Dactyl (/ˈdæktɪl/ DAK-til), formal designation (243) Ida I, is a small asteroid moon 1,200 by 1,400 by 1,600 m (3,900 by 4,600 by 5,200 ft), in size) that orbits 243 Ida, a main-belt asteroid. It was imaged by the Galileo spacecraft on August 28, 1993; Dactyl was discovered while examining the delayed image downloads from Galileo on February 17, 1994. It was provisionally designated S/1993 (243) 1. The satellite was named after the mythical creatures called dactyls, who, according to Greek mythology, lived on Mount Ida.
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Dactyl (moon) in the context of Binary asteroid
A binary asteroid is a system of two asteroids orbiting their common barycenter. The binary nature of 243 Ida was discovered when the Galileo spacecraft flew by the asteroid in 1993. Since then numerous binary asteroids and several triple asteroids have been detected.
The mass ratio of the two components – called the "primary" and "secondary" of a binary system – is an important characteristic. Most binary asteroids have a large mass ratio, i.e. a relatively small satellite in orbit around the main component. Systems with one or more small moons – also called "companions" or simply "satellites" – include 87 Sylvia, 107 Camilla and 45 Eugenia (all triples), 121 Hermione, 130 Elektra (a quadruple), 22 Kalliope, 283 Emma, 379 Huenna, 243 Ida and 4337 Arecibo (in order of decreasing primary size). Some binary systems have a mass ratio near unity, i.e., two components of similar mass. They include 90 Antiope, 2006 VW139, 2017 YE5 and 69230 Hermes, with average component diameters of 86, 1.8, 0.9 and 0.8 km, respectively.