Like the hollow wooden horse hiding Greek warriors in the Trojan War, could an entire population of asteroids be masquerading as comets? Observations of the binary Trojan asteroid (617) Patroclus taken at the W. M. Keck Observatory on Mauna Kea have astronomers wondering if asteroids caught in the gravitationally neutral zone of the Sun-Jupiter system might actually be ancient comets and space dust.

Artist's representation of binary asteroid (617) Patroclus, a trojan asteroid gravitationally locked 60-degrees behind the orbit of Jupiter in its path around the Sun. Credit: Lynette Cook and W. M. Keck Observatory

Dr. Franck Marchis of the University of California at Berkeley is leading an international team of astronomers that has discovered that the composition and density of the Patroclus system is remarkably similar to that of comets. The components are less dense than water, probably porous and probably, like snow, made out of water ice. The results, published in the February 2nd issue of Nature, are raising important questions about how this Trojan asteroid migrated to its current location in the solar system and it how it acquired its binary nature.

Trojan asteroids are those that lie 60 degrees in front or 60 degrees behind the planet Jupiter in its orbit around the Sun. They are relatively small and quite faint, making them difficult to study even with the world's largest ground-based telescopes. A new technique using sodium lasers with ground-based image-correcting technology called "Laser Guide Star Adaptive Optics" (LGS-AO) is helping scientists study asteroids with more detail than ever before.

The LGS-AO system installed on the Keck II 10-meter telescope at Mauna Kea removes the blurring effects caused by Earth's atmosphere from astronomical images and produces the finest infrared images in the world.

"Space telescopes are tremendous tools for observing remote solar system targets, but large ground-based telescopes equipped with Laser Guide Star Adaptive Optics systems provide both the power to collect more light and the ability to study objects with even more detail," said Dr. David Le Mignant, adaptive optics scientist and lead team member for the LGS-AO science operations at the W. M. Keck Observatory. "With LGS-AO, we observe a different population of solar system targets: fainter and smaller objects like Patroclus and more distant ones like the object beyond Pluto. This should lead us to many new discoveries," added Dr. Le Mignant.

Modern theories suggest that Trojan asteroids may have formed in the Solar Nebula at the same time as the rest of the solid bodies in the Solar System. To date, more than one thousand such asteroids have been discovered.

Asteroid Patroclus was previously believed to be a single object about 150 kilometers (90 miles) in diameter, but recent observations from the Gemini North telescope in Hawaii found that Patroclus is actually comprised of two objects, making it the first binary Trojan asteroid to be discovered. The discovery of a binary asteroid was not as surprising as the fact that the two objects are nearly identical in size. Dr. Marchis' team found the larger piece is 122 kilometers (76 miles) wide at its largest point, and the similar-sized partner is 112 kilometers (70 miles). The two pieces orbit their center of mass every four days, separated by a distance of about 680 kilometers (423 miles). The names of these objects are associated with the heroes of Homer's Iliad. The asteroid Patroclus was named after the best friend and companion to Achilles, the main character of the story and Greek hero of the Trojan War.

Scientists believe there may be as many Trojan asteroids as there are main-belt asteroids, but they are difficult to study with high spatial resolution because they are too faint for most adaptive optics systems.

"The Laser Guide Star system is a remarkable breakthrough in ground-based observations," said Dr. Franck Marchis of the University of California at Berkeley. "With such a capability we are able to regularly study small bodies in the solar system in ways that were not possible before. We want to thank the Keck Observatory Adaptive Optics team for their involvement in our observing program which helped make these results possible."

Since collisions of small bodies in the solar system typically happen at relatively high speeds and leave behind lots of small debris, it is unlikely that Patroclus was formed this way, or that an asteroid the size of Patroclus would have experienced a collision in the last billion years. How then, could the Patroclus binary system have formed?

An illustration of "Lagrangian points" in the Sun-Jupiter system. Lagrangian points are five positions in interplanetary space where small objects, affected only by gravity, are stationary relative to two larger objects (such as the Sun and Jupiter). Purple lines represent the L4 and L5 Lagrangian points. More than a thousand asteroids have been discovered in these positions, indicated by white dots. Credit: IMCCE/UC Berkeley

New results have some scientists theorizing that Patroclus originated from a very early time in the solar system's history, about four and a half billion years ago. Patroclus may have formed during the accretion phase of solar system formation, similar to thousands of other objects in the Kuiper Belt, an outer region of the solar system beyond the orbit of Neptune. Recent simulations suggest that the giant gas planets migrated outward and gravitationally removed neighboring planetesimals. Some of these objects were then subsequently caught into the gravitationally-stable Lagrangian points of the Jupiter-Sun system.

The story of Patroclus may be even more complex: As Patroclus encountered the planet Jupiter several billion years ago, it may have gotten a little too close to the mighty planet. The tremendous gravitational forces of Jupiter, which are three times stronger than that of Earth, may have split the small and porous body in half through an effect known as "tidal splitting."

"The Patroclus system displays similar characteristics to binary Near-Earth asteroids," said Dr. Marchis. "Near-Earth binary asteroids are believed to be formed during an encounter with a planet, which results in tidal splitting. Recent published work from our collaborators leads us to suggest that a Trojan asteroid may be formed in a similar way -- through an encounter with Jupiter. This scenario is different than what is believed to cause binary asteroid systems in the main asteroid belt, which typically feature two or more bodies of unequal size."

The team responsible for finding the mass and size of the Trojan binary asteroid Patroclus are Franck Marchis, Imke de Pater and Michael H. Wong of the University of California at Berkeley; Daniel Hestroffer, Pascal Descamps, Jerôme Berthier and Frederic Vachier of the Institut de Mecanique Celeste et de Calcul des Ephemerides (IMCCE); and Antonin Bouchez, Randall Campbell, Jason Chin, Marcos van Dam, Scott Hartman, Erik Johansson, Robert Lafon, David Le Mignant, Paul Stomski, Doug Summers and Peter Wizinowich of the W. M. Keck Observatory.

Funding for the project was provided by the National Science Foundation Science and Technology Center for Adaptive Optics and by the National Aeronautics and Space Administration (NASA) through the Science Mission Directorate Research and Analysis Programs.

Data was obtained between November 2004 and May 2005 with the second generation Near Infrared Camera (NIRC2) on the Keck II 10-meter telescope at the W. M. Keck Observatory, managed by the California Association for Research in Astronomy, a non-profit 501 (c) (3) corporation. The first Keck telescope began observations in May, 1993. Its twin joined in 1996. Together, the telescopes are the world's most powerful eyes looking into the optical and infrared universe.