[meteorite-list] Amateur Astronomers, Professionals Combine Observations to Produce Detailed Picture of Double Asteroid

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Fri, 30 Mar 2007 10:38:05 -0700 (PDT)
Message-ID: <200703301738.l2UHc5T20407_at_zagami.jpl.nasa.gov>

3/29/07 - File #17511
Contact: Robert Sanders
(510) 643-6998
rsanders at berkeley.edu

Amateur astronomers, professionals combine observations to produce
detailed picture of double asteroid


The full story is online at http://www.berkeley.edu/news/media/releases/2007/03/29_antiope.shtml.
See images at http://www.eso.org/outreach/press-rel/pr-2007/pr-18-07.html

Berkeley -- Roping together observations from the world's largest
telescopes as well as the small instrument of a local backyard
amateur, astronomers have assembled the most complete picture yet
of a pair of asteroids whirling around one another in a perpetual
pas de deux.

In a paper to be published in the April 2007 issue of the journal
Icarus, a team of University of California, Berkeley, and Paris
Observatory astronomers depict the asteroid 90 Antiope as two
slightly egg-shaped rubble piles locked in orbit, like two twirling
dancers facing one another with linked arms.

This new view of Antiope is the culmination of research that
started in 2003 and that eventually included data supplied by both
professional and amateur astronomers from around the globe.

Before the year 2000, Antiope was just another main-belt asteroid,
one of millions between the orbits of Mars and Jupiter. But that
year, it was resolved into a doublet, thanks to sharper pictures
obtained with adaptive optics (AO) on the largest ground-based
telescope, the 10-meter Keck II telescope in Hawaii. Yet, even with
AO, these two asteroids were too small for astronomers to discern
their shape or to see more than two bright blobs revolving around
their center of mass.

Two years ago, with improved images from the European Southern
Observatory's 8-meter Very Large Telescope (VLT) in Chile and Keck
II, University of California, Berkeley astronomer Franck Marchis
and colleagues in France were able to determine the orbits of the
two asteroids, each of them about 86 kilometers in diameter and
separated by about 171 kilometers.

But uncertainties remained, and in 2005 the team invited observers
around the world to turn their telescopes on the asteroid pair
during a time when they predicted a mutual eclipse or occultation
would cause a drop in brightness. In an eclipse, one of the pair
casts a shadow over the other; in an occultation, one passes in
front of, and completely blocks light from, the other.

Sure enough, at the appointed time on May 31, 2005, one of the
asteroids eclipsed the other, and team member Tadeusz Michalowski e-
mailed Marchis and their colleagues from South Africa to confirm
the eclipse. Michalowski, an astronomer at Adam Michiewicz
University's Astronomical Observatory in Poznan, Poland, recorded
the dip in Antiope's brightness from the South African Astronomical

Over the next six months, at Marchis' invitation, amateurs and
professionals from as far afield as Brazil, France, Reunion Island
in the Indian Ocean and Grass Valley, Calif., observed repeated
occultations, as well as shadows passing over one of the pair.

"This is the first publication I've had in a professional journal,
and I'm really happy about it," said amateur astronomer Peter
Dunckel, 75, a retired paper company executive who observes from
the backyard of his vacation home in Grass Valley. "What is really
a thrill is to have my little 7-inch telescope along with an 8-
meter telescope on the same paper; it is unbelievable."

Dunckel observed the binary pair for 35 hours over a period of six
weeks, recording Antiope's brightness every minute with a CCD
camera attached to his Maksutov Newtonian reflector telescope.

"Amateurs can be used for professional studies, compensating for
the small size of their telescopes by the large numbers of
observations and the frequency of observations they can do,"
Marchis said. "You can time the orbits more precisely when a mutual
event happens, which allows you to extract also the size, shape and
surface detail of each component, and also what it's made of."

The asteroid pair is itself the remnant of an ancient asteroid,
dubbed Themis, which astronomers estimate was destroyed around 2.5
million years ago, probably hit by another asteroid. The rubble
spread out from the point of impact but continued to follow
approximately the same orbit around the sun in the outer part of
the main asteroid belt. Themis was a carbonaceous chondrite left
over from the formation of the solar system 2.5 billion years ago.

Evidently, either another asteroid hit Antiope again to split it in
two, or two of the Themis pieces remained bound to one another
after the initial break up, possibly even remaining attached.
However the doublet arose, computer simulations by another group
suggest that the spinning, elongated rubble pile would have
separated into two egg-shaped rubble piles, each the shape of a
Roche ellipsoid, the theoretical shape predicted for a system if
their composition was liquid or loosely aggregated, rather than
solid, and if the components are deformed due to mutual gravitation.

The eclipse and occultation observations, combined with previous
observations of Antiope during a grazing occultation, confirmed the
ellipsoid shape of each component of the asteroid, Marchis said.
Each component differs from a sphere by less than 7 percent, or 6
kilometers out of 86. They orbit around their center of mass every
16.5 days

"Due to mutual gravitation, both components took a shape very close
to the pure hydrostatic shape, the Roche ellipsoid, as if the
asteroid was a fluid," Marchis said. "This result indicates that
the internal strength in the components must be low, so possibly a
rubble pile structure."

They were able to calculate the density as 1.25 grams per cubic
centimeter (water is one gram per cubic centimeter), which, if one
assumes that the rock component is carbonaceous chondrite, means
the asteroid pair is 30 percent empty space.

"Despite this intensive study, the origin of this unique doublet
still remains a mystery," said team member Pascal Descamps, an
astronomer at the Institut de Mecanique Celeste et de Calculs des
Ephemerides (IMCCE) of the Observatoire de Paris. "The formation of
such a large double system is an improbable event and represents a
formidable challenge to theory. It is possible that a parent body
was spun up so much that it broke apart, but this seems very hard
to do for asteroids in the main belt, unlike, for example, near-
Earth asteroids."

Marchis and his team are employing both amateur and professional
astronomers to observe more of these mutual events between
components of binary asteroid systems. These partnerships are a
powerful way to get direct and accurate insights about these
systems, he said.

As for Dunckel, who commutes from San Francisco to the Grass Valley
vacation home he refers to as "Rattlesnake Creek Observatory," he
says he's hooked on scientific amateur observing, "now that I've
broken the dam, so to speak." He has upgraded to a 10-inch
reflecting telescope and is excited about applying a new computer
program that will allow him to create 3D models of asteroids from
light curves he obtains in collaboration with others.

The VLT and Keck observations were made between 2003 and 2005,
while the subsequent mutual event observations and analyses were
made by a team that included Descamps, Frederic Vachier, Francois
Colas, Jerome Berthier, Daniel Hestroffer, Roberto Vieira-Martins
and Mirel Birlan of IMCCE; amateur astronomers Dunckel of Grass
Valley, Calif., and J.-P. Teng-Chuen-Yu, A. Peyrot, B. Payet, J.
Dorseuil, Y. Leonie and T. Dijoux of Les Makes Observatory in La
Reunion, France; M. Assafin of the Observatorio do Valongo in Rio
de Janeiro, Brazil; M. Polinska of Poland's Astronomical
Observatory in Poznan; W. Pych of Poland's Nicolaus Copernicus
Astronomical Center in Warsaw; and K.P.M. Miller of the University
of California's Lick Observatory on Mount Hamilton near San Jose,

Marchis' work was partly supported by the National Science
Foundation's Science and Technology Center for Adaptive Optics,
which is managed by UC Santa Cruz.


NOTE: Franck Marchis can be reached at his office, (510) 642-3958,
or via e-mail at fmarchis at berkeley.edu. Peter Dunckel can be
reached via e-mail at pbd2 at pacbell.net.
Received on Fri 30 Mar 2007 01:38:05 PM PDT

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