[meteorite-list] Telescopes Large and Small Team Up to Study Triple Asteroid 87 Sylvia

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu, 10 Oct 2013 13:22:27 -0700 (PDT)
Message-ID: <201310102022.r9AKMRKt007579_at_zagami.jpl.nasa.gov>


Telescopes Large and Small Team Up to Study Triple Asteroid
SETI Institute
October 7, 2013

Combining observations from the world's 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 45^th annual Division of Planetary
Sciences meeting in Denver, Colorado.

This work illustrates a new trend in astronomy in which backyard amateur
astronomers team up with professional astronomers to expand our
knowledge of our solar system. The study of multiple asteroids such as
(87) Sylvia gives astronomers an opportunity to peek through the past
history of our solar system and constrain the internal composition of
asteroids. In 2005, the triple asteroid was discovered to possess two moons.

The team, led by Franck Marchis, senior research scientist at the Carl
Sagan Center of the SETI Institute, has continued to observe this triple
asteroid system by gathering 66 adaptive optics observations from 8-10m
class telescopes including those at the W. M. Keck Observatory, the
European Southern Observatory, and Gemini North.

"Because (87) Sylvia is a large, bright asteroid located in the main
belt, it is a great target for the first generation of adaptive optics
systems available on these large telescopes. We have combined data from
our team with archival data to get a good understanding of the orbits of
these moons," Marchis said.

With expert assistance from colleagues at the Institut de Mecanique
Celeste et de Calcul des Ephemerides (IMCCE) of the Observatoire de
Paris, the team developed an accurate dynamical model of the system,
allowing them to predict the position of the moons around the asteroid
at any time.

The "drop test" of this work was the prediction of the relative
positions of the moons during an occultation on Jan. 6, 2013. Observers
equipped with small telescopes located on a narrow path across the south
of France, Italy and Greece could see the triple system (87) Sylvia
passing in front of a bright 11-mag star. Such occultations allow
exquisitely precise measurements of the relative positions and sizes of
the occulting objects.

In collaboration with EURASTER, a group of amateur and professional
astronomers, the team successfully motivated ~50 observers to watch the
event. Twelve of them detected the occultation by the primary of the
system which lasted between 4 and 10 seconds depending on the observer's
position on Earth.

"Additionally, four observers detected a two-second eclipse of the star
caused by Romulus, the outermost satellite, at a relative position close
to our prediction. This result confirmed the accuracy of our model and
provided a rare opportunity to directly measure the size and shape of
the satellite", Jerome Berthier, astronomer at IMCCE said.

The chords of this occultation observations revealed that Romulus is 24
km in diameter with an extremely elongated shape, possibly made of two
lobes joined together like a dumbbell. This is not surprising if the
satellite formed from the accretion of fragments created by the
disruption of a proto-Sylvia by an impact, several billion years ago.

The team derived the shape of the 270-km primary asteroid Sylvia by
combining data from the occultation of the asteroid with other sources
of information. These included archived recordings of the variation of
light caused by the spin of the asteroid, and direct imaging by adaptive
optics systems. Because the satellites' orbits do not seem to be
affected by the irregular shape of the asteroid, the team concluded that
the large asteroid is most likely differentiated. The asteroid likely
has a spherical core of dense material, surrounded by a fluffy or
fractured outer surface layer.

"Combined observations from small and large telescopes provide a unique
opportunity to understand the nature of this complex and enigmatic
triple asteroid system," Marchis said. "Thanks to the presence of these
moons, we can constrain the density and interior of an asteroid, without
the need for a spacecraft's visit. Knowledge of the internal structure
of asteroids is key to understanding how the planets of our solar system

Support for this work was provided by NASA through grant number NNX11AD62G.
Received on Thu 10 Oct 2013 04:22:27 PM PDT

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