An asteroid with a moon! The robot spacecraft Galileo whose primary mission is to explore the Jupiter system, encountered and photographed two asteroids during its long journey to Jupiter. The second asteroid it photographed, called Ida, was discovered to have a moon which appears as a small dot to the left of Ida in this picture. The tiny moon, named Dactyl, is only about one mile across, while the potato-shaped Ida measures about 36 miles long and 14 miles wide. Dactyl is the first moon of an asteroid ever discovered. The names Ida and Dactyl are based on characters in Greek mythology.
Credit: NASA, JPL, Galileo Project
Ida is a heavily cratered, irregularly shaped asteroid in the main asteroid belt between Mars and Jupiter -- the 243rd asteroid to be discovered since the first one was found at the beginning of the 19th century. Ida is placed by scientists in the S class (stony or stony iron meteorites). It is a member of the Koronis family, which scientists believe was created when a larger body perhaps 200 to 300 kilometers (120 to 180 miles) in diameter was smashed relatively recently -- at least considerably after the solar system formed some 4.5 billion years ago.
On August 28, 1993 Galileo came within 2,400 kilometers (1,500 miles) of 243 Ida, the second asteroid ever encountered by a spacecraft. They passed each other at a relative velocity of 12.4 km/sec (28,000 mph). At the time of the encounter, Ida and Galileo were 441 million kilometers (274 million miles) from the Sun.
Ida is about 56 x 24 x 21 kilometers (35 x 15 x 13 miles) in size, more than twice as large as Gaspra. It has a period of rotation of 4 hours, 38 minutes. Its density has been estimated to be between 2.2 and 2.9 grams per cubic centimeter. Ida's age is somewhat baffling. Its surface is heavily cratered suggesting that it has existed in its present form for at least a billion years and perhaps much longer. It is also considerably older than estimates for the Koronis breakup.
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