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RE: Asteroid 87 Sylvia
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The discovery was made with 8.2 metre Yepun telescope, one of many of ESO's telescopes that make up the Very Large Telescope Array at Cerro Paranal ,Chile.
Asteroids 87 Sylvia, was already known to be double since 2001, from observations made by Mike Brown and Jean-Luc Margot with the Keck telescope.
With the recent discovery, the astronomers used NACO to observe Sylvia on 27 occasions, over a two-month period. On each of the images, the known 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.

87 Sylvia was named after Rhea Sylvia, the mythical mother of the founders of Rome, therefore, Marchis proposed naming the twin moons, Romulus and Remus. The International Astronomical Union approved the names.
Er, which is quite strange as there is already an asteroid with that name, (10386) Romulus...
The moons are small, orbiting in nearly circular orbits and in the same plane and direction. The newly discovered moonlet, Remus, orbits only 710 km from Sylvia, and is about 7 km across orbiting with a period of 33 hours.

Romulus, orbits at about 1360 km in 87.6 hours and measures 18 km across.




The asteroid 87 Sylvia is located about 3.5 times further away from the Sun than the Earth, between the orbits of Mars and Jupiter, spinning once every 5 hours and 11 minutes.

CHART:

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.

-- Edited by Blobrana at 00:18, 2005-08-09

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One of the thousands of minor planets orbiting the Sun has been found to have its own mini planetary system. Astronomer Franck Marchis (University of California, Berkeley, USA) and his colleagues at the Observatoire de Paris (France) have discovered the first triple asteroid system – two small asteroids orbiting a larger one known since1866 as 87 Sylvia.

"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." - 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 outstanding image’ sharpness provided by the adaptive optics NACO instrument. Via the observatory's proven "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.


One of these asteroids was 87 Sylvia, which was 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. 87 Sylvia is thus not double but triple!

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 approved the names.

Sylvia's moons are considerably smaller, 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 Remus, a body only 7 km across and circling Sylvia every 33 hours. The second, Romulus, orbits at about 1360 km in 87.6 hours and measures about 18 km across.

The asteroid 87 Sylvia is one of the largest known from the asteroid main belt, and is located about 3.5 times further away from the Sun than the Earth, between the orbits of Mars and Jupiter. The wealth of details provided by the 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.
"It could be up to 60 percent empty space" - 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.
"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" - 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.

orbital data, primary (osculating elements) [JPL]:
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.)

orbital data, secondary:
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]:


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-- Edited by Blobrana at 16:55, 2005-08-10

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