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Boulder, Colorado – October 6, 1999:
Astronomers using the
adaptive optics system on the
Canada-France-Hawaii Telescope (CFHT)
have discovered a moon orbiting the asteroid (45)Eugenia.
These images are the first observations of an asteroidal satellite taken
with an Earth-based telescope.
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
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
"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.
The above infrared image (1.6 µm) is a superposition of 5 detections of the new moon
(taken – clockwise from top – on the nights of Nov 1998, 6, 7, 9, 10, and 1 UT).
The blue dashed line shows the orbit of the moon around the primary asteroid (45) Eugenia.
The period of the orbit is 4.7 days.
The moon travels in a clockwise direction (with North up and East to the left).
The radius of the orbit is 1190 km.
The main asteroid's diameter is close to 215 km (depicted here as a central white circle)
and the moon's size is estimated to be 13 km in diameter. .
The moon is 285 times fainter than the main asteroid and is very close to the main asteroid (just over 5 asteroid diameters away, or 0.77 arcseconds on the sky). Image processing (deconvolution) was used to increase the contrast between the moon and the main asteroid. The apparent variations in the moon's brightness and size are artificially produced due to changes in the atmospheric clarity from night to night. .
Utilizing adaptive optics this image (resolution of approximately 185 km or 0.12 arcseconds on the sky) is 6 times sharper than could have been otherwise obtained from the ground. Images taken with the PUEO adaptive optics system of the Canada-France-Hawaii Telescope located on Mauna Kea, Hawaii.
Image: L. Close (ESO), W. Merline (Southwest Research Institute)
"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.
W.J. Merline, L.M. Close, C. Dumas, C.R. Chapman, F. Roddier, F. Ménard,
D.C. Slater, G. Duvert, C. Shelton and T. Morgan,
"Discovery of a moon orbiting the asteroid 45 Eugenia,"
Nature 401, 565–568 (1999)
Full Press Release
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