243 Ida

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 Kuffner, 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. 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 . == 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 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 relative to the asteroid. 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, 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. 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. meteoriteBoth 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. Ida is a distinctly elongated asteroid, with an irregular surface. Ida is 2.35 times as long as it is wide, 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. 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.|216x216px About 20 large (40–150 m across) ejecta blocks have been identified, embedded in Ida's 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. Region 1 of Ida contains two major structures. One is a prominent ridge named Townsend Dorsum that stretches 150 degrees around Ida's surface. The other structure is a large indentation named Vienna Regio. They are located near, but are not connected with, the craters Mammoth, Lascaux, and Kartchner. Craters Ida is one of the most densely cratered bodies yet explored in the Solar System, 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, Region 2 contains nearly all of the craters larger than in diameter, but Region 1 has no large craters at all. Azzurra seems to be the most recent major impact on Ida. The ejecta from this collision is distributed discontinuously over Ida An exception to the crater morphology is the fresh, asymmetric Fingal, which has a sharp boundary between the floor and wall on one side. They are distributed evenly around Ida, except for a protrusion north of crater Choukoutien 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. == Composition ==

Ida was classified as an S-type asteroid based on the similarity of its reflectance spectra with similar asteroids. OC meteorites contain varying amounts of the silicates olivine and pyroxene, iron, and feldspar. Olivine and pyroxene were detected on Ida by Galileo. 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. == Origin ==

Ida originated in the breakup of the roughly diameter Koronis parent body. Ida carried away insignificant amounts of this core material. 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. == Dactyl ==

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. The spacecraft was from Ida and 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, Physical characteristics Dactyl is an "egg-shaped" These features, and Dactyl's spheroidal shape, imply that the moon is gravitationally controlled despite its small size. The small differences indicate that the space weathering process is less active on Dactyl. 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. and is inclined about 8° to Ida's equator. 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 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 from Ida. from the disruption of the Koronis parent body. 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. == See also ==