Binary Asteroid Revealed As Twin Rubble Piles

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 approximate orbit 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, Réunion 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 Mécanique Céleste et de Calculs des Éphémérides (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, Frédéric Vachier, Francois Colas, Jérôme 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. Léonie and T. Dijoux of Les Makes Observatory in La Réunion, France; M. Assafin of the Observatório 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.