DISCOVERY AND PHYSICAL-PROPERTIES OF DACTYL, A SATELLITE OF ASTEROID 243-IDA

OBSERVATIONS Of Stellar occultations by asteroids have suggested that some may have satellites(1), But given the absence of any confirmatory evidence, the prevailing view has been that although such satellites probably do exist, they are likely to be rare(2). Here we report the d iscovery(3) by the Galileo spacecraft of a satellite associated with t he asteroid 243 Ida, Although the satellite, Dactyl, is only 1.6 km ac ross, it has been imaged with sufficient resolution for geological ana lysis, We describe the physical properties of Dactyl, with emphasis on its notably smooth shape, its crater population (which includes a cra ter chain) and its photometric properties, We find that, spectroscopic ally, Dactyl resembles both Ida and the other members of the Koronis a steroid family, implying a similar composition; small spectral differe nces may reflect a space weathering process that slightly alters the c olours with time, We argue that Dactyl originated during the breakup o f the Koronis parent body, and that satellites could be common around other asteroids (particularly members of asteroid families).