[meteorite-list] VLT NACO Instrument Helps Discover First Triple Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Aug 10 17:10:18 2005
Message-ID: <200508102109.j7AL9Ia11915_at_zagami.jpl.nasa.gov>

http://www.eso.org/outreach/press-rel/pr-2005/pr-21-05.html

ESO Press Release 21/05
11 August 2005

Under Embargo until August 10, 2005 at 19:00 CET (17:00 GMT)
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 <images/phot-25a-05-preview.jpg>

ESO PR Photo 25a/05

Orbits of Twin Moonlets around 87 Sylvia

[Preview - JPEG: 400 x 516 pix - 145k] <images/phot-25a-05-preview.jpg>
[Normal - JPEG: 800 x 1032 pix - 350k] <images/phot-25a-05-normal.jpg>

ESO PR Photo 25b/05 <images/phot-25b-05-preview.jpg>

ESO PR Photo 25b/05

Artist's impression of the triple asteroid system

[Preview - JPEG: 420 x 400 pix - 98k] <images/phot-25b-05-preview.jpg>
[Normal - JPEG: 849 x 800 pix - 238k] <images/phot-25b-05-normal.jpg>
[Full Res - JPEG: 4000 x 3407 pix - 3.7M] <images/phot-25b-05-fullres.jpg>
[Full Res - TIFF: 4000 x 3000 pix - 36.0M] <images/phot-25b-05-fullres.tif>

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 <video/vid-03-05.mov>

ESO Video Clip 03/05

Asteroid Sylvia and Her Twins

[Quicktime Movie - 50 sec - 384 x 288 pix - 12.6M] <video/vid-03-05.mov>

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
<../../epr/epr-contact.html>).


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 <http://www.iau.org/> 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 Arma??o dos B?zios, 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 <pr-21-05_p2.html>, 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
<http://www.berkeley.edu/news/media/releases/2005/08/10_sylvia.shtml>
and the one from the Observatoire de Paris, here
<http://www.obspm.fr/actual/nouvelle/aug05/sylvia.en.shtml>.

Contacts

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

Pascal Descamps, Daniel Hestroffer, Jerome Berthier
IMCCE, Observatoire de Paris, France
Phone: +33 1 4051 2268 or +33 1 4051 2260
Email: descamps_at_imcce.fr, hestroffer@imcce.fr, berthier@imcce.fr


National contacts for the media:

Belgium - Dr. Rodrigo Alvarez +32-2-474 70 50 rodrigo.alvarez_at_oma.be
Finland - Ms. Terhi Loukiainen +358 9 7748 8385 terhi.loukiainen_at_aka.fi
Denmark - Dr. Michael Linden-V?rnle +45-33-18 19 97 mykal_at_tycho.dk
France - Dr. Daniel Kunth +33-1-44 32 80 85 kunth_at_iap.fr
Germany - Dr. Jakob Staude +49-6221-528229 staude_at_mpia.de
Italy - Prof. Massimo Capaccioli +39-081-55 75 511 capaccioli_at_na.astro.it
The Netherlands - Ms. Marieke Baan +31-20-525 74 80 mbaan_at_science.uva.nl
Portugal - Prof. Teresa Lago +351-22-089 833 mtlago_at_astro.up.pt
Sweden - Dr. Jesper Sollerman +46-8-55 37 85 54 jesper_at_astro.su.se
Switzerland - Dr. Martin Steinacher +41-31-324 23 82
martin.steinacher_at_sbf.admin.ch
United Kingdom - Mr. Peter Barratt +44-1793-44 20 25
Peter.Barratt_at_pparc.ac.uk
Received on Wed 10 Aug 2005 05:09:18 PM PDT