To Related PhotoAlthough Galileo flew by Asteroid Ida last August 28, some of the images it took are just now being transmitted and analyzed. That is why scientists were recently surprised to discover that Ida is not alone in space, but has a moon in orbit around it.
The discovery was first made by Ann Harch of the Galileo camera team, who noticed a bright object near Ida on some new images that were processed on February 17. The team considered and eliminated the possibility that the object was a planet, star, or something other than a moon. AÊfew days later, the Near Infrared Mapping Spectrometer (NIMS) team also noticed some odd data while analyzing Ida's mineral content. They compared notes with the camera team and realized they had, indeed, found a moon.
Using images taken by the two Galileo instruments and comparing sighting angles at different times, the scientists determined the as-yet-unnamed moon is located about 100 kilometers from Ida's center. NIMS data also indicate that the rocks and soil on the surface of the tiny moon (only about 1.5 kilometers long) have roughly equal mixtures of olivine, orthopyroxene, and clinopyroxene, while Ida's surface is predominately olivine with a bit of orthopyroxene. The two are about the same temperature--200 K. More data are needed to determine the characteristics of the moon's orbit, which in turn will help to calculate Ida's density.
Shortly after the discovery, Galileo scientists eliminated the idea that the moon is a passing body caught in Ida's gravity. They also doubt that the moon is a piece of Ida knocked loose by a smaller projectile, especially since their bulk compositions differ slightly. Instead, scientists are theorizing that the two are siblings of a "family" of asteroids formed hundreds of millions of years ago when a larger, 100-kilometer-wide asteroid was shattered in a great collision. Instead of fragments shooting straight out from the impact, the exploding asteroid may have produced jets of material carrying two or more objects out together. Those objects would then be captured, gravitationally, around each other. (See "How Can an Asteroid Have a Moon?" on page 4.)
Thus, a family of asteroids could have been created as a result of such an impact. Ida belongs to the Koronis family that travels in the main Asteroid Belt between Mars and Jupiter. Gaspra, the asteroid visited by Galileo in October 1991, is a member of the Flora family.
The discovery of Ida's moon "probably means they [asteroidal moons] are quite common," said astronomer Michael J. S. Belton, who leads the Galileo camera team. Many scientists suspect that a significant fraction of asteroids may have satellites. He noted that scientists believe they are on the verge of answering many questions about the existence and origin of asteroids and their satellites.
Galileo finished transmitting Ida data in June--including additional images of Ida's moon--and scientists expect to be able to determine more about the origin, composition, size, and orbit of Ida's moon, as well as the dynamics of collisions that played a central role in shaping the planets.
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