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The Planetary Society Blog
By Emily Lakdawalla
Antiope, a true binary asteroid
Apr. 11, 2007 | 17:48 EDT | 21:48 UTC
This appears to be my week for finally facing some complicated topics that I've been putting off for a while. Sometimes I just post pretty pictures; but I can't get away with that for all interesting stories. Many times, a press release arrives in my mailbox, and after reading it I find that there must be more to the story than they have managed to put in the release, especially if the release is tied to the publication of an article in a scholarly journal. Unless the journal is one of the ones I subscribe to (and that's a short list: Science, Nature, Eos, and the Journal of Geophysical Research - Planets), I have to send a request to one of the authors for the article, and if they're kind enough to send it I have to read the article, after which I'm usually befuddled and need to email some questions and request digital copies of figures, and when all is said and done the process may take weeks. I'm not fast, but I try to be thorough.
I've just managed to work my way through an interesting paper published recently in Icarus by Pascal Descamps, Franck Marchis, and a host of coauthors about a truly binary body, asteroid (90) Antiope. We toss around the term "double planet" for Pluto because Charon is quite large at half Pluto's diameter, but the Pluto-Charon system has nothing on Antiope. Antiope is a binary asteroid with two components that are almost identical in size (91 and 86 kilometers in diameter). What's more, the two components dance around each other at a distance of 171 kilometers. Now, think about that for a moment. When astronomers discuss the distances between bodies, they don't ordinarily have to specify, because the distinction is usually a trivial one, but they are generally referring to the distances between their centers. But how far apart are two bodies 90 kilometers in diameter, separated by twice that? That's right: the two components of Antiope are separate bodies, but they are separated by a distance that is less than the diameter of each component. Here's a diagram: The Antiope system
This diagram shows the two components to scale at their maximum separation from each other. Credit: Courtesy of Franck Marchis |
For comparison, Pluto and Charon are separated by a distance that's about eight times Pluto's diameter.
In order to determine these orbital characteristics, Descamps and Marchis needed to use the Adaptive Optics-equipped Very Large Telescope. The VLT has high enough resolution that it can separate the light from Antiope into two points of light, which is pretty amazing. However, the resolution is not good enough to resolve Antiope's components as separate disks.Very Large Telescope views the double asteroid (90) AntiopeThe Adaptive Optics-equipped Very Large Telescope was able to separate the two closely orbiting components of asteroid (90) Antiope in observations conducted in 2004. These observations enabled the precise determination of the orbital period and the distance separating the two similar-sized bodies: 16.5 hours and 171 kilometers (106 miles), respectively. Credit: © ESO |
Fortunately, the two components went through a period in late 2005 when they transited each other as seen from Earth. With the help of many, many light curves provided by a pile of amateur astronomers -- some of whom were only using 18-centimeter telescopes, which is really a fairly ordinary size for serious amateurs -- Descamps and Marchis were able to determine the shapes of the two components to fairly high precision. And those shapes were surprising: the bodies were nearly spherical, just slightly squashed into ellipsoids. To explain why this is surprising, consider a couple of similarly-sized bodies:Ida (56 by 21 kilometers)Credit: NASA / JPL | Pandora (110 by 88 kilometers)
Credit: NASA / JPL / Space Science Institute / Justin Phillips |
What makes things spherical? Self-gravity. The problem here is that bodies this small generally do not have enough self-gravity to overcome the frictional forces of their component parts rubbing up against each other. Very large bodies are helped by the fact that when they form, they heat up, making their interiors fluid. Very small bodies don't get this help, and most of them probably never melted; instead, they're loosely collected aggregates of material, usually referred to as "rubble piles." Curious about the roundness of Antiope's components, Descamps and Marchis tried out an interesting hypothesis: suppose that they were totally fluid. What would their shapes be, based on the inputs of their size, mass, their self-gravity, spin, and the gravitational forces they exert on each other? They produced a mathematical model of the shapes, and then simulated what Antiope's light curve should be if the components had that idealized shape, and lo and behold, the simulated light curves matched the observed ones quite well. Although this does not constitute proof that Antiope's components were once, or are now, fluid, it's either a surprising coincidence or a clue to the puzzling history of this object.
So, as usual, we have a scientific publication that creates as many questions as it answers. The new answers are the precise determination of the orbital characteristics, sizes, and shapes of the two components of Antiope. The new questions are how this system could possibly have formed in the first place, and how its tiny components have managed to acquire the shapes you would expect if their interiors were fluid. This kind of situation is what gives scientists job security. No matter what they find out, they always end up with more questions!
I've written up the facts about Antiope from the Icarus paper in a new page in our Asteroids and Comets section.
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