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Asteroid Ida
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.

Dactyl
The greatest discovery from the Galileo fly-by was that Ida has a natural satellite. Although Galileo flew past Ida in August 1993, the discovery wasn't made until late in February 1994 when images stored on the spacecraft's tape recorder were finally transmitted to Earth. The moon has been named Dactyl by the International Astronomical Union.
Dactyl is the first natural satellite of an asteroid ever discovered and photographed. The tiny moon is about 1.2 by 1.4 by 1.6 km (0.75 by 0.87 by 1 mile) across. The name is derived from the Dactyli, a group of mythological beings who lived on Mount Ida. The Dactyli protected the infant Zeus after the nymph Ida hid and raised the god on the mountain. Other mythological accounts say that the Dactyli were Ida's children by Zeus.
Dactyl is made more or less from the same kind of material as Ida. As an S-type asteroid, Ida is composed mostly of silicate rocks. Galileo scientists believe the moon may have been created at the same time as Ida when an older, larger asteroid was shattered in a collision with another asteroid, giving birth to dozens of smaller asteroids. Alternatively, it is possible that Ida was hit by a smaller object even more recently, leaving a crater on the asteroid and throwing off the material that became the small moon.
Galileo scientists also believe it is virtually impossible that the moon is a captured object, something created completely separately from Ida that happened to wander near the asteroid and be caught by its gravitational field. According to the laws of celestial mechanics, such an event would deflect the smaller object, but it would not be captured into orbit unless a third force of some kind slowed it down.

      MEAN RADIUS:                   (11x26) km
      MASS:                          ? (Earth=1)
      DENSITY:                       ? (gm/cm)
      GRAVITY:                       ? (Earth=1)
      ORBIT PERIOD:                  4.84 (Earth years)
      ROTATION PERIOD:               ? (Earth days)
      SEMIMAJOR AXIS OF ORBIT:       2.86 au (Earth=1)
      ECCENTRICITY OF ORBIT:         0.042 (Earth=0.017)