VLT NACO Instrument Helps Discover First Triple Asteroid (Forwarded)

ESO Education and Public Relations Dept.

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Contacts:

Franck Marchis
University of California, Berkeley, USA
Phone: +1 (510) 642 3958 or +1 (510) 599 0604

Pascal Descamps, Daniel Hestroffer, Jerome Berthier
IMCCE, Observatoire de Paris, France
Phone: +33 1 4051 2268 or +33 1 4051 2260

Under Embargo until August 10, 2005 at 19:00 CET (17:00 GMT)

ESO Press Release 21/05

Rubble-Pile Minor Planet Sylvia and Her Twins

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 25b/05
Orbits of Twin Moonlets Artist's impression of the
around 87 Sylvia triple asteroid system

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
Asteroid Sylvia and Her Twins

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.

Notes

[1]: The team is composed of Franck Marchis (University of
California, Berkeley, USA) and Pascal Descamps, Daniel
Hestroffer, and Jerome Berthier (Observatoire de Paris,
France).

[2]: 87 Sylvia is the 87th minor planet discovered. It was
first observed from the Observatory of Madras (India) on
May 16, 1866, by the Government Astronomer Norman R. Pogson.
It was common in the early days to assign a name -- mostly
feminine -- from the mythology to newly found asteroids.
Pogson selected a name from the list furnished to him by
Sir John Herschel.

[3]: In the Appendix, you can read the story of Syvia and
her sons, Romulus and Remus.

The press release of the University of California, Berkeley,
is available here and the one from the Observatoire de Paris,
here.

Appendix: The mythical Romulus and Remus

As related by the Roman historian Livy, who lived around the
time of Christ, Rhea Silvia was forced to become a vestal
virgin by her uncle, Amulius, after he overthrew her father,
Numitor, king of the ancient Italian city of Alba Longa.
Amulius wanted to ensure she would not have any sons who
could attempt to overthrow him. Despite her virgin priestess
status, Mars, the god of war, came to her in her temple and
conceived with her twin sons, Romulus and Remus. When they
were born, Amulius ordered a servant to kill the twins, but
a merciful servant set them adrift in the river Tiber.

The twin boys were found by Tiberinus, the river god, and
nursed by a female wolf underneath a fig tree. Romulus and
Remus were eventually discovered by Faustulus, a shepherd,
who brought the children to his home and, with his wife,
Acca Larentia, raised the boys as their own. Upon reaching
adulthood, Romulus and Remus returned and killed Amulius and
reinstated Numitor, their grandfather, as king of Alba Longa.

The two subsequently built a settlement on the Palatine Hill,
where they had been nursed by the she-wolf, beginning on
April 21, 753 BC, the traditional date for the founding of
Rome. During the building, Remus thoughtlessly jumped the
unfinished city wall, an omen of ill fortune, and Romulus
killed him. Romulus named the city Roma, after himself, and
made himself king.

Marchis proposed Romulus as the name of the largest moonlet
because the name means "forceful" (rhome in Latin). The
name Remus is related to the word "cautious" (remorari) and
seemed more appropriate to the smallest moon and the closest
to Sylvia.

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Italy: Prof. Massimo Capaccioli, +39-081-55 75 511
The Netherlands: Ms. Marieke Baan, +31-20-525 74 80
Portugal: Prof. Teresa Lago, +351-22-089 833
Sweden: Dr. Jesper Sollerman, +46-8-55 37 85 54
Switzerland: Dr. Martin Steinacher, +41-31-324 23 82
United Kingdom: Mr. Peter Barratt, +44-1793-44 20 25

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