243 Ida
Ida, minor planet designation 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.
This article is about an asteroid. For other uses, see Ida (disambiguation).Discovery[1]
September 29, 1884
(243) Ida
Main belt (Koronis family)[3]
Idean (Idæan) /aɪˈdiːən/[4]
2.979 AU (4.457×1011 m)
2.743 AU (4.103×1011 m)
2.861 AU (4.280×1011 m)
0.0411
1,767.644 days (4.83955 a)
0.2036°/d
38.707°
1.132°
324.016°
110.961°
Dactyl
59.8 × 25.4 × 18.6 km[6]
15.7 km[7]
4.2 ± 0.6 ×1016 kg[7]
2.6 ± 0.5 g/cm3[8]
0.3–1.1 cm/s2[9]
4.63 hours (0.193 d)[10]
168.76°[11]
−2.88°[11]
0.2383[5]
200 K (−73 °C)[3]
9.94[5]
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 31.4 km (19.5 mi). 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 1.4 kilometres (0.87 mi) 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 observations[edit]
Ida was discovered on 29 September 1884 by Austrian astronomer Johann Palisa at the Vienna Observatory.[13] It was his 45th asteroid discovery.[1] Ida was named by Moriz von Kuffner, a Viennese brewer and amateur astronomer.[14][15] In Greek mythology, Ida was a nymph of Crete who raised the god Zeus.[16] 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.[17]
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).[18] Its spectrum matched those of the asteroids in the S-type classification.[19][20] 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).[21]
Exploration[edit]
Galileo flyby[edit]
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]
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]
Composition[edit]
Ida was classified as an S-type asteroid based on the similarity of its reflectance spectra with similar asteroids.[12] S-types may share their composition with stony-iron or ordinary chondrite (OC) meteorites.[12] 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.[39][66] OC meteorites contain varying amounts of the silicates olivine and pyroxene, iron, and feldspar.[67] Olivine and pyroxene were detected on Ida by Galileo.[3] 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.[68][69] 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%.[66]
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.[69]
Origin[edit]
Ida originated in the breakup of the roughly 120 km (75 mi) diameter Koronis parent body.[10] The progenitor asteroid had partially differentiated, with heavier metals migrating to the core.[72] Ida carried away insignificant amounts of this core material.[72] 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.[72] However, this is inconsistent with the estimated age of the Ida–Dactyl system of less than 100 million years;[73] 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.[74]