243 Ida

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.[22] No prior missions had attempted such a flyby.[23] Galileo was launched into orbit by the Space Shuttle Atlantis mission STS-34 on 18 October 1989.[24] Changing Galileo's trajectory to approach Ida required that it consume 34 kg (75 lb) of propellant.[25] 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.[26]

Images from the flyby, starting 5.4 hours before closest approach and showing Ida's rotation

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.[26] 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).[16][27] Ida was the second asteroid, after Gaspra, to be imaged by a spacecraft.[28] About 95% of Ida's surface came into view of the probe during the flyby.[9]

Transmission of many Ida images was delayed due to a permanent failure in the spacecraft's high-gain antenna.[29] The first five images were received in September 1993.[30] These comprised a high-resolution mosaic of the asteroid at a resolution of 31–38 m/pixel.[31][32] The remaining images were sent in February 1994,[3] when the spacecraft's proximity to the Earth allowed higher speed transmissions.[30][33]

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.[34] Ida's relatively large surface exhibited a diverse range of geological features.[35] The discovery of Ida's moon Dactyl, the first confirmed satellite of an asteroid, provided additional insights into Ida's composition.[36]

Ida is classified as an S-type asteroid based on ground-based spectroscopic measurements.[37] 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.[12] Estimates of Ida's density are constrained to less than 3.2 g/cm³ by the long-term stability of Dactyl's orbit.[37] 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.[36]

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.[17][38] The same process affects both Ida and its moon, although Dactyl shows a lesser change.[39] 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.[23]

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.[23] OC meteorites are, likewise, the most common type of meteorite found on the Earth's surface.[23] 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.[39]

The surface of Ida is covered in a blanket of pulverized rock, called regolith, about 50–100 m (160–330 ft) thick.[30] This material is produced in impact events and redistributed across Ida's surface by geological processes.[47] Galileo observed evidence of recent downslope regolith movement.[48]

Ida's regolith is composed of the silicate minerals olivine and pyroxene.[3][49] Its appearance changes over time through a process called space weathering.[39] Because of this process, older regolith appears more red in color compared to freshly exposed material.[38]

Galileo image of a 150 m (490 ft) block at 24.8°S, 2.8°E[50]

About 20 large (40–150 m across) ejecta blocks have been identified, embedded in Ida's regolith.[30][51] Ejecta blocks constitute the largest pieces of the regolith.[52] 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.[46][53] Most of them are located within the craters Lascaux and Mammoth, but they may not have been produced there.[53] This area attracts debris due to Ida's irregular gravitational field.[46] Some blocks may have been ejected from the young crater Azzurra on the opposite side of the asteroid.[54]

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".[30] This feature may have been filled in by debris, or blasted out of the asteroid by impacts.[30][54]

Region 1 of Ida contains two major structures. One is a prominent 40 km (25 mi) ridge named Townsend Dorsum that stretches 150 degrees around Ida's surface.[55] The other structure is a large indentation named Vienna Regio.[30]

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.[30][56] They are located near, but are not connected with, the craters Mammoth, Lascaux, and Kartchner.[52] Some grooves are related to major impact events, for example a set opposite Vienna Regio.[57]

Ida is one of the most densely cratered bodies yet explored in the Solar System,[31][44] and impacts have been the primary process shaping its surface.[58] Cratering has reached the saturation point, meaning that new impacts erase evidence of old ones, leaving the total crater count roughly the same.[59] It is covered with craters of all sizes and stages of degradation,[44] and ranging in age from fresh to as old as Ida itself.[30] The oldest may have been formed during the breakup of the Koronis family parent body.[39] The largest crater, Lascaux, is almost 12 km (7.5 mi) across.[45][60] 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.[30] Some craters are arranged in chains.[32]

Asymmetric 1.5 km (0.93 mi) wide crater Fingal at 13.2°S, 39.9°E[60]

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.[61] Azzurra seems to be the most recent major impact on Ida.[51] The ejecta from this collision is distributed discontinuously over Ida[38] and is responsible for the large-scale color and albedo variations across its surface.[62] An exception to the crater morphology is the fresh, asymmetric Fingal, which has a sharp boundary between the floor and wall on one side.[63] Another significant crater is Afon, which marks Ida's prime meridian.[11]

The craters are simple in structure: bowl-shaped with no flat bottoms and no central peaks.[63] They are distributed evenly around Ida, except for a protrusion north of crater Choukoutien which is smoother and less cratered.[64] The ejecta excavated by impacts is deposited differently on Ida than on planets because of its rapid rotation, low gravity and irregular shape.[43] Ejecta blankets settle asymmetrically around their craters, but fast-moving ejecta that escapes from the asteroid is permanently lost.[65]

Dactyl was found on 17 February 1994 by Galileo mission member Ann Harch, while examining delayed image downloads from the spacecraft.[3] Galileo recorded 47 images of Dactyl over an observation period of 5.5 hours in August 1993.[77] The spacecraft was 10,760 kilometres (6,690 mi) from Ida[78] 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.[79]

Dactyl was initially designated 1993 (243) 1.[78][80] It was named by the International Astronomical Union in 1994,[80] for the mythological dactyls who inhabited Mount Ida on the island of Crete.[81][82]

Dactyl is an "egg-shaped"[36] but "remarkably spherical"[81] object measuring 1.6 by 1.4 by 1.2 kilometres (0.99 mi × 0.87 mi × 0.75 mi).[36] It is oriented with its longest axis pointing towards Ida.[36] Like Ida, Dactyl's surface exhibits saturation cratering.[36] 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.[16] At least six craters form a linear chain, suggesting that it was caused by locally produced debris, possibly ejected from Ida.[36] Dactyl's craters may contain central peaks, unlike those found on Ida.[83] These features, and Dactyl's spheroidal shape, imply that the moon is gravitationally controlled despite its small size.[83] Like Ida, its average temperature is about 200 K (−73 °C; −100 °F).[3]

Dactyl shares many characteristics with Ida. Their albedos and reflection spectra are very similar.[84] The small differences indicate that the space weathering process is less active on Dactyl.[39] Its small size would make the formation of significant amounts of regolith impossible.[39][78] This contrasts with Ida, which is covered by a deep layer of regolith.

The two largest imaged craters on Dactyl were named Acmon /ˈækmən/ and Celmis /ˈsɛlmɪs/, 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.[85]

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.[37] Dactyl orbits in the prograde direction[86] and is inclined about 8° to Ida's equator.[77] Based on computer simulations, Dactyl's pericenter must be more than about 65 km (40 mi) from Ida for it to remain in a stable orbit.[87] 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°.[88][89] 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.[37]

If in a circular orbit at the distance at which it was seen, Dactyl's orbital period would be about 20 hours.[84] Its orbital speed is roughly 10 m/s (33 ft/s), "about the speed of a fast run or a slowly thrown baseball".[37]

Dactyl may have originated at the same time as Ida,[90] from the disruption of the Koronis parent body.[53] However, it may have formed more recently, perhaps as ejecta from a large impact on Ida.[91] It is extremely unlikely that it was captured by Ida.[79] Dactyl may have suffered a major impact around 100 million years ago, which reduced its size.[72]