View source for 243 Ida

{{short description|Main-belt asteroid}}
{{About|an asteroid|other uses|Ida (disambiguation)}}
{{pp-move-indef}}
{{Infobox planet
| background = #D6D6D6
| minorplanet = yes
| mpc_name = (243) Ida
| name = Ida
|named_after=[[Ida (nurse of Zeus)]]
| pronounced = {{IPAc-en|ˈ|aɪ|d|ə}}<ref>Noah Webster (1884) ''A Practical Dictionary of the English Language''</ref>
| adjectives = Idean (Idæan) {{IPAc-en|aɪ|ˈ|d|iː|ə|n}}<ref>{{OED|Idæan}}</ref>
| image = File:243 ida crop.jpg
| image_scale =
| caption = ''[[Galileo (spacecraft)|Galileo]]'' image of 243 Ida. [[Pola Regio]] is located on the right "tip" of the asteroid. The dot to the right is its moon [[Dactyl (moon)|Dactyl]].
| discovery_ref = <ref name="Raab2002"/>
| discoverer = [[Johann Palisa]]
| discovery_site = [[Vienna Observatory]]
| discovered = September 29, 1884
| mp_category = Main belt ([[Koronis family]])<ref name="Holm1994"/>
| orbit_ref = <ref name="JPL2008">{{harvnb|JPL|2008}}</ref>
| epoch = 31 July 2016 ([[Julian day|JD]] 2457600.5)
| semimajor = {{convert|2.861|AU|m|abbr=on}}
| perihelion = {{convert|2.743|AU|m|abbr=on}}
| aphelion = {{convert|2.979|AU|m|abbr=on}}
| eccentricity = 0.0411
| period = {{convert|1767.644|d|yr}}
| inclination = 1.132°
| asc_node = 324.016°
| arg_peri = 110.961°
| mean_anomaly = 38.707°
| avg_speed = 0.2036°/d
| satellites = Dactyl
| dimensions = 59.8 × 25.4 × 18.6&nbsp;km<ref name="Belton1996">{{harvnb|Belton et al.|1996}}</ref>
| mean_radius = 15.7&nbsp;km<ref name="BrittYeomansHousenConsolmagno2002p486">{{harvnb|Britt|Yeomans|Housen|Consolmagno|2002|p=486}}</ref>
| mass = 4.2 ± 0.6 ×10<sup>16</sup>&nbsp;kg<ref name="BrittYeomansHousenConsolmagno2002p486"/>
| density = 2.6 ± 0.5&nbsp;g/cm<sup>3</sup><ref name="Belton1995"/>
| surface_grav = 0.3–1.1&nbsp;cm/s<sup>2</sup><ref name="ThomasBeltonCarcichChapman1996"/>
| rotation = {{convert|4.63|h|d}}<ref name="VokrouhlickyNesvornyBottke2003p147"/>
| spectral_type = [[S-type asteroid|S]]<ref name="WilsonKeilLove1999p479"/>
| abs_magnitude = 9.94<ref name="JPL2008"/>
| albedo = 0.2383<ref name="JPL2008"/>
| right_asc_north_pole = 168.76°<ref name="Archinal2018"/>
| declination = −87.12°<ref name="Archinal2018"/>
| single_temperature = {{convert|200|K|C}}<ref name="Holm1994"/>
}}

'''243 Ida''' is an [[asteroid]] in the [[Koronis family]] of the [[asteroid belt]]. It was discovered on 29 September 1884 by Austrian astronomer [[Johann Palisa]] at [[Vienna Observatory]] and named after [[Ida (nurse of Zeus)|a nymph from Greek mythology]]. Later telescopic observations categorized Ida as an [[S-type asteroid]], the most numerous type in the inner [[asteroid belt]]. On 28 August 1993, Ida was visited by the uncrewed ''[[Galileo (spacecraft)|Galileo]]'' spacecraft while en route to [[Jupiter]]. It was the second asteroid visited by a spacecraft and the first found to have a [[natural satellite]].

Ida's orbit lies between the planets [[Mars]] and Jupiter, like all main-belt asteroids. Its [[orbital period]] is 4.84 years, and its rotation period is 4.63 hours. Ida has an average diameter of {{convert|31.4|km|mi|abbr=on}}. It is irregularly shaped and elongated, apparently composed of two large objects connected together. Its surface is one of the most heavily cratered in the Solar System, featuring a wide variety of crater sizes and ages.

Ida's moon [[Dactyl (moon)|Dactyl]] was discovered by mission member Ann Harch in images returned from ''Galileo''.<ref>{{Cite web |title=Hero astronomer who helped save spacecraft rewarded with celestial gift {{!}} Cornell Chronicle |url=https://news.cornell.edu/stories/1999/04/asteroid-named-astronomer-who-helped-save-near-mission |access-date=2025-07-30 |website=news.cornell.edu |language=en}}</ref> It was named after the [[Dactyls (mythology)|Dactyls]], creatures which inhabited Mount Ida in Greek mythology. Dactyl is only {{convert|1.4|km|mi}} in diameter, about 1/20 the size of Ida. Its orbit around Ida could not be determined with much accuracy, but the constraints of possible orbits allowed a rough determination of Ida's density and revealed that it is depleted of metallic minerals. Dactyl and Ida share many characteristics, suggesting a common origin.

The images returned from ''Galileo'' and the subsequent measurement of Ida's mass provided new insights into the geology of S-type asteroids. Before the ''Galileo'' flyby, many different theories had been proposed to explain their mineral composition. Determining their composition permits a correlation between meteorites falling to the Earth and their origin in the asteroid belt. Data returned from the flyby pointed to S-type asteroids as the source for the [[ordinary chondrite]] meteorites, the most common type found on the Earth's surface.

== Discovery and observations ==
Ida was discovered on 29 September 1884 by Austrian astronomer [[Johann Palisa]] at the [[Vienna Observatory]].<ref name="Ridpath1897p206">{{harvnb|Ridpath|1897|p=206}}</ref> It was his 45th asteroid discovery.<ref name="Raab2002">{{harvnb|Raab|2002}}</ref> Ida was [[List of minor planets#Numbering and naming conventions|named]] by [[Moriz von Kuffner]], a Viennese brewer and amateur astronomer.<ref name="Schmadel2003p36">{{harvnb|Schmadel|2003|p=36}}</ref><ref name="Berger2003p241">{{harvnb|Berger|2003|p=241}}</ref> In [[Greek mythology]], [[Idaea|Ida]] was a [[nymph]] of [[Crete]] who raised the god [[Zeus]].<ref name="NASA2005">{{harvnb|NASA|2005}}</ref> Ida was recognized as a member of the [[Koronis family]] by [[Kiyotsugu Hirayama]], who proposed in 1918 that the group comprised the remnants of a destroyed precursor body.<ref name="Chapman1996p700"/>

Ida's [[Astronomical spectroscopy#Planets, asteroids, and comets|reflection spectrum]] was measured on 16 September 1980 by astronomers [[David J. Tholen]] and Edward F. Tedesco as part of the eight-color asteroid survey (ECAS).<ref name="ZellnerTholenTedesco1985p357p373">{{harvnb|Zellner|Tholen|Tedesco|1985|pp=357, 373}}</ref> Its spectrum matched those of the asteroids in the S-type classification.<ref>{{harvnb|Zellner|Tholen|Tedesco|1985|p=404}} {{quote|The Eos and Koronis families&nbsp;... are entirely of type S, which is rare at their heliocentric distances ...}}</ref><ref name="ZellnerTholenTedesco1985p410">{{harvnb|Zellner|Tholen|Tedesco|1985|p=410}}</ref> Many observations of Ida were made in early 1993 by the [[United States Naval Observatory Flagstaff Station|US Naval Observatory in Flagstaff]] and the [[Oak Ridge Observatory]]. These improved the measurement of Ida's orbit around the Sun and reduced the uncertainty of its position during the ''Galileo'' flyby from {{convert|78|to|60|km|mi|abbr=on}}.<ref name="OwenYeomans1994p2295">{{harvnb|Owen|Yeomans|1994|p=2295}}</ref>

== Exploration ==
=== ''Galileo'' flyby ===
Ida was visited in 1993 by the [[Jupiter]]-bound space probe ''[[Galileo (spacecraft)|Galileo]]''. Its encounters of the asteroids [[951 Gaspra|Gaspra]] and Ida were secondary to the Jupiter mission. These were selected as targets in response to a new NASA policy directing mission planners to consider asteroid flybys for all spacecraft crossing the belt.<ref name="D'AmarioBrightWolf1992p26">{{harvnb|D'Amario|Bright|Wolf|1992|p=26}}</ref> No prior missions had attempted such a flyby.<ref name="Chapman1996p699">{{harvnb|Chapman|1996|p=699}}</ref> ''Galileo'' was launched into orbit by the [[Space Shuttle Atlantis|Space Shuttle ''Atlantis'']] mission [[STS-34]] on 18 October 1989.<ref name="D'AmarioBrightWolf1992p24">{{harvnb|D'Amario|Bright|Wolf|1992|p=24}}</ref> Changing ''Galileo's'' trajectory to approach Ida required that it consume {{convert|34|kg|lb|abbr=on}} of [[propellant]].<ref name="D'AmarioBrightWolf1992p72">{{harvnb|D'Amario|Bright|Wolf|1992|p=72}}</ref> Mission planners delayed the decision to attempt a flyby until they were certain that this would leave the spacecraft enough propellant to complete its Jupiter mission.<ref name="D'AmarioBrightWolf1992p36">{{harvnb|D'Amario|Bright|Wolf|1992|p=36}}</ref>

''Galileo's'' trajectory carried it into the asteroid belt twice on its way to Jupiter. During its second crossing, it flew by Ida on 28 August 1993 at a speed of {{convert|12400|m/s|abbr=on}} relative to the asteroid.<ref name="D'AmarioBrightWolf1992p36"/> The onboard imager observed Ida from a distance of {{convert|240350|km|mi|abbr=on|sigfig=5}} to its closest approach of {{convert|2390|km|mi|abbr=on|sigfig=3}}.<ref name="NASA2005"/><ref name="SullivanGreeleyPappalardoAsphaug1996p120">{{harvnb|Sullivan|Greeley|Pappalardo|Asphaug|1996|p=120}}</ref> Ida was the second asteroid, after Gaspra, to be imaged by a spacecraft.<ref name="Cowen1993p215">{{harvnb|Cowen|1993|p=215}} {{quote|Nearly a month after a successful photo session, the Galileo spacecraft last week finished radioing to Earth a high-resolution portrait of the second asteroid ever to be imaged from space. Known as 243 Ida, the asteroid was photographed from an average distance of just 3,400 kilometers some 3.5 minutes before Galileo's closest approach on Aug. 28.}}</ref> About 95% of Ida's surface came into view of the probe during the flyby.<ref name="ThomasBeltonCarcichChapman1996">{{harvnb|Thomas|Belton|Carcich|Chapman|1996}}</ref>

Transmission of many Ida images was delayed due to a permanent failure in the spacecraft's [[directional antenna|high-gain antenna]].<ref name="Chapman1994p358">{{harvnb|Chapman|1994|p=358}}</ref> The first five images were received in September 1993.<ref name="Chapman1996p707">{{harvnb|Chapman|1996|p=707}}</ref> These comprised a high-resolution [[Image stitching|mosaic]] of the asteroid at a resolution of 31–38&nbsp;m/[[pixel]].<ref name="ChapmanBeltonVeverkaNeukum1994p237">{{harvnb|Chapman|Belton|Veverka|Neukum|1994|p=237}}</ref><ref name="GreeleySullivanPappalardoVeverka1994p469">{{harvnb|Greeley|Sullivan|Pappalardo|Veverka|1994|p=469}}</ref> The remaining images were sent in February 1994,<ref name="Holm1994" /> when the spacecraft's proximity to the Earth allowed higher speed transmissions.<ref name="Chapman1996p707"/><ref name="MonetStoneMonetDahn1994p2293">{{harvnb|Monet|Stone|Monet|Dahn|1994|p=2293}}</ref>

{{Multiple image
| direction         = horizontal
| image1            = Animation of Galileo trajectory.gif
| image2            = Galileo trajectory Ida.svg
| total_width       = 600
| image3            = Ida-approach.gif
| alt1              = Animation of Galileo's trajectory
| caption1          = Animation of ''Galileo''{{'s}} trajectory from 19 October 1989 to 30 September 2003 <br /> {{legend2|magenta|''Galileo''}}{{·}}{{legend2|lime|[[Jupiter]]}}{{·}}{{legend2|royalblue|[[Earth]]}}{{·}}{{legend2|PaleGreen| [[Venus]]}}{{·}}{{legend2| Gold |[[951 Gaspra]]}}{{·}}{{legend2| Cyan |243 Ida}}
| alt2              = Trajectory of Galileo
| caption2          = Trajectory of Galileo from launch to Jupiter orbital insertion
| caption3          = Images from the flyby, starting 5.4 hours before closest approach and showing Ida's rotation
| alt3              = Images from the flyby
| align             = center
}}

=== Discoveries ===
The data returned from the ''Galileo'' flybys of Gaspra and Ida, and the later ''[[NEAR Shoemaker]]'' asteroid mission, permitted the first study of asteroid [[geology]].<ref name="GeisslerPetitGreenberg1996p57">{{harvnb|Geissler|Petit|Greenberg|1996|p=57}}</ref> Ida's relatively large surface exhibited a diverse range of geological features.<ref name="ChapmanBeltonVeverkaNeukum1994p238">{{harvnb|Chapman|Belton|Veverka|Neukum|1994|p=238}}</ref> The discovery of Ida's moon [[Dactyl (moon)#Dactyl|Dactyl]], the first confirmed satellite of an asteroid, provided additional insights into Ida's composition.<ref name="Chapman1996p709">{{harvnb|Chapman|1996|p=709}}</ref>

Ida is classified as an [[S-type asteroid]] based on ground-based [[Spectroscopy|spectroscopic measurements]].<ref name="ByrnesD'Amario1994">{{harvnb|Byrnes|D'Amario|1994}}</ref> The composition of S-types was uncertain before the ''Galileo'' flybys, but was interpreted to be either of two minerals found in meteorites that had fallen to the Earth: [[ordinary chondrite]] (OC) and [[Stony-iron meteorite|stony-iron]].<ref name="WilsonKeilLove1999p479">{{harvnb|Wilson|Keil|Love|1999|p=479}}</ref> Estimates of Ida's density are constrained to less than 3.2&nbsp;g/cm<sup>3</sup> by the long-term stability of Dactyl's orbit.<ref name="ByrnesD'Amario1994" /> This all but rules out a [[Stony-iron meteorite|stony-iron]] composition; were Ida made of 5&nbsp;g/cm<sup>3</sup> iron- and nickel-rich material, it would have to contain more than 40% empty space.<ref name="Chapman1996p709" />

The Galileo images also led to the discovery that [[space weathering]] was taking place on Ida, a process which causes older regions to become more red in color over time.<ref name="Chapman1996p700">{{harvnb|Chapman|1996|p=700}}</ref><ref name="Chapman1996p710">{{harvnb|Chapman|1996|p=710}}</ref> The same process affects both Ida and its moon, although Dactyl shows a lesser change.<ref name="Chapman1995p496">{{harvnb|Chapman|1995|p=496}}</ref> The weathering of Ida's surface revealed another detail about its composition: the reflection spectra of freshly exposed parts of the surface resembled that of OC meteorites, but the older regions matched the spectra of S-type asteroids.<ref name="Chapman1996p699" />[[File:NWA869Meteorite.jpg|thumb|Polished section of an [[ordinary chondrite]] meteorite]]Both of these discoveries—the space weathering effects and the low density—led to a new understanding about the relationship between S-type asteroids and OC meteorites. S-types are the most numerous kind of asteroid in the inner part of the asteroid belt.<ref name="Chapman1996p699"/> OC meteorites are, likewise, the most common type of meteorite found on the Earth's surface.<ref name="Chapman1996p699"/> The reflection spectra measured by remote observations of S-type asteroids, however, did not match that of OC meteorites. The ''Galileo'' flyby of Ida found that some S-types, particularly the [[Koronis family]], could be the source of these meteorites.<ref name="Chapman1995p496"/>

== Physical characteristics ==
{{Multiple image
| image1            = Asteroid size comparison.jpg
| image2            = 243 Ida rotation.jpg
| total_width       = 400
| align             = left
| direction         = horizontal
| alt1              = Size comparison of Ida
| caption1          = Size comparison of Ida, several other asteroids, the dwarf planet Ceres, and Mars
| alt2              = Images of Ida rotating
| caption2          = Successive images of a rotating Ida
}}
Ida's mass is between 3.65 and 4.99&nbsp;×&nbsp;10<sup>16</sup>&nbsp;kg.<ref name="PetitDurdaGreenbergHurford1997p179t180">{{harvnb|Petit|Durda|Greenberg|Hurford|1997|pp=179–180}}</ref> Its [[gravitational field]] produces an acceleration of about 0.3 to 1.1&nbsp;cm/s<sup>2</sup> over its surface.<ref name="ThomasBeltonCarcichChapman1996"/> This field is so weak that an astronaut standing on its surface could leap from one end of Ida to the other, and an object moving in excess of {{convert|20|m/s|ft/s|sigfig=1|abbr=on}} could [[escape velocity|escape]] the asteroid entirely.<ref name="GeisslerPetitDurdaGreenberg1996p142">{{harvnb|Geissler|Petit|Durda|Greenberg|1996|p=142}}</ref><ref name="LeeVeverkaThomasHelfenstein1996p99">{{harvnb|Lee|Veverka|Thomas|Helfenstein|1996|p=99}}</ref>

Ida is a distinctly elongated asteroid,<ref name="GeisslerPetitGreenberg1996p58">{{harvnb|Geissler|Petit|Greenberg|1996|p=58}}</ref> with an irregular surface.<ref name="Chapman1994p363">{{harvnb|Chapman|1994|p=363}}</ref><ref name="BottkeCellinoPaolicchiBinzel2002p10">{{harvnb|Bottke|Cellino|Paolicchi|Binzel|2002|p=10}}</ref> Ida is 2.35 times as long as it is wide,<ref name="GeisslerPetitGreenberg1996p58"/> and a "waist" separates it into two geologically dissimilar halves.<ref name="Chapman1996p707"/> This constricted shape is consistent with Ida being made of two large, solid components, with loose [[debris]] filling the gap between them. However, no such debris was seen in high-resolution images captured by ''Galileo''.<ref name="BottkeCellinoPaolicchiBinzel2002p10"/> Although there are a few steep slopes tilting up to about 50° on Ida, the slope generally does not exceed 35°.<ref name="ThomasBeltonCarcichChapman1996"/> Ida's irregular shape is responsible for the asteroid's very uneven gravitational field.<ref name="Cowen1995">{{harvnb|Cowen|1995}}</ref> The surface acceleration is lowest at the extremities because of their high rotational speed. It is also low near the "waist" because the mass of the asteroid is concentrated in the two halves, away from this location.<ref name="ThomasBeltonCarcichChapman1996"/>

== Surface features ==
[[File:243 Ida large.jpg|thumb|left|Mosaic of images recorded by ''Galileo'' 3.5&nbsp;minutes before its closest approach]]
Ida's surface appears heavily [[Impact crater|cratered]] and mostly gray, although minor color variations mark newly formed or uncovered areas.<ref name="NASA2005"/> Besides craters, other features are evident, such as grooves, ridges, and protrusions. Ida is covered by a thick layer of [[regolith]], loose debris that obscures the solid rock beneath. The largest, boulder-sized, debris fragments are called ''[[ejecta block]]s'', several of which have been observed on the surface.

=== Regolith ===
The surface of Ida is covered in a blanket of pulverized rock, called ''[[regolith]]'', about {{convert|50|-|100|m|ft|abbr=on}} thick.<ref name="Chapman1996p707" /> This material is produced in [[impact event]]s and redistributed across Ida's surface by geological processes.<ref name="LeeVeverkaThomasHelfenstein1996p96">{{harvnb|Lee|Veverka|Thomas|Helfenstein|1996|p=96}}</ref> ''Galileo'' observed evidence of recent [[Mass wasting|downslope regolith movement]].<ref name="GreeleySullivanPappalardoVeverka1994p470">{{harvnb|Greeley|Sullivan|Pappalardo|Veverka|1994|p=470}}</ref>

Ida's regolith is composed of the [[silicate]] minerals [[olivine]] and [[pyroxene]].<ref name="Holm1994">{{harvnb|Holm|1994}}</ref><ref name="Chapman1996p701">{{harvnb|Chapman|1996|p=701}}</ref> Its appearance changes over time through a process called [[space weathering]].<ref name="Chapman1995p496" /> Because of this process, older regolith appears more red in color compared to freshly exposed material.<ref name="Chapman1996p710" />

[[File:Ejecta block on 243 Ida.svg|thumb|''Galileo'' image of a {{convert|150|m|ft|abbr=on}} block at 24.8°S, 2.8°E<ref name="LeeVeverkaThomasHelfenstein1996p90">{{harvnb|Lee|Veverka|Thomas|Helfenstein|1996|p=90}}</ref>|216x216px]]
About 20 large (40–150&nbsp;m across) ejecta blocks have been identified, embedded in Ida's regolith.<ref name="Chapman1996p707" /><ref name="GeisslerPetitDurdaGreenberg1996p141" /> Ejecta blocks constitute the largest pieces of the regolith.<ref name="SullivanGreeleyPappalardoAsphaug1996p132">{{harvnb|Sullivan|Greeley|Pappalardo|Asphaug|1996|p=132}}</ref> Because ejecta blocks are expected to break down quickly by impact events, those present on the surface must have been either formed recently or uncovered by an impact event.<ref name="Cowen1995" /><ref name="LeeVeverkaThomasHelfenstein1996p97">{{harvnb|Lee|Veverka|Thomas|Helfenstein|1996|p=97}}</ref> Most of them are located within the craters Lascaux and Mammoth, but they may not have been produced there.<ref name="LeeVeverkaThomasHelfenstein1996p97" /> This area attracts debris due to Ida's irregular gravitational field.<ref name="Cowen1995" /> Some blocks may have been ejected from the young crater Azzurra on the opposite side of the asteroid.<ref name="Stooke1997p1385" />

=== Structures ===
Several major structures mark Ida's surface. The asteroid appears to be split into two halves, here referred to as ''region 1'' and ''region 2'', connected by a "waist".<ref name="Chapman1996p707" /> This feature may have been filled in by debris, or blasted out of the asteroid by impacts.<ref name="Chapman1996p707" /><ref name="Stooke1997p1385">{{harvnb|Stooke|1997|p=1385}}</ref>

Region 1 of Ida contains two major structures. One is a prominent {{convert|40|km|mi|abbr=on}} ridge named ''Townsend Dorsum'' that stretches 150 degrees around Ida's surface.<ref name="SárneczkyKereszturi2002">{{harvnb|Sárneczky|Kereszturi|2002}}</ref> The other structure is a large indentation named ''Vienna Regio''.<ref name="Chapman1996p707" />

Ida's region 2 features several sets of grooves, most of which are {{convert|100|m|ft|abbr=on}} wide or less and up to {{convert|4|km|mi|abbr=on}} long.<ref name="Chapman1996p707" /><ref name="SullivanGreeleyPappalardoAsphaug1996p131">{{harvnb|Sullivan|Greeley|Pappalardo|Asphaug|1996|p=131}}</ref> They are located near, but are not connected with, the craters Mammoth, Lascaux, and Kartchner.<ref name="SullivanGreeleyPappalardoAsphaug1996p132" /> Some grooves are related to major impact events, for example a set opposite Vienna Regio.<ref name="ThomasProckter2004">{{harvnb|Thomas|Prockter|2004}}</ref>

=== Craters ===
Ida is one of the most densely cratered bodies yet explored in the Solar System,<ref name="ChapmanBeltonVeverkaNeukum1994p237" /><ref name="Chapman1994p363" /> and impacts have been the primary process shaping its surface.<ref>{{harvnb|Geissler|Petit|Greenberg|1996|pp=57–58}}</ref> Cratering has reached the saturation point, meaning that new impacts erase evidence of old ones, leaving the total crater count roughly the same.<ref name="Chapman1996p707p708">{{harvnb|Chapman|1996|pp=707–708}}</ref> It is covered with craters of all sizes and stages of degradation,<ref name="Chapman1994p363" /> and ranging in age from fresh to as old as Ida itself.<ref name="Chapman1996p707" /> The oldest may have been formed during the breakup of the [[Koronis family]] [[parent body]].<ref name="Chapman1995p496" /> The largest crater, Lascaux, is almost {{convert|12|km|mi|abbr=on}} across.<ref name="BottkeCellinoPaolicchiBinzel2002p10" /><ref name="USGS">{{harvnb|USGS}}</ref> Region 2 contains nearly all of the craters larger than {{convert|6|km|mi|abbr=on}} in diameter, but Region 1 has no large craters at all.<ref name="Chapman1996p707" /> Some craters are arranged in chains.<ref name="GreeleySullivanPappalardoVeverka1994p469" />
[[File:Fingal on 243 Ida.svg|left|thumb|360x360px|Asymmetric {{convert|1.5|km|mi|abbr=on}} wide crater Fingal at 13.2°S, 39.9°E<ref name="USGS" />]]
Ida's major craters are named after caves and [[lava tube]]s on Earth. The crater Azzurra, for example, is named after a submerged cave on the island of [[Capri]], also known as the ''[[Blue Grotto (Capri)|Blue Grotto]]''.<ref name="GreeleyBatson2001p393">{{harvnb|Greeley|Batson|2001|p=393}}</ref> Azzurra seems to be the most recent major impact on Ida.<ref name="GeisslerPetitDurdaGreenberg1996p141">{{harvnb|Geissler|Petit|Durda|Greenberg|1996|p=141}}</ref> The ejecta from this collision is distributed discontinuously over Ida<ref name="Chapman1996p710" /> and is responsible for the large-scale color and [[albedo]] variations across its surface.<ref>{{harvnb|Bottke|Cellino|Paolicchi|Binzel|2002|p=9}}</ref> An exception to the crater morphology is the fresh, asymmetric Fingal, which has a sharp boundary between the floor and wall on one side.<ref name="pappalardo124" /> Another significant crater is Afon, which marks Ida's [[prime meridian]].<ref name="Archinal2018"/>

The craters are simple in structure: bowl-shaped with no flat bottoms and no central peaks.<ref name="pappalardo124">{{harvnb|Sullivan|Greeley|Pappalardo|Asphaug|1996|p=124}}</ref> They are distributed evenly around Ida, except for a protrusion north of crater Choukoutien which is smoother and less cratered.<ref>{{harvnb|Sullivan|Greeley|Pappalardo|Asphaug|1996|p=128}}</ref> The [[ejecta]] excavated by impacts is deposited differently on Ida than on planets because of its rapid rotation, low gravity and irregular shape.<ref name="GeisslerPetitGreenberg1996p58" /> [[Ejecta blanket]]s settle asymmetrically around their craters, but fast-moving ejecta that escapes from the asteroid is permanently lost.<ref>{{harvnb|Geissler|Petit|Durda|Greenberg|1996|p=155}}</ref>

== Composition ==
Ida was classified as an [[S-type asteroid]] based on the similarity of its reflectance spectra with similar asteroids.<ref name="WilsonKeilLove1999p479" /> S-types may share their composition with stony-iron or [[ordinary chondrite]] (OC) meteorites.<ref name="WilsonKeilLove1999p479" /> The composition of the interior has not been directly analyzed, but is assumed to be similar to OC material based on observed surface color changes and Ida's [[bulk density]] of 2.27–3.10 g/cm<sup>3</sup>.<ref name="Chapman1995p496" /><ref name="WilsonKeilLove1999p480">{{harvnb|Wilson|Keil|Love|1999|p=480}}</ref> OC meteorites contain varying amounts of the silicates [[olivine]] and [[pyroxene]], [[iron]], and [[feldspar]].<ref name="Lewis1996p89">{{harvnb|Lewis|1996|p=89}} {{quote|The chondrites fall naturally into five composition classes, of which three have very similar mineral contents, but different proportions of metal and silicates. All three contain abundant iron in three different forms (ferrous iron oxide in silicates, metallic iron, and ferrous sulfide), usually with all three abundant enough to be classified as potential ores. All three contain feldspar (an aluminosilicate of calcium, sodium, and potassium), pyroxene (silicates with one silicon atom for each atom of magnesium, iron, or calcium), olivine (silicates with two iron or magnesium atoms per silicon atom), metallic iron, and iron sulfide (the mineral [[troilite]]). These three classes, referred to collectively as the ordinary chondrites, contain quite different amounts of metal.}}</ref> Olivine and pyroxene were detected on Ida by ''Galileo''.<ref name="Holm1994" /> The mineral content appears to be homogeneous throughout its extent. ''Galileo'' found minimal variations on the surface, and the asteroid's spin indicates a consistent density.<ref name="ThomasProckter2004p21">{{harvnb|Thomas|Prockter|2004|p=21}}</ref><ref name="SullivanGreeleyPappalardoAsphaug1996p135">{{harvnb|Sullivan|Greeley|Pappalardo|Asphaug|1996|p=135}}</ref> Assuming that its composition is similar to OC meteorites, which range in density from 3.48 to 3.64 g/cm<sup>3</sup>, Ida would have a [[porosity]] of 11–42%.<ref name="WilsonKeilLove1999p480" />

Ida's interior probably contains some amount of impact-fractured rock, called ''[[regolith|megaregolith]]''. The megaregolith layer of Ida extends between hundreds of meters below the surface to a few kilometers. Some rock in Ida's core may have been fractured below the large craters Mammoth, Lascaux, and Undara.<ref name="SullivanGreeleyPappalardoAsphaug1996p135" />

== Orbit and rotation ==
[[File:Ida orbit.svg|thumb|Orbit and positions of Ida and five planets as of 9 March 2009]]
Ida is a member of the [[Koronis family]] of [[asteroid belt|asteroid-belt]] asteroids.<ref name="Chapman1996p700" /> Ida orbits the Sun at an average distance of {{convert|2.862|AU|Gm|abbr=on}}, between the orbits of [[Mars]] and [[Jupiter]].<ref name="Holm1994" /><ref name="JPL2008" /> Ida takes 4.84089 years to complete one orbit.<ref name="JPL2008" />

Ida rotates in the [[retrograde motion|retrograde direction]] with a [[rotation period]] of 4.63 hours (roughly 5 hours).<ref name="VokrouhlickyNesvornyBottke2003p147">{{harvnb|Vokrouhlicky|Nesvorny|Bottke|2003|p=147}}</ref><ref name="GeisslerPetitGreenberg1996p58" /><ref name="Archinal2018"/> The calculated maximum [[moment of inertia]] of a uniformly dense object the same shape as Ida coincides with the spin axis of the asteroid. This suggests that there are no major variations of density within the asteroid.<ref name="ThomasProckter2004" /> Ida's axis of rotation [[precession#Astronomy|precesses]] with a period of 77 thousand years, due to the gravity of the Sun acting upon the nonspherical shape of the asteroid.<ref name="Slivan1995p134">{{harvnb|Slivan|1995|p=134}}</ref>

== Origin ==
Ida originated in the breakup of the roughly {{convert|120|km|mi|abbr=on|sigfig=2}} diameter Koronis parent body.<ref name="VokrouhlickyNesvornyBottke2003p147" /> The progenitor asteroid had partially differentiated, with heavier metals migrating to the core.<ref name="GreenbergBottkeNolanGeissler1996p117">{{harvnb|Greenberg|Bottke|Nolan|Geissler|1996|p=117}}</ref> Ida carried away insignificant amounts of this core material.<ref name="GreenbergBottkeNolanGeissler1996p117" /> It is uncertain how long ago the disruption event occurred. According to an analysis of Ida's cratering processes, its surface is more than a billion years old.<ref name="GreenbergBottkeNolanGeissler1996p117" /> However, this is inconsistent with the estimated age of the Ida–Dactyl system of less than 100&nbsp;million years;<ref name="HurfordGreenberg2000p1595">{{harvnb|Hurford|Greenberg|2000|p=1595}}</ref> it is unlikely that Dactyl, due to its small size, could have escaped being destroyed in a major collision for longer. The difference in age estimates may be explained by an increased rate of cratering from the debris of the Koronis parent body's destruction.<ref name="CarrollOstlie1996p878">{{harvnb|Carroll|Ostlie|1996|p=878}}</ref>

== 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=https://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" />

== See also ==
* [[List of geological features on Ida and Dactyl|List of geological features on 243 Ida and Dactyl]]
* [[List of minor planets]]

== Notes ==
<references>

<ref name="Archinal2018">{{harvnb|Archinal, Acton, A'Hearn et al.|2018|p=6, 15–16}}</ref>

<ref name="Belton1995">{{cite journal |display-authors=6 |last1=Belton |first1=M. J. S. |last2=Chapman |first2=C. R. |last3=Thomas |first3=P. C. |last4=Davies |first4=M. E. |last5=Greenberg |first5=R. |last6=Klaasen |first6=K. |last7=Byrnes |first7=D. |last8=D'Amario |first8=L. |last9=Synnott |first9=S. |last10=Johnson |first10=T. V. |last11=McEwen |first11=A. |last12=Merline |first12=W. J. |last13=Davis |first13=D. R. |last14=Petit |first14=J.-M. |last15=Storrs |first15=A. |last16=Veverka |first16=J. |last17=Zellner |first17=B. |year=1995 |title=Bulk density of asteroid 243 Ida from the orbit of its satellite Dactyl |journal=Nature |volume=374 |issue=6525 |pages=785–788 |doi=10.1038/374785a0|bibcode = 1995Natur.374..785B |s2cid=4333634 }}</ref>

</references>

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{{Refend}}

===Books===
{{Refbegin|colwidth=60em}}
*{{cite book|last=Berger|first=Peter|editor-last=Gourvish|editor-first=Terry|year=2003|title=Business and Politics in Europe, 1900–1970|chapter=The Gildemeester Organisation for Assistance to Emigrants and the expulsion of Jews from Vienna, 1938–1942|publisher=Cambridge University Press|location=Cambridge, UK|url=https://books.google.com/books?id=uK99Gg2zBRQC|isbn=978-0-521-82344-9|access-date=3 June 2020|archive-date=15 June 2024|archive-url=https://web.archive.org/web/20240615051928/https://books.google.com/books?id=uK99Gg2zBRQC|url-status=live}}
*{{cite book|last = Carroll|first = Bradley W.|author2= Ostlie, Dale A.|title = An Introduction to Modern Astrophysics|publisher = Addison-Wesley Publishing Company|year = 1996|isbn = 978-0-201-54730-6|ref=CITEREFCarrollOstlie1996}}
*{{cite book|last=Greeley|first=Ronald|author2=Batson, Raymond M.|year=2001|title=The Compact NASA Atlas of the Solar System|url=https://archive.org/details/compactnasaatlas0000gree|url-access=registration|publisher=Cambridge University Press|location=Cambridge, UK|isbn=978-0-521-80633-6|ref=CITEREFGreeleyBatson2001}}
*{{cite book|last=Lewis|first=John S.|author-link=John S. Lewis|year=1996|title=Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets|publisher=Addison-Wesley|location=Reading, MA|isbn=978-0-201-47959-1|title-link=Mining the Sky: Untold Riches from the Asteroids, Comets, and Planets}}
*{{cite book|author=Pausanias|translator=Jones, W. H. S.|translator2=Omerod, H. A.|publisher=Loeb Classical Library|isbn=978-0-674-99104-0|title=Description of Greece|year=1916|url=https://www.theoi.com/Text/Pausanias1A.html|ref=CITEREFPausanias5.7.6|access-date=28 February 2009|archive-date=18 November 2019|archive-url=https://web.archive.org/web/20191118212723/https://www.theoi.com/Text/Pausanias1A.html|url-status=live}}
*{{cite book|last=Ridpath|first=John Clark|title=The Standard American Encyclopedia of Arts, Sciences, History, Biography, Geography, Statistics, and General Knowledge|publisher=Encyclopedia Publishing|year=1897|url=https://books.google.com/books?id=1GMMAAAAYAAJ|access-date=3 June 2020|archive-date=15 June 2024|archive-url=https://web.archive.org/web/20240615051928/https://books.google.com/books?id=1GMMAAAAYAAJ|url-status=live}}
*{{cite book|last=Schmadel|first=Lutz D.|chapter=Catalogue of Minor Planet Names and Discovery Circumstances|title=Dictionary of minor planet names|series=IAU commission|volume=20|publisher=Springer|year=2003|chapter-url=https://books.google.com/books?id=KWrB1jPCa8AC|isbn=978-3-540-00238-3|access-date=23 October 2015|archive-date=15 June 2024|archive-url=https://web.archive.org/web/20240615051928/https://books.google.com/books?id=KWrB1jPCa8AC|url-status=live}}
*{{cite thesis|last=Slivan|first=Stephen Michael|title=Spin-Axis Alignment of Koronis Family Asteroids|publisher=Massachusetts Institute of Technology|date=June 1995|hdl=1721.1/11867|oclc=32907677|type=Thesis}}
*{{cite book|last=Thomas|first=Peter C.|author2=Prockter, Louise M.|chapter=Tectonics of Small Bodies|title=Planetary Tectonics|series=Cambridge Planetary Science|volume=11|publisher=Cambridge University Press|date=28 May 2004|chapter-url=http://mahi.ucsd.edu/johnson/erth01/Asteroid_tectonics.pdf|access-date=2008-11-29|isbn=978-0-521-76573-2|ref=CITEREFThomasProckter2004|url-status=dead|archive-url=https://web.archive.org/web/20090304170518/http://mahi.ucsd.edu/johnson/erth01/Asteroid_tectonics.pdf|archive-date=4 March 2009}}
{{Refend}}

===Other===
{{Refbegin|colwidth=60em}}
*{{cite journal|last=Belton|first=Michael J. S.|author2=Carlson, R.|title=1993 (243) 1|journal=IAU Circular|issue=5948|pages=2|date=12 March 1994|url=http://www.cbat.eps.harvard.edu/iauc/05900/05948.html|bibcode=1994IAUC.5948....2B|ref=CITEREFBeltonCarlson1994|access-date=5 July 2011|archive-date=1 February 2019|archive-url=https://web.archive.org/web/20190201172138/http://www.cbat.eps.harvard.edu/iauc/05900/05948.html|url-status=live}}
*{{cite journal |last1=Byrnes |first1=Dennis V. |author2=D'Amario, Louis A. |author3=Galileo Navigation Team |title=Solving for Dactyl's Orbit and Ida's Density |journal=The Galileo Messenger |issue=35 |date=December 1994 |url=http://www2.jpl.nasa.gov/galileo/mess35/DACTYL.html |archive-url=https://web.archive.org/web/19970105233902/http://www.jpl.nasa.gov/galileo/mess35/DACTYL.html |url-status=dead |archive-date=1997-01-05 |access-date=2008-10-23 |ref=CITEREFByrnesD'Amario1994 }}
*{{cite journal |last1=Chapman |first1=Clark R. |author2=Belton, Michael J. S. |author3=Veverka, Joseph |author4=Neukum, G. |author5=Head, J. |author6=Greeley, Ronald |author7=Klaasen, K. |author8=Morrison, D.|title=First Galileo image of asteroid 243 Ida|journal=Abstracts of the 25th Lunar and Planetary Science Conference|date=March 1994|pages=237–238|bibcode=1994LPI....25..237C|ref=CITEREFChapmanBeltonVeverkaNeukum1994}}
*{{cite magazine|last=Cowen|first=Ron|title=Close-up of an asteroid: Galileo eyes Ida|magazine=Science News|volume=144|issue=14|date=2 October 1993|page=215|issn=0036-8423}}
*{{cite magazine|last=Cowen|first=Ron|title=Idiosyncrasies of Ida—asteroid 243 Ida's irregular gravitational field|magazine=Science News|volume=147|issue=15|date=1 April 1995|page=207|url=http://www.sciencenews.org/pages/pdfs/data/1995/147-13/14713-14.pdf|access-date=2009-03-26|issn=0036-8423|archive-date=27 March 2012|archive-url=https://web.archive.org/web/20120327181135/http://www.sciencenews.org/pages/pdfs/data/1995/147-13/14713-14.pdf|url-status=dead}}
*{{cite journal |last1=Greeley |first1=Ronald |author2=Sullivan, Robert J. |author3=Pappalardo, R. |author4=Head, J. |author5=Veverka, Joseph |author6=Thomas, Peter C. |author7=Lee, P. |author8=Belton, M. |author9=Chapman, Clark R. |title=Morphology and Geology of Asteroid Ida: Preliminary Galileo Imaging Observations|journal=Abstracts of the 25th Lunar and Planetary Science Conference|date=March 1994|pages=469–470|bibcode=1994LPI....25..469G |ref=CITEREFGreeleySullivanPappalardoVeverka1994}}
*{{cite journal|last=Green|first=Daniel W. E.|title=1993 (243) 1 = (243) Ida I (Dactyl)|journal=IAU Circular|issue=6082|pages=2|date=26 September 1994|url=http://www.cbat.eps.harvard.edu/iauc/06000/06082.html|bibcode=1994IAUC.6082....2G|access-date=5 July 2011|archive-date=1 February 2019|archive-url=https://web.archive.org/web/20190201171944/http://www.cbat.eps.harvard.edu/iauc/06000/06082.html|url-status=live}}
*{{cite journal|last=Holm|first=Jeanne|editor=Jones, Jan|title=Discovery of Ida's Moon Indicates Possible "Families" of Asteroids|journal=The Galileo Messenger|issue=34|date=June 1994|url=http://www2.jpl.nasa.gov/galileo/mess34/Moon2.html|access-date=2008-10-23|archive-date=24 June 2010|url-status=dead|archive-url=https://web.archive.org/web/20100624204322/http://www2.jpl.nasa.gov/galileo/mess34/Moon2.html}} [http://planetary.s3.amazonaws.com/galileo_messenger/GM034.pdf Alt URL] {{Webarchive|url=https://web.archive.org/web/20190806230230/http://planetary.s3.amazonaws.com/galileo_messenger/GM034.pdf |date=6 August 2019 }}
*{{cite journal|last=Raab|first=Herbert|title=Johann Palisa, the most successful visual discoverer of asteroids|journal=Meeting on Asteroids and Comets in Europe|year=2002|url=http://www.astrometrica.at/Papers/Palisa.pdf|access-date=2008-10-23|archive-url=https://web.archive.org/web/20081030190303/http://www.astrometrica.at/Papers/Palisa.pdf|archive-date=2008-10-30|url-status=dead}}
*{{cite journal|last=Sárneczky|first=K|author2=Kereszturi, Á.|title='Global' Tectonism on Asteroids?|journal=33rd Annual Lunar and Planetary Science Conference|pages=1381|date=March 2002|url=http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1381.pdf|archive-url=https://web.archive.org/web/20050126211723/http://www.lpi.usra.edu/meetings/lpsc2002/pdf/1381.pdf|archive-date=2005-01-26|url-status=live|access-date=2008-10-22|bibcode=2002LPI....33.1381S|ref=CITEREFSárneczkyKereszturi2002}}
*{{cite journal|last=Stooke|first=P. J.|title=Reflections on the Geology of 243 Ida|journal=Lunar and Planetary Science XXVIII|year=1997|pages=1385–1386|url=http://www.lpi.usra.edu/meetings/lpsc97/pdf/1045.PDF|archive-url=https://web.archive.org/web/20090304170519/http://www.lpi.usra.edu/meetings/lpsc97/pdf/1045.PDF|archive-date=2009-03-04|url-status=live|access-date=2008-11-29|bibcode=1997LPI....28.1385S}}
*{{cite web|title=JPL Small-Body Database Browser: 243 Ida|publisher=Jet Propulsion Laboratory|url=https://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Ida|date=25 August 2008|ref=CITEREFJPL2008|access-date=24 October 2019|archive-date=7 August 2011|archive-url=https://web.archive.org/web/20110807052147/http://ssd.jpl.nasa.gov/sbdb.cgi?sstr=Ida|url-status=live}}
*{{cite web |title=Images of Asteroids Ida & Dactyl |publisher=[[NASA|National Aeronautics and Space Administration]] |date=23 August 2005 |url=http://neo.jpl.nasa.gov/images/ida.html |access-date=2008-12-04 |archive-url=https://web.archive.org/web/20081021211759/http://neo.jpl.nasa.gov/images/ida.html |archive-date=21 October 2008 |url-status=dead |ref=CITEREFNASA2005 }}
*{{cite web|title=Gazetteer of Planetary Nomenclature: Ida|publisher=[[United States Geological Survey]] Astrogeology Research Program|url=https://planetarynames.wr.usgs.gov/jsp/FeatureTypes2.jsp?system=Asteroid%20Belt&body=Ida&systemID=12&bodyID=51&sort=AName&show=Fname&show=Lat&show=Long&show=Diam&show=Stat&show=Orig|access-date=2009-04-15|ref=CITEREFUSGS|archive-date=23 September 2006|archive-url=https://web.archive.org/web/20060923084513/http://planetarynames.wr.usgs.gov/jsp/FeatureTypes2.jsp?system=Asteroid|url-status=live}}
{{Refend}}

== External links ==
{{Commons}}
* [https://www.johnstonsarchive.net/astro/asteroidmoons.html Asteroids with Satellites], Robert Johnston, johnstonsarchive.net
* {{AstDys|243}}
* {{JPL small body}}

{{Use dmy dates|date=October 2019}}
{{Minor planets navigator |242 Kriemhild |number=243 |244 Sita}}
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{{DEFAULTSORT:000243}}
[[Category:Koronis asteroids]]
[[Category:Discoveries by Johann Palisa|Ida]]
[[Category:Named minor planets|Ida]]
[[Category:Binary asteroids]]
[[Category:Minor planets visited by spacecraft|19930828]]
[[Category:Galileo program|243 Ida]]
[[Category:S-type asteroids (Tholen)]]
[[Category:S-type asteroids (SMASS)]]
[[Category:Astronomical objects discovered in 1884|18840929]]