First Triple Asteroid System Found

University of California, Berkeley, assistant research astronomerFranck Marchis and his colleagues at the Observatoire de Paris havediscovered the first triple asteroid system - two small asteroidsorbiting a larger one known since 1866 as 87 Sylvia.

Because 87 Sylvia was named after Rhea Sylvia, the mythicalmother of the founders of Rome, Marchis proposed naming the twin moonsafter those founders: Romulus and Remus. The International AstronomicalUnion (IAU) approved the names, to be announced in its Aug. 11circular.

Marchis and his colleagues will report their discovery in theAug. 11 issue of the journal Nature simultaneously with an announcementthat day at theAsteroid Comet Meteor conference in Armação dos Búzios, in the Brazilian state of Rio de Janeiro.

The asteroid 87 Sylvia is one of the largest known from theasteroid main belt, which is located between the orbits of Mars andJupiter. Shaped like a lumpy potato, Sylvia is about 280 kilometers(175 miles) in diameter and is located in the Cybele outer part of thebelt, about 3.5 astronomical units (AU) from the sun. An AU is 93million miles, the average distance between the sun and Earth.

Four years ago, Sylvia was discovered to have a moon, making itone of some 60 known binary asteroids in various asteroid populationsof the solar system. Seventeen of these binary systems are in the mainasteroid belt and have been imaged directly either by adaptive opticssystems on large, ground-based telescopes or by the Hubble SpaceTelescope.

Now, a second moon has been seen around Sylvia, making it atriple asteroid system. Sylvia's newly discovered moons orbit in nearlycircular orbits in the same plane and direction (prograde) as the moonorbits the Earth. The closest moonlet, orbiting about 710 km (450miles) from Sylvia, is Remus, a body only 7 km (4.4 miles) across andcircling Sylvia every 33 hours. The second, Romulus, orbits at about1360 km (860 miles), measures about 18 km (11.3 miles) across, andorbits in 87.6 hours. The asteroid Sylvia spins at a rapid rate, onceevery 5 hours and 11 minutes.

"People have been looking for multiple asteroid systems for along time, because binary asteroid systems in the main belt seem to becommon and formation scenarios, such as a collision between twoasteroids followed by disruption and re-accretion, suggest thatfragments should be orbiting bigger asteroids," Marchis said. "Icouldn't believe we found one."

From two months' of observations of the moonlets' orbits,Marchis and his Paris colleagues were able to precisely calculate themass and density of Sylvia, which shows it to be a "rubble-pile"asteroid, Marchis said. These asteroids are loose aggregations of rockpresumably created when one asteroid smacked into another, disruptingone or both of them. A new asteroid formed later by accretion of largefragments from the disruption. The moonlets probably are debris leftover from the collision that were gravitationally captured by the newlyformed asteroid and which eventually settled into orbits around it.

"That's why most main-belt asteroids with companions have arubble-pile structure," he said. "Because of the scenarios of theirformation, we expect to see more multiple asteroid systems like this."

The density, 1.2 grams per cubic centimeter, is 20 percenthigher than the density of water, which suggests it is composed ofwater, ice and rubble from a primordial asteroid, probably a hydratedcarbonaceous chondrite, based on previous spectroscopic studies of theasteroid.

"It could be up to 60 percent empty space," said astronomer Daniel Hestroffer, one of three coauthors from the Institutde Mécanique Céleste et Calculs d'Éphémérides at the Observatoire deParis.

The discovery was made with one of the European SouthernObservatory's 8-meter telescopes (Yepun) of the Very Large Telescope atCerro Paranal, using the telescope's infrared camera and the highangular resolution provided by the adaptive optics system (NACO). Viathe observatory's promising "service observing mode," Marchis and hiscolleagues were able to obtain sky images of many asteroids over asix-month period without actually having to travel to Chile. DVD dataof the observations were sent regularly via mail to Berkeley.

Marchis had the discovery sitting on his shelf for months,since November 2004, because he waited for the completion of theproject before starting to process the data and before sending them tocolleague Pascal Descamps of the Observatoire de Paris. Just as Marchiswas set to go on vacation in March 2005, Descamps sent him a brief noteentitled "87 Sylvia est triple?" pointing out that he could see twomoonlets around several images of Sylvia. The entire team then focusedquickly on analysis of the data, wrote a paper, submitted an abstractto the August meeting in Rio de Janeiro and submitted a naming proposalto the IAU.

Marchis and his colleagues hope to use the adaptive optics ofthe Keck and the Gemini telescopes to obtain better images of thetriple-asteroid system in order to pin down the precise orbits, verifySylvia's formation scenario and chart the system's evolution. Alreadythey see precession of the moon's orbits resulting from the irregularshape of Sylvia.

The work was partially supported by the National ScienceFoundation and the Technology Center for Adaptive Optics and by theChretien International Research Grant of the American AstronomicalSociety.

The fourth author with Marchis, Descamps and Hestroffer was JérômeBerthier, also of the Institut de Mécanique Céleste et Calculsd'Éphémérides at the Observatoire de Paris. The moon now designatedRomulus was discovered in 2001 by M. E. Brown and J. L. Margot usingthe Keck II telescope atop Mauna Kea in Hawaii.