Editing 243 Ida (section)

== Dactyl ==
{{Main|Dactyl (moon)}}
[[File:Dactyl1.jpg|thumb|upright|An image of Dactyl captured by the ''Galileo'' spacecraft during its 1993 flyby.]]
Ida has a [[minor-planet moon|moon]] named '''Dactyl''', official designation '''(243) Ida I'''. It was discovered in images taken by the ''Galileo'' spacecraft during its flyby in 1993. These images provided the first direct confirmation of an asteroid moon.<ref name="Chapman1996p709" /> At the time, it was separated from Ida by a distance of {{convert|90|km}}, moving in a [[retrograde and prograde motion|prograde orbit]]. Dactyl is heavily cratered, like Ida, and consists of similar materials. Its origin is uncertain, but evidence from the flyby suggests that it originated as a fragment of the Koronis parent body.

=== Discovery ===
Dactyl was found on 17 February 1994 by ''Galileo'' mission member Ann Harch, while examining delayed image downloads from the spacecraft.<ref name="Holm1994" /> ''Galileo'' recorded 47 images of Dactyl over an observation period of 5.5 hours in August 1993.<ref name="PetitDurdaGreenbergHurford1997p177">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=177}}</ref> The spacecraft was {{convert|10760|km|mi}} from Ida<ref name="BeltonCarlson1994">{{harvnb|Belton|Carlson|1994}}</ref> and {{convert|10870|km|mi}} from Dactyl when the first image of the moon was captured, 14 minutes before ''Galileo'' made its closest approach.<ref name="Mason1994p108">{{harvnb|Mason|1994|p=108}}</ref>

Dactyl was initially [[Astronomical naming conventions#Natural satellites of planets|designated]] 1993 (243) 1.<ref name="BeltonCarlson1994" /><ref name="Green1994">{{harvnb|Green|1994}}</ref> It was named by the [[International Astronomical Union]] in 1994,<ref name="Green1994" /> for the mythological [[dactyls (mythology)|dactyls]] who inhabited [[Mount Ida (Crete)|Mount Ida]] on the island of Crete.<ref name="Schmadel2003p37">{{harvnb|Schmadel|2003|p=37}}</ref><ref name="Pausanias576">{{harvnb|Pausanias|5.7.6}} {{quote|When Zeus was born, Rhea entrusted the guardianship of her son to the Dactyls of Ida, who are the same as those called Curetes. They came from Cretan Ida&nbsp;– Heracles, Paeonaeus, Epimedes, Iasius and Idas.}}</ref>

=== Physical characteristics ===
Dactyl is an "egg-shaped"<ref name="Chapman1996p709" /> but "remarkably spherical"<ref name="Schmadel2003p37" /> object measuring {{convert|1.6 by 1.4 by 1.2|km|mi}}.<ref name="Chapman1996p709" /> It is oriented with its longest axis pointing towards Ida.<ref name="Chapman1996p709" /> Like Ida, Dactyl's surface exhibits saturation cratering.<ref name="Chapman1996p709" /> It is marked by more than a dozen craters with a diameter greater than {{convert|80|m|ft|abbr=on}}, indicating that the moon has suffered many collisions during its history.<ref name="NASA2005" /> At least six craters form a linear chain, suggesting that it was caused by locally produced debris, possibly ejected from Ida.<ref name="Chapman1996p709" /> Dactyl's craters may contain central peaks, unlike those found on Ida.<ref name="AsphaugRyanZuber2003p463">{{harvnb|Asphaug|Ryan|Zuber|2003|p=463}}</ref> These features, and Dactyl's [[spheroid]]al shape, imply that the moon is gravitationally controlled despite its small size.<ref name="AsphaugRyanZuber2003p463" /> Like Ida, its average temperature is about {{convert|200|K|C F|abbr=on}}.<ref name="Holm1994" />

Dactyl shares many characteristics with Ida. Their [[albedo]]s and reflection [[Spectrum|spectra]] are very similar.<ref name="ChapmanKlaasenBeltonVeverka1994p455">{{harvnb|Chapman|Klaasen|Belton|Veverka|1994|p=455}}</ref> The small differences indicate that the space weathering process is less active on Dactyl.<ref name="Chapman1995p496" /> Its small size would make the formation of significant amounts of [[regolith]] impossible.<ref name="Chapman1995p496" /><ref name="BeltonCarlson1994" /> This contrasts with Ida, which is covered by a deep layer of regolith.

The two largest imaged craters on Dactyl were named Acmon {{IPAc-en|ˈ|æ|k|m|ə|n}} and Celmis {{IPAc-en|ˈ|s|ɛ|l|m|ᵻ|s}}, after two of the mythological dactyls. Acmon is the largest crater in the above image, and Celmis is near the bottom of the image, mostly obscured in shadow. The craters are 300 and 200 meters in diameter, respectively.<ref name=dactylnamedfeatures>{{cite web|title=Planetary Names: Dactyl|url=http://planetarynames.wr.usgs.gov/Page/DACTYL/target|publisher=IAU|access-date=18 July 2015|archive-url=https://web.archive.org/web/20150701152709/http://planetarynames.wr.usgs.gov/Page/DACTYL/target|archive-date=2015-07-01|url-status=dead}}</ref>

=== Orbit ===
[[File:Dactyl potential orbits.svg|thumb|Diagram of potential orbits of Dactyl around Ida|left]]
Dactyl's orbit around Ida is not precisely known. ''Galileo'' was in the [[orbital plane|plane]] of Dactyl's orbit when most of the images were taken, which made determining its exact orbit difficult.<ref name="ByrnesD'Amario1994" /> Dactyl orbits in the [[retrograde and prograde motion|prograde]] direction<ref name="PetitDurdaGreenbergHurford1997p179">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=179}}</ref> and is inclined about 8° to Ida's equator.<ref name="PetitDurdaGreenbergHurford1997p177" /> Based on computer simulations, Dactyl's [[apsis|pericenter]] must be more than about {{convert|65|km|mi|abbr=on}} from Ida for it to remain in a stable orbit.<ref name="PetitDurdaGreenbergHurford1997p195">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=195}}</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 {{convert|90|km|mi|abbr=on}} from Ida at longitude 85°.<ref name="PetitDurdaGreenbergHurford1997p188">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=188}}</ref><ref name="PetitDurdaGreenbergHurford1997p193">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=193}}</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 {{convert|700|km|mi|abbr=on}} 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" /> Its orbital speed is roughly {{convert|10|m/s|ft/s|abbr=on}}, "about the speed of a fast run or a slowly thrown baseball".<ref name="ByrnesD'Amario1994" />

=== Age and origin ===
Dactyl may have originated at the same time as Ida,<ref name="GreenbergBottkeNolanGeissler1996p116">{{harvnb|Greenberg|Bottke|Nolan|Geissler|1996|p=116}}</ref> from the disruption of the Koronis parent body.<ref name="LeeVeverkaThomasHelfenstein1996p97" /> However, it may have formed more recently, perhaps as ejecta from a large impact on Ida.<ref name="PetitDurdaGreenbergHurford1997p182">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|p=182}}</ref> It is extremely unlikely that it was captured by Ida.<ref name="Mason1994p108" /> Dactyl may have suffered a major impact around 100&nbsp;million years ago, which reduced its size.<ref name="GreenbergBottkeNolanGeissler1996p117" />