Rubble-Pile Minor Planet Sylvia and Her Twins

Details Rubble-Pile Minor Planet Sylvia and Her Twins Rubble-Pile Minor Planet Sylvia and Her Twins

Aug 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005eso..pres...21.&link_type=abstract

ESO Press Release, 08/2005

Physics

Optics

Scientific paper

VLT NACO Instrument Helps Discover First Triple Asteroid
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) [1] have discovered the first triple asteroid system - two small asteroids orbiting a larger one known since 1866 as 87 Sylvia [2].
"Since double asteroids seem to be common, people have been looking for multiple asteroid systems for a long time," said Marchis. "I couldn't believe we found one."
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.
ESO PR Photo 25a/05
ESO PR Photo 25a/05
Orbits of Twin Moonlets around 87 Sylvia
[Preview - JPEG: 400 x 516 pix - 145k] [Normal - JPEG: 800 x 1032 pix - 350k]
ESO PR Photo 25b/05
ESO PR Photo 25b/05
Artist's impression of the triple asteroid system
[Preview - JPEG: 420 x 400 pix - 98k] [Normal - JPEG: 849 x 800 pix - 238k] [Full Res - JPEG: 4000 x 3407 pix - 3.7M] [Full Res - TIFF: 4000 x 3000 pix - 36.0M]
Caption: ESO PR Photo 25a/05 is 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. The inset shows the potato shape of 87 Sylvia. The field of view is 2 arcsec. North is up and East is left. ESO PR Photo 25b/05 is an artist rendering of the triple system: Romulus, Sylvia, and Remus.
ESO Video Clip 03/05
ESO Video Clip 03/05
Asteroid Sylvia and Her Twins
[Quicktime Movie - 50 sec - 384 x 288 pix - 12.6M]
Caption: ESO PR Video Clip 03/05 is an artist rendering of the triple asteroid system showing the large asteroid 87 Sylvia spinning at a rapid rate and surrounded by two smaller asteroids (Remus and Romulus) in orbit around it. This computer animation is also available in broadcast quality to the media (please contact Herbert Zodet).
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 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 [3], 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 (see ESO PR Photo 25a/05). It is 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," said co-discoverer Daniel Hestroffer (Observatoire de Paris, France).
"It is most probably a "rubble-pile" asteroid", Marchis added. These asteroids are loose aggregations of rock, presumably the result of a collision. Two asteroids smacked into each other and got disrupted. 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."
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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