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Astronomers Discover Moon Orbiting Asteroid 45 Eugenia

Southwest Research Institute

CONTACT:
Dr. William J. Merline
(303) 546-0487 or merline@boulder.swri.edu

Maria Martinez
(210) 522-3305 or mmartinez@swri.org

EMBARGOED FOR RELEASE UNTIL OCTOBER 6, 1999 AT 2:00 PM EDT
 
Astronomers discover moon orbiting asteroid

New technology promises to revolutionize our understanding of asteroid
makeup

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.

EDITORS: Images to support this story are available after 2 PM EDT,
Wednesday, October 6, 1999, on the Internet at: 

   www.boulder.swri.edu/~merline/press_release

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