

    OH NO!

    That’s only 44 times the distance from Earth as TU24, and we know what THAT did to us..

    Sorry. Had to :)

  6. Jarrod Henry

    Gasp! I mean 22 times. Not 44! It’s half as far away as I thought!

    (Stupid typos :P)

  7. Blinky

    Michael H-

    It’s not illuminated from the front because they used accidentally used the wrong lighting in the secret soundstage in Area 51.

  8. Leon

    See how all the dinosaurs on display are dead, and no live ones are around?

    Point well taken of course, but on a technical note: I understand recent research suggests the dinosaurs actually weren’t wiped out by the effects of the impacting body, that the bulk of the extinctions occurred several million years later. The Earth was undergoing a substantial amount of volcanic activity, which was putting a strain on life for a long time. The asteroid collision was merely one of the environmental hardships our scaly/feathery friends faced during that period.

  9. Yoshi_3up

    I’m just gonna sit back and wait until starts making dumb claims about it.

    You think they got enough after TU24? Now they’re talking about another one which, even if it is 0.300 AU from Earth, it’s going to cause wreak havoc. (“The magnetic field is being pushed from INWARDS?!”).

    Fear not. It hit it’s closest point 2 days ago.

    By the way, it’s 2 KM in diameter, and ONLY 11 million KM away. I’m starting to get a little scared.


  10. My personal theory about the extinction of the dinosaurs is that God was punishing them for legalizing gay marriage. Of course, I have absolutely no evidence to support this theory, but lack of evidence has never stopped the intelligent crowd, so I’m sticking to my story.

  11. Leon

    Or maybe it was because the One-World Dinosaur Government (DinoGov for short) had legalized dinosaur abortions. Most of the dinosaur egg fossils we’ve found were broken open, haven’t they? Proof positive!

  12. Mark Martin

    The reason it’s illuminated that way is because they hired a professional photographer to consult on some slick looking three-point lighting. The only difference is that it was lit with radar-floodlamps. But they were HUMONGOUS lamps, the size of whole asteroids and positioned at just the right spots about the solar system to yield the rim lighting you see in the pic.

  13. Blinky
  14. Think of the astronomers as bats,

    Just go to


  15. You think they got enough after TU24? Now they’re talking about another one which, even if it is 0.300 AU from Earth, it’s going to cause wreak havoc. (”The magnetic field is being pushed from INWARDS?!”).

    I’m surprised that missed out on the fact that a large, stellar mass object gets to within less than 1 AU of the earth EVERY YEAR. Radiation from this body has been linked to cancers on Earth. Gravitational effects from the proximity of this body are believed to higher tides every couple weeks. Plasma discharges from the object are may be linked to auroral displays, as well as power grid and radio blackouts. There may be evidence that infrared radiation from this object causes convective air currents that give rise to small and large-scale storms. The object has been directly linked to the death of a man during a prison break in ancient Greece.

    Be warned!

  16. Buzz Parsec

    Squid – If you think that’s bad news, get a load of Oxygen Dihydride.

    It’s an isomer of Dihydrogen Monoxide, but with even more pernicious side effects. In addition to the well-known health effects, it can solidify into dangerous spear-like crystals at temperatures and pressures commonly found on the Earth, crystals that have been known to fall on people and kill them (as well as causing temporary blindness by fracturing personal optical systems.) It is also capable of disrupting computer systems. Even Mac O/S X is not immune; in this very post, it has caused Safari’s spell-checker to malfunction, calling out both “Dihydride” and “Dihydrogen” as errors.

  17. autumn

    Idle question, but at what mass does a rocky body collapse to a spheroid, and what would the approximate diameter be?

  18. Ronn!

    For autumn: IIRC typically something larger than about 400 km across will be pretty much spherical, depending on its composition. Which means that 2001 SN263 is obviously artificial . . . 😉

  19. RickJ

    I’ve posted a shot of it taken Feb. 15, 2008 at 1:47 to 2:10 UTC on the forum at:

    This one is well positioned in Gemini. While it will be a tiny bit closer in a week the difference is very small so now is a good a time to look for it. Motion should be easy to see as it is moving at a tad over 8″ of arc per minute to the southeast. By the 26th it will be near the head of Hydra so both hemispheres have a good look at this guy. It visible in a 6″ or larger scope.


  20. Wouter Lievens

    How far apart are the sattelite asteroids from the main body? I haven’t found that info anywhere.

  21. Mark S.

    Does anyone know the orbital period of its satellites? I would imagine that they couldn’t be anywhere near the same distance from the main mass.

    I’m wondering how big in the sky they would look if you were sitting on the big chunk, and how often they would pass over…

  22. Allen Thomson

    The technique used to produce the image is a form of inverse synthetic aperture radar (ISAR), also called range-doppler and delay-doppler radar.


  23. StevoR

    Thanks for that news BA – just a couple of minor quibbles :

    “The more we know about near-Earth asteroids, the better! If you disagree, I suggest going to your local natural history museum … ”

    Unless your unluckly enough that your local “Natural History Museum” happens to be a creationist one in which case you’ll get badly MIS-informed that the Dinos were around withAdam & Eve & got killed in Noah’s flood! :- (

    ” … and checking out the dinosaur bones. Look around. See how all the dinosaurs on display are dead, and no live ones are around?”

    Uh … Birds? 😉


    NB. Haven’t read the comments yet – gotta fly – sorry if these pts made already …

  24. blf

    Think of the astronomers as bats

    I’m now sure the time I see a bat I’ll be thinking “There’s The BAt!”

  25. bozzle

    This is probably a stupid question – prepare yourselves: Do the two smaller bodies need to be in orbit around the larger rock? Is it possible that all three are traveling as a cluster with a stationary formation?


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