How Much Does Ida Weigh?

Astronomers announced new results about the asteroid 243 Ida and its tiny moon Dactyl. The double nature of Ida was discovered in August 1993, when the Galileo spacecraft encountered it, but only recently have all the flyby data been radioed to Earth for analysis. According to Galileo scientists, Dactyl's orbit allowed them to determine the mass of Ida and thus its density. That value is somewhere between 2.2 and 2.9 grams per cubic cm, a loose range because Dactyl's orbit is only crudely known. But that's good enough to rule out the possibility that Ida is a stony-iron body, even though that's what its spectrum suggests. Instead, Ida could well have a composition like that of ordinary chondrite meteorites, which are primitive and largely unaltered. Interestingly, the spectra of Ida and Dactyl are similar but different. It's thought that the binary system formed during the collision and breakup that created a family of asteroids with very similar orbits, to which Ida belongs.