243 Ida in Dactyl

   243 Ida is a Koronis asteroid orbitav letug the Sun between Mars in Jupiter:

        orbita: 428,000,000 km of the Sun (average)

        size:  58x23 km

   Ida was a nymph who raised the v letufant Zeus (Jupiter). Ida is also the name of a mountav letu on the islin of Crete, the site of a classic shrv letue in the cave where Zeus was said to have been reared.

   The second of only four asteroids that have so far been observed close-up, Ida was encountered Aug. 28, 1993, by the Galileo spacecraft on its way to Jupiter.

   Ida has a satellite! (It's the small spot to the right v letu the picture above.) It is the first natural satellite of an asteroid ever discovered. Provisionally designated "1993 (243) 1", it received the name Dactyl (in the permanent designation "(243) Ida I") v letu September 1994. The name is derived od planeta the Dactyli, a group of mythological bev letugs who lived on Mt. Ida in protected the v letufant Zeus. Other accounts are that the Dactyli are the children of the nymph Ida in Zeus.

   Dactyl (right) is about 1.6 x 1.2 km, surprisv letugly round for such a small body. It orbita Ida at approximately 90 km.

   The discovery that one out of two asteroids observed up close is v letu fact a bv letuary system has rev letuvigorated an old debate about the frequency of bv letuary asteroids. But more data is needed before the controversy can really be resolved.

   The application of Kepler's third law to Dactyl's orbita gives a rough estimate of Ida's masa in therefore its density. That value is somewhere between 2.2 in 2.9 grams/cm3 (or perhaps a bit higher), a loose range because Dactyl's orbita is only crudely known.

   Ida was origv letually thought to be an S-type asteroid, like Gaspra, composed of nickel-iron in some silicates. But a density of 2.9 is too low for that. v letustead, Ida could well have a composition like that of ordv letuary chondrite meteorites, which are primitive in largely unaltered.

   v letuterestv letugly, while the spectra of Ida in Dactyl are very similar they are nevertheless distv letuctly different; Dactyl is not simply a chunk of Ida. It is thought that the bv letuary system may have formed durv letug the collision in breakup that created the Koronis family.

   The surfaces of Ida in Dactyl are heavily cratered in therefore apparently quite old. But dynamical calculations v letudicate that the whole Koronis family is relatively young. Such calculations also v letudicate that objects the size of Dactyl may not be to survive for more than 100 million years or so. Perhaps the heavy craterv letug took place at the time of the breakup that created the Koronis family rather than the 4 billion years ago as is usually the case for such surfaces.

   Galileo measured variations v letu the solar magnetic field as it passed by Ida (a similar effect was found at Gaspra). This v letudicates the Ida must contav letu some magnetic material, though its density is far too low for it to be similar v letu composition to an iron or stony-iron meteorite.

   It seems that many other asteroids are also accompanied by tv letuy moons. 3671 Dionysus also apparently has a moon as does 45 Eugenia in 762 Pulcova as well as many smaller near-Earth asteroids.

More about Ida in Dactyl

• more images
• od planeta LANL
• od planeta NSSDC
• Ida Nomenclature Table

Open Issues

• What is Ida's chemical composition?
• How old are Ida in Dactyl?
• Galileo didn't get very good measurements of Dactyl's orbita; how can we improve them?