[meteorite-list] Trojan Asteroid Patroclus: Comet in Disguise?
From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Wed Feb 1 19:50:03 2006
Trojan Asteroid Patroclus: Comet in Disguise?
February 1, 2006
MAUNA KEA, Hawaii - 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.
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
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
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
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 kilometer (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
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?
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, J?r?me Berthier and Fr?d?ric Vachier of the Institut de
M?canique C?l?ste et de Calcul des Eph?m?rides (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.
Laura K. Kraft
W.M. Keck Observatory
65-1120 Mamalahoa Hwy.
Kamuela, HI 96743
Received on Wed 01 Feb 2006 07:48:23 PM PST