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Boulder, Colorado. October 7, 1999. An international team of astronomers has discovered a moon orbiting the asteroid (45)Eugenia. The pictures, taken with the Canada-France-Hawaii Telescope (CFHT) on Mauna Kea, Hawaii, are the first images of an asteroidal satellite taken from Earth. The team's findings will be reported in the October 7 issue of Nature.

Previous attempts to photograph such satellites, using both ground-based telescopes and the Hubble Space Telescope, found no satellites. The only other such picture came from an interplanetary spacecraft, Galileo, when it discovered the small moon, now known as Dactyl, around asteroid (243)Ida in 1993. The observations could only be accomplished because of a new technique, called adaptive optics, that reduces the blurring caused by the Earth's atmosphere.

A surprising result of this discovery is the very low density of the primary asteroid -- only about 20 percent denser than water. Most asteroids appear dark and were thought to be composed primarily of rock, which is about three times denser than water. "A picture is emerging that some asteroids are real lightweights," said Dr. William Merline, leader of the team, and a senior research scientist at the Boulder office of San Antonio-based Southwest Research Institute (SwRI). A recent flyby of the NEAR spacecraft confirmed that another asteroid, (253)Mathilde, also has a low density. "Either these objects are highly porous rubble-piles of rock, or they are mostly water ice," said Dr. Clark Chapman, another team member, also from SwRI.

The presence of a moon allows scientists to determine the mass of an asteroid because of the effect of the primary asteroid's gravity on its small moon. The size of most asteroids is known from standard astronomical studies. If both the mass and the size are known, researchers can learn the asteroid's density. The density then gives a clue to the asteroid's makeup -- either in terms of composition or structure.

"If these asteroids are rubble-piles, it tells us about the severity of collisions in the asteroid belt and its subsequent evolution. If the objects are largely ice, covered with a dark-coating, then these objects may be remnants of burned-out comets and will further our understanding of the connection between comets and asteroids," said Dr. Christophe Dumas of the Jet Propulsion Lab in Pasadena.

"It is almost certain that the satellite was formed by a collision," said Merline. "As we know from the formation of our own moon and the craters on planetary surfaces, collisions played a large role in the formation of our solar system. Satellites of asteroids give us a window into these collisions, and help us understand how and why our solar system looks like it does."

The light from stars and other celestial objects is distorted by the atmosphere, much as water distorts our view of an underwater object. The new technique, pioneered at the University of Hawaii by team member Dr. Francois Roddier, analyzes the distortions and corrects the light beam by means of what is essentially a "fun-house mirror" back into its previous, undistorted form. "CFHT's exceptional site, telescope, and adaptive optics now allow us to see far sharper detail through the Earth's atmosphere. In many cases we can now compete with the clarity of space-based telescopes," said Roddier. The instrument used was built by the CFHT Corporation.

Previously, faint and close satellites would have been lost in the glare of the primary asteroid. "It is similar to taking a photo of a candle located 400 km away and then discovering a firefly (that is 300 times fainter) flying within two meters of the flame," said Dr. Laird Close, a participant from the European Southern Observatory (ESO) in Germany.

The results are the first from a program to search for satellites around nearly 200 asteroids. "If more satellites are found, it will revolutionize our understanding of the makeup of asteroids," said Merline.

"Except for a few of the very largest asteroids, this is the only way that asteroid densities can be determined other than by spacecraft flybys," according to Close.

Eugenia orbits the sun in the main asteroid belt, a collection of thousands of asteroids that exists between the orbits of Mars and Jupiter. Asteroids are thought to be bodies that never formed a planet; the gravity of the giant planet Jupiter may have stirred up the bodies enough that they collided with each other at fast speeds, perhaps either fragmenting or forming satellites, rather than colliding gently, adhering, and gradually building up a planet.

Researchers estimate that the diameter of the satellite is about 13 kilometers. Eugenia's diameter is about 215 kilometers. The researchers have determined that the satellite has a circular orbit about 1,190 km away from Eugenia. It orbits about once every five days.

While awaiting assignment of a permanent name, the satellite has been given provisional designation, by the International Astronomical Union, of S/1998(45)1, the first satellite of asteroid (45) that was discovered during 1998.

This work was funded by NASA and the U.S. National Science Foundation. A portion of the image processing and data analysis was carried out using facilities at the ESO. CFHT is funded by the National Research Council (NRC) of Canada, the Centre National de la Recherche Scientifique (CNRS) of France, and the University of Hawaii.

Other team members and affiliations are Dr. Francois Menard, CFHT; Dr. David Slater, SwRI headquarters in San Antonio; Dr. Gilles Duvert, Laboratoire d'Astrophysique in Grenoble, France; Dr. Chris Shelton, W.M. Keck Observatory, Hawaii; and Dr. Tom Morgan, NASA Headquarters, Washington, D.C.


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