The Asteroid Moon

Astronomers use ground-based instrumentation to detect a moon orbiting an asteroid.

The use of adaptive optics enabled researchers to detect a moon orbiting asteroid (45)Eugenia. The presence of a moon enables researchers to determine the density of the asteroid. Scattered light from the telescope caused the bright spikes on each side of the image.

An international team of astronomers led by Southwest Research Institute (SwRI) 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.

Previous attempts to photograph such satellites, using both ground-based telescopes and the Hubble Space Telescope, found none. The only other such picture came from the interplanetary spacecraft Galileo when it discovered a small moon, now known as Dactyl, around asteroid (243)Ida in 1993. The latest observations could only be accomplished because of a new technique, called adaptive optics, which reduces the blurring caused by the Earth's atmosphere.

A surprising result of this discovery is the very low density (the heaviness in relation to its size) of the primary asteroid -- only about 20 percent denser than water. Asteroids are 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," says Dr. William Merline, leader of the team and senior research scientist at the SwRI Department of Space Studies in Boulder, Colorado. The new observations indicate that the asteroid may be a rubble-pile of low density rocks or largely water ice. The NEAR spacecraft recently confirmed that another asteroid, (253)Mathilde, also has a low density.

The presence of a moon enables scientists to determine the mass of an asteroid because of the effect of the primary asteroid's gravity on its 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, which can give clues about the asteroid's composition and structure.

"It is almost certain that the satellite was formed by a collision," says 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 as it does."

The light from stars and other celestial objects is distorted by Earth's atmosphere, much as water distorts the view of an underwater object. The adaptive optics technique, pioneered at the University of Hawaii by Dr. Francois Roddier, analyzes the distortions and corrects the light beam by means of what is essentially a "fun-house mirror" back into undistorted form.

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," says Merline.

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.

This work was funded by NASA and the U.S. National Science Foundation. CFHT is funded by the National Research Council of Canada, the Centre National de la Recherche Scientifique of France, and the University of Hawaii.

As this issue went to press, Merline and the asteroid team announced the discovery of a satellite orbiting a second asteroid. Comments about this article? Contact Merline at (303) 546-0487 or merline@boulder.swri.edu.

Reference

W.J. Merline, L.M. Close, C. Dumas, C.R. Chapman, F. Roddier, F. Ménard, D.C. Slater, G. Duvert, C. Shelton, T. Morgan, "Discovery of a Moon Orbiting the Asteroid (45)Eugenia," Nature 401, 565-568, 1999.

Published in the Spring 2000 issue of Technology Today®, published by Southwest Research Institute. For more information, contact Maria Martinez.

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