Triple Asteroid System

Discussion in 'Astronomy, Exobiology, & Cosmology' started by blobrana, Aug 9, 2005.

  1. blobrana Registered Senior Member

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    Astronomer Franck Marchis (University of California, Berkeley, USA) and his colleagues at the Observatoire de Paris (France) have discovered the first triple asteroid system.

    Asteroid <b>87 Sylvia</b> has two small asteroids orbiting it.


    "<i>Since double asteroids seem to be common, people have been looking for multiple asteroid systems for a long time. I couldn't believe we found one</i>." - Franck Marchis.

    The discovery was made with Yepun, one of ESO's 8.2-m telescopes of the Very Large Telescope Array at Cerro Paranal (Chile), using the adaptive optics NACO instrument.
    The asteroid was already known to be double since 2001, from observations made by Mike Brown and Jean-Luc Margot with the Keck telescope.
    The astronomers used NACO to observe Sylvia on 27 occasions, over a two-month period. On each of the images, the known small companion was seen, allowing Marchis and his French colleagues to precisely compute its orbit. But on 12 of the images, the astronomers also found a closer and smaller companion.

    Because 87 Sylvia was named after Rhea Sylvia, the mythical mother of the founders of Rome, Marchis proposed naming the twin moons after those founders: Romulus and Remus. The International Astronomical Union has seemingly approved the names.

    Sylvia's moons orbiting in nearly circular orbits and in the same plane and direction. The closest and newly discovered moonlet, orbiting about 710 km from Sylvia, is <b>Remus</b>, a body only 7 km across and orbits with a period of 33 hours. The second, <b>Romulus</b>, orbits at about 1360 km in 87.6 hours and measures about 18 km across.

    87 Sylvia is one of the larger asteroids in the asteroid belt, and is located about 3.5 times further away from the Sun than the Earth, between the orbits of Mars and Jupiter.
    NACO images show that 87 Sylvia is shaped like a lumpy potato, measuring 380 x 260 x 230 km and spinning at a rapid rate, once every 5 hours and 11 minutes.

    The observations of the moonlets' orbits allow the astronomers to precisely calculate the mass and density of Sylvia. With a density only 20% higher than the density of water, it is likely composed of water ice and rubble from a primordial asteroid.
    "<i>It could be up to 60 percent empty space</i>" - Daniel Hestroffer, co-discoverer (Observatoire de Paris, France).

    These asteroids are loose aggregations of rock, presumably the result of a collision. Two asteroids smacked into each other and got disrupted.
    "<i>It is most probably a "rubble-pile" asteroid. The new rubble-pile asteroid formed later by accumulation of large fragments while the moonlets are probably debris left over from the collision that were captured by the newly formed asteroid and eventually settled into orbits around it. Because of the way they form, we expect to see more multiple asteroid systems like this</i>" - Franck Marchis.

    Marchis and his colleagues will report their discovery in the August 11 issue of the journal Nature, simultaneously with an announcement that day at the Asteroid Comet Meteor conference in Armação dos Búzios, Rio de Janeiro state, Brazil.

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    <pre>
    <b>Orbital data, primary (osculating elements) [JPL]:</b>
    semimajor axis a: 3.488913506 AU
    orbital period P: 6.5169 y (=2380.3 d)
    eccentricity e: 0.079392375
    perihelion distance q: 3.211920378 AU
    aphelion distance Q: 3.76590664 AU
    inclination to ecliptic i: 10.8570621°
    argument of perihelion omega: 266.4476418°
    ascending node OMEGA: 73.3517081°
    mean anomaly M: 292.040254°
    perihelion passage: 01 SEP 2004
    Epoch: 10 JUN 2003
    data arc: 1894 to 2002 (786 obs.)

    <b>Orbital data, secondary:</b>
    semimajor axis a: 1356 ± 5 km [MAc]
    semimajor axis/primary radius a/Rp: (10.5)
    orbital period P: 3.6496 ± 0.0007 d [MAc]
    eccentricity e: 0.001 ± 0.001 [MAc]
    inclination i: 1.7° ± 1° [MAc]:</pre>
     
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  3. superluminal I am MalcomR Valued Senior Member

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    Cool. Must have happened in the fairly recent past. I can't imagine such a loosely bound system to be very stable over long periods, especially situated between Mars and Jupiter.
     
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  5. blobrana Registered Senior Member

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    Hum,
    I’m not sure about that...
    looking at the individual orbits and they do seem to be quite similar - something that may be a fluke - but i imaging that they have been like for many millions of years - if not longer...

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    <sub>Image credit: F. Marchis </sub>

    The orbits of both moons are traced in this composite image taken over nine nights of observations with the Very Large Telescope in Chile

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    <sub>Image credit: F. Marchis </sub>

    A small, faint moon named Remus can just be seen around the large asteroid Sylvia, along with its larger moon, Romulus.
     
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  7. superluminal I am MalcomR Valued Senior Member

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    Millions of years? A mere eyeblink in astronomical terms, right? What I meant was the relatively recent past (tens of millions of years?). Anyway, very nifty pictures!
     

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