Астероид Ида: Исследование аппаратом "Галилео" и открытие спутника Дактиль.

Main belt asteroid
an asteroid

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,

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. 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. 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.

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.

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%. 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 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.