Dactyl (moon)

Dactyl (Template:IPAc-en Template:Respell), formal designation (243) Ida I, is a small asteroid moon Template:Cvt, 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.<ref>Template:Cite news</ref> The satellite was named after the mythical creatures called dactyls, who, according to Greek mythology, lived on Mount Ida.<ref>Template:Cite web</ref>

Orbit

File:Dactyl potential orbits.svg

Diagram of potential orbits of Dactyl around Ida

Dactyl's orbit around Ida is not precisely known. Galileo was in the plane of Dactyl's orbit when most of the images were taken, which made determining its exact orbit difficult.<ref name="ByrnesD'Amario1994">Template:Harvnb</ref> Dactyl orbits in the prograde direction<ref name="PetitDurdaGreenbergHurford1997p179">Template:Harvnb</ref> and is inclined about 8° to Ida's equator.<ref name="PetitDurdaGreenbergHurford1997p177">Template:Harvnb</ref> Based on computer simulations, Dactyl's pericenter must be more than about Template:Convert from Ida for it to remain in a stable orbit.<ref name="PetitDurdaGreenbergHurford1997p195">Template:Harvnb</ref> The range of orbits generated by the simulations was narrowed down by the necessity of having the orbits pass through points at which Galileo observed Dactyl to be at 16:52:05 UT on 28 August 1993, about Template:Convert from Ida at longitude 85°.<ref name="PetitDurdaGreenbergHurford1997p188">Template:Harvnb</ref><ref name="PetitDurdaGreenbergHurford1997p193">Template:Harvnb</ref> On 26 April 1994, the Hubble Space Telescope observed Ida for eight hours and was unable to spot Dactyl. It would have been able to observe it if it were more than about Template:Convert from Ida.<ref name="ByrnesD'Amario1994" />

If in a circular orbit at the distance at which it was seen, Dactyl's orbital period would be about 20 hours.<ref name="ChapmanKlaasenBeltonVeverka1994p455">Template:Harvnb</ref> Its orbital speed is roughly Template:Convert, "about the speed of a fast run or a slowly thrown baseball".<ref name="ByrnesD'Amario1994" />

Origin

The origins of Dactyl are unclear, but two main hypotheses exist. The first is that Dactyl and Ida formed at the same time, and the second is that Dactyl was knocked loose by a later impact.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>

Dactyl was the first asteroid moon discovered. Its discovery settled the long debate over the existence of asteroid moons.