New images and data from Galileo suggest that although the asteroid Ida and its natural satellite are similar in color and brightness, they appear to be composed of different material. The International Astronomical Union (IAU) has approved the name Dactyl for the tiny one-mile diameter moon discovered in orbit around the asteroid Ida by NASA's Galileo mission. Dactyl's motion, in the same direction as Ida's rotation, appears to be in a plane viewed nearly edge-on by the spacecraft, making it difficult to determine the exact orbital shape and period.
"A circular orbit at 60 miles (90 kilometers), nearly in Ida's equatorial plane, with a period of about one Earth day, appears to fit the observations we have now," said Kenneth P. Klaasen of the imaging team. "However, a range of elliptical orbits cannot be ruled out yet," he added. "Other observations that are still on Galileo's onboard tape recorder to be played back next month should permit us to improve the calculation."
There are different theories about the origin of Dactyl. Dactyl is the first natural satellite of an asteroid ever discovered and photographed, obtained by the Galileo spacecraft during its flyby of the asteroid on Aug. 28, 1993. It might be a large block thrown off during an impact that formed one of the large craters on Ida's surface. "More likely," said imaging team member Dr. Clark Chapman, "the moon was formed during the cataclysmic fragmentation and disruption of a larger asteroid in which Ida itself was formed. In this scenario, the little moon was ejected from the explosion in practically the same orbit as Ida, and was captured in the larger object's gravitational field," Chapman explained, "while most other fragments went into independent orbits around the Sun."
Galileo's near-infrared mapping spectrometer, which initially confirmed discovery of Dactyl, provided the data for thermal and mineralogical maps of the surface of Ida and mineralogical studies of its moon. "We have good data on what minerals make up these bodies," said Dr. Robert Carlson, principal investigator for the spectrometer. "The areas on Ida's surface where we have our best data appear to be predominantly olivine, with a bit of orthopyroxene, while its moon is quite different, with a roughly equal mixture of olivine, orthopyroxene, and clinopyroxene. This suggests the moon is not a chip off the asteroid." Ida orbits the Sun in about the middle of the asteroid belt between Mars and Jupiter. The asteroid is about 36 miles (58 kilometers) long and 14 miles (23 kilometers) wide, and rotates once every 4 hours, 40 minutes. One of only two asteroids ever observed close-up, it was encountered Aug. 28, 1993, by the Galileo spacecraft on its way to Jupiter.
The name Dactyl is derived from the Dactyli, a group of mythological beings who lived on Mount Ida, where the infant Zeus was hidden -- and raised, in some accounts -- by the nymph Ida and protected by the Dactyli. Other mythological accounts say that the Dactyli were Ida's children by Zeus.
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