Engenia: Discovery Image
			The unprocessed, "raw" image from which the discovery was made on Nov 1, 1998. This shows the primary asteroid, with the satellite (white blob to upper left at about 10:30 position).
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Image Credit: Canada-France-Hawaii Telescope, Southwest Research Institute.
A surprising result is the discovery that the primary asteroid has very low density -- 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. 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		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 km. Eugenia's diameter is about 215 km. 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.
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