Artistic representation of the triple asteroid system showing the large 270-km asteroid Sylvia surrounded by its two moons, Romulus and Remus. The differentiated interior of the asteroid is shown through a cutaway diagram. The primary asteroid of the system may have a dense, regularly-shaped core, surrounding by fluffy or fractured material. The two moons are shown to be strongly elongated, and composed of two lobes, as suggested by the recently observed occultation data by the moon Romulus.
Credit: Danielle Futselaar/SETI Institute
Sylvia and its moons as seen from the 8-10m class telescopes with adaptive optics. The dark circle shows the irregular shape of the asteroid. The small moons can be seen at various positions on these images. More than 66 observations were collected, some were recorded by our team others were extracted from Keck Observatory, Gemini and VLT archive data.
Credit: Franck Marchis
A map of the path
of the occultation on January 6, 2013. The blue lines indicate the limit
of the path by the primary asteroid Sylvia (centered on the black
line). The green and orange lines correspond to the paths of Romulus and
Remus, respectively. Credit: IMCCE
Kamuela, HI -- Combining observations from the world’s largest telescopes with small telescopes used by amateur astronomers, a team of scientists discovered that the large main-belt asteroid (87) Sylvia has a complex interior, thanks to the presence of two moons orbiting the main asteroid, and probably linked to the way the multiple system was formed. The findings are being revealed today, October 7, at the 45th 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. The two moons of (87) Sylvia were discovered in 2005.
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 Mécanique
Céleste et de Calcul des Éphémérides(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. Observers at different locations see different parts of the
asteroid, or its moons, passing in front of the 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 moon, 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 moon”, Jérôme 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 moon 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 moons’ 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 formed”.
The W. M. Keck Observatory operates the largest, most scientifically
productive telescopes on Earth. The two, 10-meter optical/infrared
telescopes on the summit of Mauna Kea on the Island of Hawaii feature a
suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field
spectroscopy and a world-leading laser guide star adaptive optics
system. The Observatory is a private 501(c) 3 non-profit organization
and a scientific partnership of the California Institute of Technology,
the University of California and NASA.
Media Contact:
Karen Randall
krandall@seti.org
+1 650 960-4537
Steve Jefferson
sjefferson@keck.hawaii.edu
+1 808 881-3827
Science Contacts:
Franck Marchis
fmarchis@seti.org,
+1 650 810 0236
Jérôme Berthier,
berthier@imcce.fr
+33 1 4051 2261
