243 Ida

Template:Short description Template:About Template:Pp-move-indef {{#invoke:infobox|infoboxTemplate | class = vcard | titleclass = fn org | title = Ida | image = {{#invoke:InfoboxImage|InfoboxImage|image=File:243 ida crop.jpg|upright={{#if:||1.1}}|alt=}} | caption = 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. | headerstyle = {{#if:#D6D6D6|background-color:#D6D6D6|background-color:#E0CCFF}} | labelstyle = max-width:{{#if:||11em}}; | autoheaders = y

| header1 = Discovery<ref name="Raab2002"/>

| header10 = {{#if:|Designations|Designations}}

| header20 = Orbital characteristics{{#ifeq:|yes| (barycentric)}}<ref name="JPL2008">Template:Harvnb</ref>

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| header60 = Proper orbital elements

| label61 = Template:Longitem | data61 = {{#if: |{{{p_semimajor}}} AU}} | label62 = Template:Longitem | data62 = | label63 = Template:Longitem | data63 = | label64 = Template:Longitem | data64 = {{#if: |{{{p_mean_motion}}} deg Template:\yr}} | label65 = Template:Longitem | data65 = {{#if:|{{#expr:360/1 round 5}} yr
({{#expr:365.25*360/1 round 3}} d) }} | label66 = Template:Longitem | data66 = {{#if:|{{{perihelion_rate}}} arcsec Template:\yr }} | label67 = Template:Longitem | data67 = {{#if:|{{{node_rate}}} arcsec Template:\yr}}

| header70 = Template:Anchor{{#if:yes| Physical characteristics|Physical characteristics}}

| label71 = Dimensions | data71 = 59.8 × 25.4 × 18.6 km<ref name="Belton1996">Template:Harvnb</ref> | label72 = Template:Longitem | data72 = | label73 = Template:Longitem | data73 = 15.7 km<ref name="BrittYeomansHousenConsolmagno2002p486">Template:Harvnb</ref> | label74 = Template:Longitem | data74 = | label75 = Template:Longitem | data75 = | label76 = Flattening | data76 = | label77 = Circumference | data77 = | label78 = Template:Longitem | data78 = | label79 = Volume | data79 = | label80 = Mass | data80 = 4.2 ± 0.6 ×10¹⁶ kg<ref name="BrittYeomansHousenConsolmagno2002p486"/> | label81 = Template:Longitem | data81 = 2.6 ± 0.5 g/cm³<ref name="Belton1995"/> | label82 = Template:Longitem | data82 = 0.3–1.1 cm/s²<ref name="ThomasBeltonCarcichChapman1996"/> | label83 = Template:Longitem | data83 = | label84 = Template:Longitem | data84 = | label85 = Template:Longitem | data85 = Template:Convert<ref name="VokrouhlickyNesvornyBottke2003p147"/> | label86 = Template:Longitem | data86 = | label87 = Template:Longitem | data87 = | label88 = Template:Longitem | data88 = | label89 = Template:Longitem | data89 = 168.76°<ref name="Archinal2018"/> | label90 = Template:Longitem | data90 = −87.12°<ref name="Archinal2018"/> | label91 = Template:Longitem | data91 = | label92 = Template:Longitem | data92 = | label93 = {{#if:yes |Template:Longitem |Albedo}} | data93 = 0.2383<ref name="JPL2008"/> | label94 = Temperature | data94 = Template:Convert<ref name="Holm1994"/>

| data100 = {{#if:|

Surface temp.	min	mean	max
{{{temp_name1}}}
{{{temp_name2}}}
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| header110 = Atmosphere

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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 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 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 Template:Convert. 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 was discovered by mission member Ann Harch in images returned from Galileo. It was named after the Dactyls, creatures which inhabited Mount Ida in Greek mythology. Dactyl is only Template:Convert 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 observationsEdit

Ida was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory.<ref name="Ridpath1897p206">Template:Harvnb</ref> It was his 45th asteroid discovery.<ref name="Raab2002">Template:Harvnb</ref> Ida was named by Moriz von Kuffner, a Viennese brewer and amateur astronomer.<ref name="Schmadel2003p36">Template:Harvnb</ref><ref name="Berger2003p241">Template:Harvnb</ref> In Greek mythology, Ida was a nymph of Crete who raised the god Zeus.<ref name="NASA2005">Template:Harvnb</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 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">Template:Harvnb</ref> Its spectrum matched those of the asteroids in the S-type classification.<ref>Template:Harvnb Template:Quote</ref><ref name="ZellnerTholenTedesco1985p410">Template:Harvnb</ref> Many observations of Ida were made in early 1993 by the 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 Template:Convert.<ref name="OwenYeomans1994p2295">Template:Harvnb</ref>

ExplorationEdit

Galileo flybyEdit

Ida was visited in 1993 by the Jupiter-bound space probe Galileo. Its encounters of the asteroids 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">Template:Harvnb</ref> No prior missions had attempted such a flyby.<ref name="Chapman1996p699">Template:Harvnb</ref> Galileo was launched into orbit by the Space Shuttle Atlantis mission STS-34 on 18 October 1989.<ref name="D'AmarioBrightWolf1992p24">Template:Harvnb</ref> Changing Galileo's trajectory to approach Ida required that it consume Template:Convert of propellant.<ref name="D'AmarioBrightWolf1992p72">Template:Harvnb</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">Template:Harvnb</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 Template:Convert relative to the asteroid.<ref name="D'AmarioBrightWolf1992p36"/> The onboard imager observed Ida from a distance of Template:Convert to its closest approach of Template:Convert.<ref name="NASA2005"/><ref name="SullivanGreeleyPappalardoAsphaug1996p120">Template:Harvnb</ref> Ida was the second asteroid, after Gaspra, to be imaged by a spacecraft.<ref name="Cowen1993p215">Template:Harvnb Template:Quote</ref> About 95% of Ida's surface came into view of the probe during the flyby.<ref name="ThomasBeltonCarcichChapman1996">Template:Harvnb</ref>

Transmission of many Ida images was delayed due to a permanent failure in the spacecraft's high-gain antenna.<ref name="Chapman1994p358">Template:Harvnb</ref> The first five images were received in September 1993.<ref name="Chapman1996p707">Template:Harvnb</ref> These comprised a high-resolution mosaic of the asteroid at a resolution of 31–38 m/pixel.<ref name="ChapmanBeltonVeverkaNeukum1994p237">Template:Harvnb</ref><ref name="GreeleySullivanPappalardoVeverka1994p469">Template:Harvnb</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">Template:Harvnb</ref>

Template:Multiple image

DiscoveriesEdit

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">Template:Harvnb</ref> Ida's relatively large surface exhibited a diverse range of geological features.<ref name="ChapmanBeltonVeverkaNeukum1994p238">Template:Harvnb</ref> The discovery of Ida's moon Dactyl, the first confirmed satellite of an asteroid, provided additional insights into Ida's composition.<ref name="Chapman1996p709">Template:Harvnb</ref>

Ida is classified as an S-type asteroid based on ground-based spectroscopic measurements.<ref name="ByrnesD'Amario1994">Template:Harvnb</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.<ref name="WilsonKeilLove1999p479">Template:Harvnb</ref> Estimates of Ida's density are constrained to less than 3.2 g/cm³ by the long-term stability of Dactyl's orbit.<ref name="ByrnesD'Amario1994" /> This all but rules out a stony-iron composition; were Ida made of 5 g/cm³ 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">Template:Harvnb</ref><ref name="Chapman1996p710">Template:Harvnb</ref> The same process affects both Ida and its moon, although Dactyl shows a lesser change.<ref name="Chapman1995p496">Template:Harvnb</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

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 characteristicsEdit

Template:Multiple image Ida's mass is between 3.65 and 4.99 × 10¹⁶ kg.<ref name="PetitDurdaGreenbergHurford1997p179t180">Template:Harvnb</ref> Its gravitational field produces an acceleration of about 0.3 to 1.1 cm/s² 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 Template:Convert could escape the asteroid entirely.<ref name="GeisslerPetitDurdaGreenberg1996p142">Template:Harvnb</ref><ref name="LeeVeverkaThomasHelfenstein1996p99">Template:Harvnb</ref>

Ida is a distinctly elongated asteroid,<ref name="GeisslerPetitGreenberg1996p58">Template:Harvnb</ref> with an irregular surface.<ref name="Chapman1994p363">Template:Harvnb</ref><ref name="BottkeCellinoPaolicchiBinzel2002p10">Template:Harvnb</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">Template:Harvnb</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 featuresEdit

File:243 Ida large.jpg

Mosaic of images recorded by Galileo 3.5 minutes before its closest approach

Ida's surface appears heavily 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 blocks, several of which have been observed on the surface.

RegolithEdit

The surface of Ida is covered in a blanket of pulverized rock, called regolith, about Template:Convert thick.<ref name="Chapman1996p707" /> This material is produced in impact events and redistributed across Ida's surface by geological processes.<ref name="LeeVeverkaThomasHelfenstein1996p96">Template:Harvnb</ref> Galileo observed evidence of recent downslope regolith movement.<ref name="GreeleySullivanPappalardoVeverka1994p470">Template:Harvnb</ref>

Ida's regolith is composed of the silicate minerals olivine and pyroxene.<ref name="Holm1994">Template:Harvnb</ref><ref name="Chapman1996p701">Template:Harvnb</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

Galileo image of a Template:Convert block at 24.8°S, 2.8°E<ref name="LeeVeverkaThomasHelfenstein1996p90">Template:Harvnb</ref>

About 20 large (40–150 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">Template:Harvnb</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">Template:Harvnb</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" />

StructuresEdit

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">Template:Harvnb</ref>

Region 1 of Ida contains two major structures. One is a prominent Template:Convert ridge named Townsend Dorsum that stretches 150 degrees around Ida's surface.<ref name="SárneczkyKereszturi2002">Template:Harvnb</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 Template:Convert wide or less and up to Template:Convert long.<ref name="Chapman1996p707" /><ref name="SullivanGreeleyPappalardoAsphaug1996p131">Template:Harvnb</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">Template:Harvnb</ref>

CratersEdit

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>Template:Harvnb</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">Template:Harvnb</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 Template:Convert across.<ref name="BottkeCellinoPaolicchiBinzel2002p10" /><ref name="USGS">Template:Harvnb</ref> Region 2 contains nearly all of the craters larger than Template:Convert 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

Asymmetric Template:Convert wide crater Fingal at 13.2°S, 39.9°E<ref name="USGS" />

Ida's major craters are named after caves and lava tubes on Earth. The crater Azzurra, for example, is named after a submerged cave on the island of Capri, also known as the Blue Grotto.<ref name="GreeleyBatson2001p393">Template:Harvnb</ref> Azzurra seems to be the most recent major impact on Ida.<ref name="GeisslerPetitDurdaGreenberg1996p141">Template:Harvnb</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>Template:Harvnb</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">Template:Harvnb</ref> They are distributed evenly around Ida, except for a protrusion north of crater Choukoutien which is smoother and less cratered.<ref>Template:Harvnb</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 blankets settle asymmetrically around their craters, but fast-moving ejecta that escapes from the asteroid is permanently lost.<ref>Template:Harvnb</ref>

CompositionEdit

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/cm³.<ref name="Chapman1995p496" /><ref name="WilsonKeilLove1999p480">Template:Harvnb</ref> OC meteorites contain varying amounts of the silicates olivine and pyroxene, iron, and feldspar.<ref name="Lewis1996p89">Template:Harvnb Template:Quote</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">Template:Harvnb</ref><ref name="SullivanGreeleyPappalardoAsphaug1996p135">Template:Harvnb</ref> Assuming that its composition is similar to OC meteorites, which range in density from 3.48 to 3.64 g/cm³, Ida would have a porosity of 11–42%.<ref name="WilsonKeilLove1999p480" />

Ida's interior probably contains some amount of impact-fractured rock, called 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 rotationEdit

File:Ida orbit.svg

Orbit and positions of Ida and five planets as of 9 March 2009

Ida is a member of the Koronis family of asteroid-belt asteroids.<ref name="Chapman1996p700" /> Ida orbits the Sun at an average distance of Template:Convert, 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 direction with a rotation period of 4.63 hours (roughly 5 hours).<ref name="VokrouhlickyNesvornyBottke2003p147">Template:Harvnb</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 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">Template:Harvnb</ref>

OriginEdit

Ida originated in the breakup of the roughly Template:Convert diameter Koronis parent body.<ref name="VokrouhlickyNesvornyBottke2003p147" /> The progenitor asteroid had partially differentiated, with heavier metals migrating to the core.<ref name="GreenbergBottkeNolanGeissler1996p117">Template:Harvnb</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 million years;<ref name="HurfordGreenberg2000p1595">Template:Harvnb</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">Template:Harvnb</ref>

DactylEdit

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File:Dactyl1.jpg

An image of Dactyl captured by the Galileo spacecraft during its 1993 flyby.

Ida has a 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 Template:Convert, moving in a 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.

DiscoveryEdit

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">Template:Harvnb</ref> The spacecraft was Template:Convert from Ida<ref name="BeltonCarlson1994">Template:Harvnb</ref> and Template:Convert from Dactyl when the first image of the moon was captured, 14 minutes before Galileo made its closest approach.<ref name="Mason1994p108">Template:Harvnb</ref>

Dactyl was initially designated 1993 (243) 1.<ref name="BeltonCarlson1994" /><ref name="Green1994">Template:Harvnb</ref> It was named by the International Astronomical Union in 1994,<ref name="Green1994" /> for the mythological dactyls who inhabited Mount Ida on the island of Crete.<ref name="Schmadel2003p37">Template:Harvnb</ref><ref name="Pausanias576">Template:Harvnb Template:Quote</ref>

Physical characteristicsEdit

Dactyl is an "egg-shaped"<ref name="Chapman1996p709" /> but "remarkably spherical"<ref name="Schmadel2003p37" /> object measuring Template:Convert.<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 Template:Convert, 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">Template:Harvnb</ref> These features, and Dactyl's spheroidal shape, imply that the moon is gravitationally controlled despite its small size.<ref name="AsphaugRyanZuber2003p463" /> Like Ida, its average temperature is about Template:Convert.<ref name="Holm1994" />

Dactyl shares many characteristics with Ida. Their albedos and reflection spectra are very similar.<ref name="ChapmanKlaasenBeltonVeverka1994p455">Template:Harvnb</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 Template:IPAc-en and Celmis Template:IPAc-en, 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>{{#invoke:citation/CS1|citation |CitationClass=web }}</ref>

OrbitEdit

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" /> Dactyl orbits in the prograde direction<ref name="PetitDurdaGreenbergHurford1997p179">Template:Harvnb</ref> and is inclined about 8° to Ida's equator.<ref name="PetitDurdaGreenbergHurford1997p177" /> 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" /> Its orbital speed is roughly Template:Convert, "about the speed of a fast run or a slowly thrown baseball".<ref name="ByrnesD'Amario1994" />

Age and originEdit

Dactyl may have originated at the same time as Ida,<ref name="GreenbergBottkeNolanGeissler1996p116">Template:Harvnb</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">Template:Harvnb</ref> It is extremely unlikely that it was captured by Ida.<ref name="Mason1994p108" /> Dactyl may have suffered a major impact around 100 million years ago, which reduced its size.<ref name="GreenbergBottkeNolanGeissler1996p117" />

NotesEdit

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ReferencesEdit

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External linksEdit

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Asteroids with Satellites, Robert Johnston, johnstonsarchive.net
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243 Ida

Contents

Discovery and observationsEdit

ExplorationEdit

Galileo flybyEdit

DiscoveriesEdit

Physical characteristicsEdit

Surface featuresEdit

RegolithEdit

StructuresEdit

CratersEdit

CompositionEdit

Orbit and rotationEdit

OriginEdit

DactylEdit

DiscoveryEdit

Physical characteristicsEdit

OrbitEdit

Age and originEdit

See alsoEdit

NotesEdit

ReferencesEdit

Journal articlesEdit

BooksEdit

OtherEdit

External linksEdit