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RE: Asteroid 87 Sylvia
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Combining observations from the worlds largest telescopes with small telescopes used by amateur astronomers, a team of astronomers discovered that the large main-belt asteroid (87) Sylvia has a complex interior, probably linked to the way the multiple system was formed. The findings are being revealed today at the 45th annual Division of Planetary Sciences meeting in Denver, Colorado.
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Telescopes Large and Small Team Up to Study Triple Asteroid

Combined observations have shown that an asteroid called (87) Sylvia, orbiting in the main belt between Mars and Jupiter, has a complex interior. The object is a triple asteroid because it has two moons and their structure is probably linked to the way this little family of rocks was formed.
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Asteroid 87 Sylvia was discovered by N. R. Pogson on May 16, 1866 from Madras (Chennai), India.
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Title: Instability zones for satellites of asteroids. The example of the (87) Sylvia system
Authors: Julien Frouard, Audrey Compère

We study the stability of the (87) Sylvia system and of the neighbourhood of its two satellites. We use numerical integrations considering the non-sphericity of Sylvia, as well as the mutual perturbation of the satellites and the solar perturbation. Two numerical models have been used, which describe respectively the short and long-term evolution of the system. We show that the actual system is in a deeply stable zone, but surrounded by both fast and secular chaotic regions due to resonances. We then investigate how tidal and BYORP effects modify the location of the system over time with respect to the instability zones. Finally, we briefly generalize this study to other known triple systems and to satellites of asteroids in general, and discuss about their distance from mean-motion and evection resonances.

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Title: On the Stability of the Satellites of Asteroid 87 Sylvia
Authors: O. C. Winter, L.A.G. Boldrin, E. Vieira Neto, R. Vieira Martins, S.M. Giuliatti Winter, R. S. Gomes, F. Marchis, P. Descamps

The triple asteroidal system (87) Sylvia is composed of a 280-km primary and two small moonlets named Romulus and Remus (Marchis et al 2005). Sylvia is located in the main asteroid belt. The satellites are in nearly equatorial circular orbits around the primary. In the present work we study the stability of the satellites Romulus and Remus, in order to identify the effects and the contribution of each perturber. The results from the 3-body problem, Sylvia-Romulus-Remus, show no significant variation of their orbital elements. However, the inclinations of the satellites present a long period evolution, when the Sun is included in the system. Such amplitude is amplified when Jupiter is included. An analysis of these results show that Romulus and Remus are librating in a secular resonance and their longitude of the nodes are locked to each other. The satellites get caught in an evection resonance with Jupiter. However, the orbital evolutions of the satellites became completely stable when the oblateness of Sylvia is included in the simulations.

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Computer animation of the asteroid system - "87 Slyvia" by ESO.
Music: Pachelbel, Canon in D

[youtube=http://youtube.com/watch?v=uyRVxUxESrQ]



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The magnitude of (87) Sylvia will show a drop of 5% in its light curve due to mutual occultation, eclipse events, which would be detectable with amateur photometry.
The next mutual event will occur in summer 2008.

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This is a link to a composite image showing the positions of Remus and Romulus around 87 Sylvia on 9 different nights as seen on NACO images.
It clearly reveals the orbits of the two moonlets.

Image of the Orbits of Twin Moonlets around 87 Sylvia 800 x 1032 - 350k

The inset shows the potato shape of 87 Sylvia. The field of view is 2 arcsec. North is up and East is left.


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The discovery was made with one of the European Southern Observatory's 8-meter telescopes (Yepun) of the Very Large Telescope at Cerro Paranal, using the telescope's infrared camera and the high angular resolution provided by the adaptive optics system (NACO).
Via the observatory's promising "service observing mode," Marchis and his colleagues were able to obtain sky images of many asteroids over a six-month period without actually having to travel to Chile. DVD data of the observations were sent regularly via mail to Berkeley.

Marchis had the discovery sitting on his shelf for months, since November 2004, because he waited for the completion of the project before starting to process the data and before sending them to colleague Pascal Descamps of the Observatoire de Paris. Just as Marchis was set to go on vacation in March 2005, Descamps sent him a brief note entitled "87 Sylvia est triple?" pointing out that he could see two moonlets around several images of Sylvia.
The entire team then focused quickly on analysis of the data, wrote a paper, submitted an abstract to the August meeting in Rio de Janeiro and submitted a naming proposal to the IAU.

Marchis and his colleagues hope to use the adaptive optics of the Keck and the Gemini telescopes to obtain better images of the triple-asteroid system in order to pin down the precise orbits, verify Sylvia's formation scenario and chart the system's evolution. Already they see precession of the moon's orbits resulting from the irregular shape of Sylvia.
The work was partially supported by the National Science Foundation and the Technology Centre for Adaptive Optics and by the Chretien International Research Grant of the American Astronomical Society.

The fourth author with Marchis, Descamps and Hestroffer was Jerome Berthier, also of the Institut de Mecanique Celeste et Calculs d'Ephemerides at the Observatoire de Paris. The moon now designated Romulus was discovered in 2001 by M. E. Brown and J. L. Margot using the Keck II telescope atop Mauna Kea in Hawaii.

Animation of apparent orbits of Romulus and Remus (2.9 Mb .MPG file)

Animated artist's rendering of asteroids (8 Mb .MOV file)


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Image credit: F. Marchis

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



Image credit: F. Marchis

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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