243 Ida
Encyclopedia
243 Ida (ˈ ) is an asteroid
in the Koronis family
of the asteroid belt
. It was discovered on 29 September 1884 by 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 spacecraft Galileo, bound for Jupiter
. It was the second asteroid to be visited by a spacecraft and the first found to possess a 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 in a shape reminiscent of a croissant
. 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, being only 1.4 kilometres (4,593.2 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 mineral
s. 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 meteorite
s 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.
n 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.
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). 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 (48.5 to 37.3 mi).
-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 12400 m/s relative to the asteroid. The onboard imager observed Ida from a distance of 240350 km (149,346.9 mi) to its closest approach of 2390 km (1,485.1 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 the next spring, when the spacecraft's proximity to the Earth allowed higher speed transmissions.
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.
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 (65.6 ft/s) could escape
the asteroid entirely.
Ida is a distinctly elongated asteroid, with an irregular surface, and is somewhat "croissant
-shaped". 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. Whilst 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 highly uneven gravitational field. The surface acceleration is lowest at the extremities because of their fast 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.
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.
, about 50–100 m (164–328.1 ft) thick. This material is produced in impact event
s 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.
Region 1 of Ida contains two major structures. One is a prominent 40 km (24.9 mi) ridge named Townsend Dorsum 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 (328.1 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 Kartchner. Some grooves are related to major impact events, for example a set opposite Vienna Regio.
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 tube
s 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 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. Ejecta blanket
s settle asymmetrically around their craters, but fast-moving ejecta that escapes from the asteroid is permanently lost.
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.
Ida is a member of the Koronis family
of asteroid-belt asteroids. Ida orbits the Sun at an average distance of 2.862 AU, between the orbits of Mars
and Jupiter
. Ida takes 4.84089 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 nonspherical shape of the asteroid.
named Dactyl orbits around Ida. Dactyl, officially (243) Ida I Dactyl, ˈdæktɨl , 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 (55.9 mi), moving in a prograde orbit. Dactyl's surface 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.
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.
al shape, imply that the moon is gravitationally controlled despite its small size. Like Ida, its average temperature is about 200 K (-73.2 °C; -99.7 °F).
Dactyl shares many characteristics with Ida. Their albedo
s 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.
Dactyl's orbit around Ida is not precisely known. Galileo was in the plane
of Dactyl's orbit when most of the images of the moon 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 pericenter must be more than about 65 km (40.4 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 (55.9 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 the moon if it was more than about 700 km (435 mi) from Ida.
Dactyl's orbital period is about 20 hours, assuming it is in a circular orbit around Ida. Its orbital speed is roughly 10 m/s (32.8 ft/s), "about the speed of a fast run or a slowly thrown baseball".
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
in the Koronis family
Koronis family
]The Koronis family is a family of asteroids in the main belt between Mars and Jupiter. They are thought to have been formed at least two billion years ago in a catastrophic collision between two larger bodies. The largest known is about in diameter. The Koronis family travels in a cluster along...
of the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
. It was discovered on 29 September 1884 by Johann Palisa
Johann Palisa
Johann Palisa was an Austrian astronomer, born in Opava in Austrian Silesia .He was a prolific discoverer of asteroids, discovering 122 in all, from 136 Austria in 1874 to 1073 Gellivara in 1923...
and named after a nymph
Nymph
A nymph in Greek mythology is a female minor nature deity typically associated with a particular location or landform. Different from gods, nymphs are generally regarded as divine spirits who animate nature, and are usually depicted as beautiful, young nubile maidens who love to dance and sing;...
from Greek mythology. Later telescopic observations categorized Ida as an S-type asteroid
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
, the most numerous type in the inner asteroid belt. On 28 August 1993, Ida was visited by the spacecraft Galileo, bound for Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. It was the second asteroid to be visited by a spacecraft and the first found to possess a satellite
Asteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
.
Like all main-belt asteroids, Ida's orbit lies between the planets Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
and Jupiter. Its orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
is 4.84 years, and its rotation period
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...
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 in a shape reminiscent of a croissant
Croissant
A croissant is a buttery flaky pastry named for its distinctive crescent shape. It is also sometimes called a crescent, from the French word for "crescent". Croissants are made of a leavened variant of puff pastry...
. 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
Dactyl (mythology)
In Greek mythology, the Dactyls were the archaic mythical race of small phallic male beings associated with the Great Mother, whether as Cybele or Rhea. Their numbers vary, but often they were ten spirit-men so like the three Curetes, the Cabiri or the Korybantes that they were often interchangeable...
, creatures which inhabited Mount Ida in Greek mythology. Dactyl, being only 1.4 kilometres (4,593.2 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 mineral
Mineral
A mineral is a naturally occurring solid chemical substance formed through biogeochemical processes, having characteristic chemical composition, highly ordered atomic structure, and specific physical properties. By comparison, a rock is an aggregate of minerals and/or mineraloids and does not...
s. 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 meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s 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
Ordinary chondrite
The Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...
meteorites, the most common type found on the Earth's surface.
Discovery and observations
Ida was discovered on 29 September 1884 by AustriaAustria
Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...
n astronomer Johann Palisa
Johann Palisa
Johann Palisa was an Austrian astronomer, born in Opava in Austrian Silesia .He was a prolific discoverer of asteroids, discovering 122 in all, from 136 Austria in 1874 to 1073 Gellivara in 1923...
at the Vienna Observatory
Vienna Observatory
Universitäts-Sternwarte Wien is an astronomical observatory in Vienna, Austria. It is part of the University of Vienna. The first observatory was built in 1753–1754 on the roof of one of the university buildings....
. It was his 45th asteroid discovery. Ida was named by Moriz von Kuffner
Moriz von Kuffner
Moriz von Kuffner was a Jewish-Austrian industrialist, art collector, mountaineer and philanthropist. From the 1880s to the early 1910s he made a fortune in the brewery business, and became a significant sponsor of Vienna's social and cultural life as well as a mentor of astronomy...
, a Viennese brewer and amateur astronomer. In Greek mythology
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
, Ida was a nymph
Mount Ida, Crete
Mount Ida, known variously as Idha, Ídhi, Idi, Ita and now Psiloritis , is the highest mountain on Crete. Located in the Rethymno Prefecture, it is sacred to the Greek Titaness Rhea, and on its slopes, according to legend, lies the cave, Idaion Andron, in which Zeus was born...
of Crete
Crete
Crete is the largest and most populous of the Greek islands, the fifth largest island in the Mediterranean Sea, and one of the thirteen administrative regions of Greece. It forms a significant part of the economy and cultural heritage of Greece while retaining its own local cultural traits...
who raised the god Zeus
Zeus
In the ancient Greek religion, Zeus was the "Father of Gods and men" who ruled the Olympians of Mount Olympus as a father ruled the family. He was the god of sky and thunder in Greek mythology. His Roman counterpart is Jupiter and his Etruscan counterpart is Tinia.Zeus was the child of Cronus...
. Ida was recognized as a member of the Koronis family
Koronis family
]The Koronis family is a family of asteroids in the main belt between Mars and Jupiter. They are thought to have been formed at least two billion years ago in a catastrophic collision between two larger bodies. The largest known is about in diameter. The Koronis family travels in a cluster along...
by Kiyotsugu Hirayama
Kiyotsugu Hirayama
was a Japanese astronomer, best known for his discovery that many asteroid orbits were more similar to one another than chance would allow, leading to the concept of asteroid families, now called "Hirayama families" in his honour....
, 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 (ECAS). 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
United States Naval Observatory Flagstaff Station
The United States Naval Observatory Flagstaff Station , is a scientific astronomical observatory operated as a Navy Echelon V command and the national dark-sky observing Facility/observatory subordinate to the United States Naval Observatory . USNO and NOFS are commands within the CNMOC claimancy,...
and the Oak Ridge Observatory
Oak Ridge Observatory
The Oak Ridge Observatory, also known as the George R. Agassiz Station, is located at 42 Pinnacle Road, Harvard, Massachusetts, and was formerly operated by the Harvard-Smithsonian Center for Astrophysics as a facility of the Smithsonian Astrophysical Observatory .The observatory was established in...
. 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 (48.5 to 37.3 mi).
Exploration
Galileo flyby
Ida was visited in 1993 by the JupiterJupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
-bound space probe Galileo. Its encounters of the asteroids Gaspra
951 Gaspra
951 Gaspra is an S-type asteroid that orbits very close to the inner edge of the asteroid belt. Gaspra was the first asteroid ever to be closely approached when it was visited by the Galileo spacecraft, which flew by on its way to Jupiter on 29 October 1991.-Characteristics:Apart from a multitude...
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
Space Shuttle Atlantis
The Space Shuttle Atlantis is a retired Space Shuttle orbiter in the Space Shuttle fleet belonging to the National Aeronautics and Space Administration , the spaceflight and space exploration agency of the United States...
mission STS-34
STS-34
STS-34 was a NASA Space Shuttle mission using Space Shuttle Atlantis. It was the 31st shuttle mission overall, and the 5th flight for Atlantis. During the mission, the Jupiter-bound Galileo probe was deployed into space...
on 18 October 1989. Changing Galileo's trajectory to approach Ida required that it consume 34 kg (75 lb) of propellant
Propellant
A propellant is a material that produces pressurized gas that:* can be directed through a nozzle, thereby producing thrust ;...
. 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.
Transmission of many Ida images was delayed due to a permanent failure in the spacecraft's high-gain antenna
High-gain antenna
A high-gain antenna is an antenna with a focused, narrow radiowave beam width. This narrow beam width allows more precise targeting of the radio signal - also known as a directional antenna...
. The first five images were received in September 1993. These comprised a high-resolution mosaic
Image stitching
Image stitching or photo stitching is the process of combining multiple photographic images with overlapping fields of view to produce a segmented panorama or high-resolution image. Commonly performed through the use of computer software, most approaches to image stitching require nearly exact...
of the asteroid at a resolution of 31–38 m/pixel
Pixel
In digital imaging, a pixel, or pel, is a single point in a raster image, or the smallest addressable screen element in a display device; it is the smallest unit of picture that can be represented or controlled....
. The remaining images were sent the next spring, 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 ShoemakerNEAR Shoemaker
The Near Earth Asteroid Rendezvous - Shoemaker , renamed after its 1996 launch in honor of planetary scientist Eugene M. Shoemaker, was a robotic space probe designed by the Johns Hopkins University Applied Physics Laboratory for NASA to study the near-Earth asteroid Eros from close orbit over a...
asteroid mission, permitted the first study of asteroid geology
Geology
Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...
. 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
S-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
based on ground-based spectroscopic measurements
Spectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
. 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
Ordinary chondrite
The Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...
(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.
Physical characteristics
Gravitational field
The gravitational field is a model used in physics to explain the existence of gravity. In its original concept, gravity was a force between point masses...
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 (65.6 ft/s) could escape
Escape velocity
In physics, escape velocity is the speed at which the kinetic energy plus the gravitational potential energy of an object is zero gravitational potential energy is negative since gravity is an attractive force and the potential is defined to be zero at infinity...
the asteroid entirely.
Croissant
A croissant is a buttery flaky pastry named for its distinctive crescent shape. It is also sometimes called a crescent, from the French word for "crescent". Croissants are made of a leavened variant of puff pastry...
-shaped". 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
Debris
Debris is rubble, wreckage, ruins, litter and discarded garbage/refuse/trash, scattered remains of something destroyed, or, in geology, large rock fragments left by a melting glacier etc. The singular form of debris is debris...
filling the gap between them. However, no such debris was seen in high resolution images captured by Galileo. Whilst 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 highly uneven gravitational field. The surface acceleration is lowest at the extremities because of their fast 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
Impact crater
In the broadest sense, the term impact crater can be applied to any depression, natural or manmade, resulting from the high velocity impact of a projectile with a larger body...
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
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
, 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.
Regolith
The surface of Ida is covered in a blanket of pulverized rock, called regolithRegolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
, about 50–100 m (164–328.1 ft) thick. This material is produced in impact event
Impact event
An impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Throughout recorded history, hundreds of minor impact events have been reported, with some occurrences causing deaths, injuries, property damage or other significant...
s and redistributed across Ida's surface by geological processes. Galileo observed evidence of recent downslope regolith movement
Mass wasting
Mass wasting, also known as slope movement or mass movement, is the geomorphic process by which soil, regolith, and rock move downslope under the force of gravity. Types of mass wasting include creep, slides, flows, topples, and falls, each with its own characteristic features, and taking place...
.
Ida's regolith is composed of the silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
minerals olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
. Its appearance changes over time through a process called space weathering
Space weathering
Space weathering is a blanket term used for a number of processes that act on any body exposed to the harsh space environment. Airless bodies incur many weathering processes:* collisions of galactic cosmic rays and solar cosmic rays,* irradiation, implantation, and sputtering from solar wind...
. Because of this process, older regolith appears more red in color compared to freshly exposed material.
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 (24.9 mi) ridge named Townsend Dorsum 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 (328.1 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 Kartchner. 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 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 familyKoronis family
]The Koronis family is a family of asteroids in the main belt between Mars and Jupiter. They are thought to have been formed at least two billion years ago in a catastrophic collision between two larger bodies. The largest known is about in diameter. The Koronis family travels in a cluster along...
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.
Lava tube
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow, expelled by a volcano during an eruption. They can be actively draining lava from a source, or can be extinct, meaning the lava flow has ceased and the rock has cooled and left a long, cave-like...
s on Earth. The crater Azzurra, for example, is named after a submerged cave on the island of Capri
Capri
Capri is an Italian island in the Tyrrhenian Sea off the Sorrentine Peninsula, on the south side of the Gulf of Naples, in the Campania region of Southern Italy...
, also known as the Blue Grotto
Blue Grotto (Biševo)
The Blue Grotto or Blue Cave , is a water-logged sea cave located in a small bay called Balun, on the east side of the island of Biševo and about from Komiža, in the Croatian Adriatic. It is situated in the central Dalmatian archipelago, 5 kilometers south-west of the island of Vis...
. 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
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
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
Prime Meridian
The Prime Meridian is the meridian at which the longitude is defined to be 0°.The Prime Meridian and its opposite the 180th meridian , which the International Date Line generally follows, form a great circle that divides the Earth into the Eastern and Western Hemispheres.An international...
.
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 Choukoutien which is smoother and less cratered. The ejecta
Ejecta
Ejecta can mean:*In volcanology, particles that came out of a volcanic vent, traveled through the air or under water, and fell back on the ground surface or on the ocean floor...
excavated by impacts is deposited differently on Ida than on planets because of its rapid rotation, low gravity and irregular shape. Ejecta blanket
Ejecta blanket
An ejecta blanket is a generally symmetrical apron of ejecta that surrounds crater; it is layered thickly at the crater’s rim and thin to discontinuous at the blanket’s outer edge....
s 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 asteroidS-type asteroid
S-type asteroids are of a stony composition, hence the name. Approximately 17% of asteroids are of this type, making it the second most common after the C-type.-Characteristics:...
based on the similarity of its reflectance spectra with similar asteroids. S-types may share their composition with stony-iron or ordinary chondrite
Ordinary chondrite
The Ordinary chondrites are a class of stony chondritic meteorites. They are by far the most numerous group and comprise about 87% of all finds...
(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
Bulk density
Bulk density is a property of powders, granules and other "divided" solids, especially used in reference to mineral components , chemical substances, ingredients, foodstuff or any other masses of corpuscular or particulate matter. It is defined as the mass of many particles of the material...
of 2.27–3.10 g/cm3. OC meteorites contain varying amounts of the silicates olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
and pyroxene
Pyroxene
The pyroxenes are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. They share a common structure consisting of single chains of silica tetrahedra and they crystallize in the monoclinic and orthorhombic systems...
, iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, and feldspar
Feldspar
Feldspars are a group of rock-forming tectosilicate minerals which make up as much as 60% of the Earth's crust....
. 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
Porosity
Porosity or void fraction is a measure of the void spaces in a material, and is a fraction of the volume of voids over the total volume, between 0–1, or as a percentage between 0–100%...
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
Koronis family
]The Koronis family is a family of asteroids in the main belt between Mars and Jupiter. They are thought to have been formed at least two billion years ago in a catastrophic collision between two larger bodies. The largest known is about in diameter. The Koronis family travels in a cluster along...
of asteroid-belt asteroids. Ida orbits the Sun at an average distance of 2.862 AU, between the orbits of Mars
Mars
Mars is the fourth planet from the Sun in the Solar System. The planet is named after the Roman god of war, Mars. It is often described as the "Red Planet", as the iron oxide prevalent on its surface gives it a reddish appearance...
and Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. Ida takes 4.84089 years to complete one orbit.
Ida's rotation period
Rotation period
The rotation period of an astronomical object is the time it takes to complete one revolution around its axis of rotation relative to the background stars...
is 4.63 hours, making it one of the fastest rotating asteroids yet discovered. The calculated maximum moment of inertia
Moment of inertia
In classical mechanics, moment of inertia, also called mass moment of inertia, rotational inertia, polar moment of inertia of mass, or the angular mass, is a measure of an object's resistance to changes to its rotation. It is the inertia of a rotating body with respect to its rotation...
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 nonspherical shape of the asteroid.
Origin
Ida originated in the breakup of the roughly 120 km (74.6 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
A small satelliteAsteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
named Dactyl orbits around Ida. Dactyl, officially (243) Ida I Dactyl, ˈdæktɨl , 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
Asteroid moon
A minor planet moon is an astronomical body that orbits a minor planet as its natural satellite. It is thought that many asteroids and Kuiper belt objects may possess moons, in some cases quite substantial in size...
. At the time, it was separated from Ida by a distance of 90 kilometres (55.9 mi), moving in a prograde orbit. Dactyl's surface 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 Harch, 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 10760 kilometres (6,686 mi) from Ida and 10870 kilometres (6,754.3 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
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
in 1994, for the mythological dactyls
Dactyl (mythology)
In Greek mythology, the Dactyls were the archaic mythical race of small phallic male beings associated with the Great Mother, whether as Cybele or Rhea. Their numbers vary, but often they were ten spirit-men so like the three Curetes, the Cabiri or the Korybantes that they were often interchangeable...
who inhabited Mount Ida
Mount Ida, Crete
Mount Ida, known variously as Idha, Ídhi, Idi, Ita and now Psiloritis , is the highest mountain on Crete. Located in the Rethymno Prefecture, it is sacred to the Greek Titaness Rhea, and on its slopes, according to legend, lies the cave, Idaion Andron, in which Zeus was born...
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 (262.5 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 spheroidSpheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....
al shape, imply that the moon is gravitationally controlled despite its small size. Like Ida, its average temperature is about 200 K (-73.2 °C; -99.7 °F).
Dactyl shares many characteristics with Ida. Their albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...
s 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
Regolith
Regolith is a layer of loose, heterogeneous material covering solid rock. It includes dust, soil, broken rock, and other related materials and is present on Earth, the Moon, some asteroids, and other terrestrial planets and moons.-Etymology:...
impossible. This contrasts with Ida, which is covered by a deep layer of regolith.
Orbit
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...
of Dactyl's orbit when most of the images of the moon 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 pericenter must be more than about 65 km (40.4 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 (55.9 mi) from Ida at longitude 85°. On 26 April 1994, the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
observed Ida for eight hours and was unable to spot Dactyl. It would have been able to observe the moon if it was more than about 700 km (435 mi) from Ida.
Dactyl's orbital period is about 20 hours, assuming it is in a circular orbit around Ida. Its orbital speed is roughly 10 m/s (32.8 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.External links
- Orbital simulation from JPL (Java) / Ephemeris