text: 243 ida 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 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 unmanned 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 . like all main - belt asteroids , ida 's orbit lies between the planets mars and jupiter . its orbital period is 4 . 84 years , and its rotation period is 4 . 63 hours . ida has an average diameter of 31 . 4 km ( 19 . 5 mi ) . it is irregularly shaped and elongated , and 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 in images returned from galileo . it was named after the , creatures which inhabited mount ida in greek mythology . dactyl , being only 1 . 4 kilometres ( 4 , 600 ft ) in diameter , is about one - twentieth the size of ida . its orbit around ida could not be determined with much accuracy . however , the constraints of possible orbits allowed a rough determination of ida 's density , which 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 . it was his 45th asteroid discovery . ida was named by moriz von , a viennese brewer and amateur astronomer . in greek mythology , ida was a nymph of crete who raised the god zeus . 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 . 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 ( ) . its spectrum matched those of the asteroids in the s - type classification . 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 78 to 60 km ( 48 to 37 mi ) . exploration galileo flyby 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 . no prior missions had attempted such a flyby . galileo was launched into orbit by the space shuttle atlantis mission sts - 34 on 18 october 1989 . changing galileo 's trajectory to approach ida required that it consume 34 kg ( 75 lb ) of propellant . 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 . 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 12 , 400 m / s ( 41 , 000 ft / s ) relative to the asteroid . the onboard imager observed ida from a distance of 240 , 350 km ( 149 , 350 mi ) to its closest approach of 2 , 390 km ( 1 , 490 mi ) . ida was the second asteroid , after gaspra , to be imaged by a spacecraft . about 95 % of ida 's surface came into view of the probe during the flyby . transmission of many ida images was delayed due to a permanent failure in the spacecraft 's high - gain antenna . the first five images were received in september 1993 . these comprised a high - resolution mosaic of the asteroid at a resolution of 31 – 38 m / pixel . the remaining images were sent in february 1994 , when the spacecraft 's proximity to the earth allowed higher speed transmissions . 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 . ida 's relatively large surface exhibited a diverse range of geological features . the discovery of ida 's moon dactyl , the first confirmed satellite of an asteroid , provided additional insights into ida 's composition . ida is classified as an s - type asteroid based on ground - based spectroscopic measurements . 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 . estimates of ida 's density are constrained to less than 3 . 2 g / cm3 by the long - term stability of dactyl 's orbit . 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 . 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 . the same process affects both ida and its moon , although dactyl shows a lesser change . 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 . 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 . oc meteorites are , likewise , the most common type of meteorite found on the earth 's surface . 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 . physical characteristics ida 's mass is between 3 . 65 and 4 . 99 × 1016 kg . its gravitational field produces an acceleration of about 0 . 3 to 1 . 1 cm / s2 over its surface . 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 20 m / s ( 70 ft / s ) could escape the asteroid entirely . ida is a distinctly elongated asteroid , with an irregular surface . ida is 2 . 35 times as long as it is wide , and a " waist " separates it into two geologically dissimilar halves . 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 . although there are a few steep slopes tilting up to about 50 ° on ida , the slope generally does not exceed 35 ° . ida 's irregular shape is responsible for the asteroid 's very uneven gravitational field . 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 . surface features ida 's surface appears heavily cratered and mostly gray , although minor color variations mark newly formed or uncovered areas . 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 . the surface of ida is covered in a blanket of pulverized rock , called regolith , about 50 – 100 m ( 160 – 330 ft ) thick . this material is produced in impact events and redistributed across ida 's surface by geological processes . galileo observed evidence of recent downslope regolith movement . ida 's regolith is composed of the silicate minerals olivine and pyroxene . its appearance changes over time through a process called space weathering . because of this process , older regolith appears more red in color compared to freshly exposed material . about 20 large ( 40 – 150 m across ) ejecta blocks have been identified , embedded in ida 's regolith . ejecta blocks constitute the largest pieces of the regolith . 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 . most of them are located within the craters lascaux and mammoth , but they may not have been produced there . this area attracts debris due to ida 's irregular gravitational field . some blocks may have been ejected from the young crater azzurra on the opposite side of the asteroid . 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 " . this feature may have been filled in by debris , or blasted out of the asteroid by impacts . region 1 of ida contains two major structures . one is a prominent 40 km ( 25 mi ) ridge named townsend that stretches 150 degrees around ida 's surface . the other structure is a large indentation named vienna regio . ida 's region 2 features several sets of grooves , most of which are 100 m ( 330 ft ) wide or less and up to 4 km ( 2 . 5 mi ) long . they are located near , but are not connected with , the craters mammoth , lascaux , and . some grooves are related to major impact events , for example a set opposite vienna regio . craters ida is one of the most densely cratered bodies yet explored in the solar system , and impacts have been the primary process shaping its surface . cratering has reached the saturation point , meaning that new impacts erase evidence of old ones , leaving the total crater count roughly the same . it is covered with craters of all sizes and stages of degradation , and ranging in age from fresh to as old as ida itself . the oldest may have been formed during the breakup of the koronis family parent body . the largest crater , lascaux , is almost 12 km ( 7 . 5 mi ) across . region 2 contains nearly all of the craters larger than 6 km ( 3 . 7 mi ) in diameter , but region 1 has no large craters at all . some craters are arranged in chains . 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 . azzurra seems to be the most recent major impact on ida . the ejecta from this collision is distributed discontinuously over ida and is responsible for the large - scale color and albedo variations across its surface . an exception to the crater morphology is the fresh , asymmetric fingal , which has a sharp boundary between the floor and wall on one side . another significant crater is afon , which marks ida 's prime meridian . the craters are simple in structure : bowl - shaped with no flat bottoms and no central peaks . they are distributed evenly around ida , except for a protrusion north of crater which is smoother and less cratered . the ejecta excavated by impacts is deposited differently on ida than on planets because of its rapid rotation , low gravity and irregular shape . ejecta blankets settle asymmetrically around their craters , but fast - moving ejecta that escapes from the asteroid is permanently lost . composition ida was classified as an s - type asteroid based on the similarity of its reflectance spectra with similar asteroids . s - types may share their composition with stony - iron or ordinary chondrite ( oc ) meteorites . 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 . oc meteorites contain varying amounts of the silicates olivine and pyroxene , iron , and feldspar . olivine and pyroxene were detected on ida by galileo . 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 . 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 % . 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 . orbit and rotation ida is a member of the koronis family of asteroid - belt asteroids . ida orbits the sun at an average distance of 2 . 862 au ( 428 . 1 gm ) , between the orbits of mars and jupiter . ida takes 4 . years to complete one orbit . ida 's rotation period is 4 . 63 hours , making it one of the fastest rotating asteroids yet discovered . 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 . ida 's axis of rotation precesses with a period of 77 thousand years , due to the gravity of the sun acting upon the shape of the asteroid . origin ida originated in the breakup of the roughly 120 km ( 75 mi ) diameter koronis parent body . the progenitor asteroid had partially differentiated , with heavier metals migrating to the core . ida carried away insignificant amounts of this core material . 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 . however , this is inconsistent with the estimated age of the ida – dactyl system of less than 100 million years ; 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 . moon ida has a moon named dactyl , official designation ( 243 ) ida i dactyl ( / / - til ) . 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 . at the time , it was separated from ida by a distance of 90 kilometres ( 56 mi ) , 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 . discovery dactyl was found on 17 february 1994 by galileo mission member ann , while examining delayed image downloads from the spacecraft . galileo recorded 47 images of dactyl over an observation period of 5 . 5 hours in august 1993 . the spacecraft was 10 , 760 kilometres ( 6 , 690 mi ) from ida and 10 , 870 kilometres ( 6 , 750 mi ) from dactyl when the first image of the moon was captured , 14 minutes before galileo made its closest approach . dactyl was initially designated 1993 ( 243 ) 1 . it was named by the international astronomical union in 1994 , for the mythological dactyls who inhabited mount ida on the island of crete . physical characteristics dactyl is an " egg - shaped " but " remarkably spherical " object measuring 1 . 6 by 1 . 4 by 1 . 2 kilometres ( 0 . 99 mi × 0 . 87 mi × 0 . 75 mi ) . it was oriented with its longest axis pointing towards ida . like ida , dactyl 's surface exhibits saturation cratering . it is marked by more than a dozen craters with a diameter greater than 80 m ( 260 ft ) , indicating that the moon has suffered many collisions during its history . at least six craters form a linear chain , suggesting that it was caused by locally produced debris , possibly ejected from ida . dactyl 's craters may contain central peaks , unlike those found on ida . these features , and dactyl 's spheroidal shape , imply that the moon is gravitationally controlled despite its small size . like ida , its average temperature is about 200 k ( − 73 ° c ; − 100 ° f ) . dactyl shares many characteristics with ida . their albedos and reflection spectra are very similar . the small differences indicate that the space weathering process is less active on dactyl . its small size would make the formation of significant amounts of regolith impossible . this contrasts with ida , which is covered by a deep layer of regolith . the two largest imaged craters on dactyl were named and , after two of the mythological dactyls . is the largest crater in the above image , and is near the bottom , mostly obscured in shadow . the craters are 300 and 200 meters in diameter , respectively . orbit 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 . dactyl orbits in the prograde direction and is inclined about 8 ° to ida 's equator . based on computer simulations , dactyl 's must be more than about 65 km ( 40 mi ) from ida for it to remain in a stable orbit . 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 90 km ( 56 mi ) from ida at longitude 85 ° . 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 700 km ( 430 mi ) from ida . if in a circular orbit at the distance at which it was seen , dactyl 's orbital period is about 20 hours . its orbital speed is roughly 10 m / s ( 33 ft / s ) , " about the speed of a fast run or a slowly thrown baseball " . age and origin dactyl may have originated at the same time as ida , from the disruption of the koronis parent body . however , it may have formed more recently , perhaps as ejecta from a large impact on ida . it is extremely unlikely that it was captured by ida . dactyl may have suffered a major impact around 100 million years ago , which reduced its size .