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

[Ron Baalke]

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

FOR IMMEDIATE RELEASE

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  
Observatory.

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,  
Calif.

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.