So the Asteroid Belts Are All Stripped Again

Asteroid Belts

The Origin of the Solar System

John E. Chambers , Alex N. Halliday , in Encyclopedia of the Solar Arrangement (Second Edition), 2007

7. The Asteroid Belt

The Asteroid Belt currently contains only enough material to make a planet 2000 times less massive than Earth, even though the spatial extent of the belt is huge. It seems likely that this region once contained much more mass than information technology does today. A smoothen interpolation of the amount of solid fabric needed to grade the inner planets and the gas giants would place about 2 Earth-masses in the Asteroid Belt. Even if most of this mass was lost at an early stage, the surface density of solid material must accept been at least 100 times higher than it is today in lodge to abound bodies the size of Ceres and Vesta (roughly 900 and 500 km in bore, respectively) in simply a few million years.

Several regions of the Asteroid Belt comprise clusters of asteroids with similar orbits and similar spectral features, suggesting they are fabricated of the same material. These clusters are fragments from the collisional breakup of larger asteroids. In that location are relatively few of these asteroid families, which implies that catastrophic collisions are quite rare. This suggests the Asteroid Belt has contained relatively little mass for most of its history. The spectrum of asteroid Vesta, located 2.4 AU from the Sun, shows that it has a basaltic crust. The HED meteorites, which probably come up from Vesta, show this chaff formed simply a few million years after the solar organisation, co-ordinate to several isotopic systems. The survival of Vesta'southward chaff suggests that the chaff formed the bear on rate in the chugalug has never been much higher than it is today. For these reasons, information technology is idea that most of the Asteroid Belt's original mass was removed at a very early stage by a dynamical process rather than by collisional erosion.

The Asteroid Belt currently contains a number of orbital resonances associated with the giant planets. Resonances occur when either the orbital period or precession period of an asteroid has a simple ratio with the corresponding period for one of the planets. Many resonances induce large changes in orbital eccentricity, causing asteroids to fall into the Sunday, or to come close to Jupiter, leading to close encounters and ejection from the solar organization. For this reason, there are very few asteroids that orbit the Sun twice every time Jupiter orbits the Sun in one case, for example. When the nebular gas was still nowadays, small asteroids moving on eccentric orbits would take drifted inward rapidly due to gas drag. After the giant planets had formed, a combination of resonances and gas drag may have transferred most objects smaller than a few hundred kilometers from the Asteroid Belt into the terrestrial-planet region. Larger planetary embryos would not take drifted very far. All the same, in one case oligarchic growth ceased, embryos began to gravitationally scatter one another beyond the belt. Numerical simulations show that most or all of these bodies would somewhen enter a resonance and be removed, leaving an Asteroid Belt profoundly depleted in mass and containing no objects bigger than Ceres. The timescale for the depletion of the belt depends sensitively on the orbital eccentricities of the giant planets at the time, which are poorly known. The chugalug may have been cleared in only a few million years, just it may have required every bit much as several hundred million years if the giant planets had nearly circular orbits.

The albedos and spectral features of asteroids vary widely from i body to another, but articulate trends are apparent equally i moves beyond the Asteroid Belt. S-type asteroids, which generally lie in the inner Asteroid Belt, announced to be more than thermally candy than the C-blazon asteroids that dominate the center belt. These may include the parent bodies of ordinary and carbonaceous chondrites respectively. C-types in plow seem more processed than the P-blazon asteroids that mostly lie in the outer belt. These differences may reflect differences in the composition of solid materials in different parts of the nebula, or differences in the time at which asteroids formed. Ordinary and enstatite chondrites, which probably come from the inner Asteroid Belt, tend to be dry out, while carbonaceous chondrites from the center and outer belt contain up to x% water past mass in the form of hydrated minerals. This suggests that temperatures were cold enough in the outer Asteroid Belt for water ice to form and become incorporated into asteroids where it reacted with dry rock. Temperatures were apparently besides loftier for water ice to condense in the inner Asteroid Chugalug. It is possible that some of the objects currently in the Asteroid Belt formed elsewhere. For example, it has been proposed that many of the parent bodies of the atomic number 26 meteorites, and possibly Vesta, formed in the terrestrial-planet region and were after gravitationally scattered outward to their current orbits.

Iron meteorites from the cores of melted asteroids are common, whereas meteorites from the mantles of these asteroids are rarely seen. This suggests that a substantial corporeality of collisional erosion took place at an early stage, with only the strong, iron-rich cores of many bodies surviving. A number of other meteorites besides show signs that their parent asteroids experienced fierce collisions early in their history. Chondrites presumably formed somewhat later than the differentiated asteroids, when the primary radioactive heat sources had mostly decayed. Chondrites are mostly composed of chondrules, which typically formed 1-3 Ma after CAIs. Chondrite parent bodies cannot exist older than the youngest chondrules they contain, so they must have formed several million years after the start of the solar system. For this reason, it appears that the early stages of planet germination were prolonged in the Asteroid Belt. Chondrites have experienced some degree of thermal processing, but their late formation meant that their parent bodies never grew hot enough to melt, which has allowed chondrules, CAIs, and matrix grains to survive.

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Planetary Radar Astronomy

Steven J. Ostro , in Encyclopedia of Concrete Scientific discipline and Technology (Third Edition), 2003

Iii.J Asteroids

Echoes from 37 main-belt asteroids (MBAs) and 58 most-Earth asteroids (NEAs) have provided a wealth of new information about these objects' sizes, shapes, spin vectors, and surface characteristics such as decimeter-scale morphology, topographic relief, regolith porosity, and metallic concentration. During the by decade, radar has been established as the most powerful Globe-based technique for determining the physical properties of asteroids that come close enough to yield strong echoes.

The polarization signatures of some of the largest MBAs (e.g., 1 Ceres and 2 Pallas) reveal surfaces that are smoother than that of the Moon at decimeter scales but much rougher at some much larger scale. For case, for Pallas, μ_c is but ∼0.05 and, equally noted higher up, surface slopes exceed 20°. For asteroids in the 200-km-diameter range, the echoes provide show for big-scale topographic irregularities. For example, brightness spikes within narrow ranges of rotation phase advise large, apartment regions on 7 Iris (Fig. 2s0), nine Metis, and 654 Zelinda, and filibuster-Doppler images of 216 Kleopatra reveal a dumbbell shape.

In that location is a 10-fold variation in the radar albedos of MBAs, implying substantial variations in these objects' surface porosities or metallic concentrations or both. The lowest MBA albedo estimate, 0.04 for Ceres, indicates a lower surface bulk density than that on the Moon. The highest MBA albedo estimates, 0.31 for xvi Psyche and 0.44 for Kleopatra, are consistent with metal concentrations about unity and lunar porosities. These objects might be the collisionally stripped cores of differentiated asteroids and by far the largest pieces of refined metal in the solar arrangement. Surprisingly, there is no reason to believe that the two largest classes of radar-observed asteroids (C and Due south) have different radar-albedo distributions.

The diversity of NEA radar signatures is extreme (see Table II). Some small NEAs are much rougher at decimeter scales than MBAs, comets, or the terrestrial planets. The radar albedo of the 2-km object 6178 (1986DA), 0.58, strongly suggests that this World-approacher is a regolith-costless metallic fragment, presumably derived from the interior of a much larger object that melted, differentiated, cooled, and subsequently was disrupted in a catastrophic collision. This asteroid, which appears extremely irregular at 10- to 100-m scales and shows hints of existence bifurcated, might be (or have been a function of) the parent body of some iron meteorites. At the other farthermost, an interval estimate for 1986JK'due south radar albedo (0.005 to 0.07) suggests a surface bulk density within a factor of 2 of 0.9 m cm⁻³. Similarly, the distribution of NEA circular polarization ratios runs from near zero to near unity. The highest values, for 2101 Adonis, 1992QN, 3103 Eger, and 3980 1980PA, bespeak extreme almost-surface structural complexity, but nosotros cannot distinguish betwixt multiple scattering from subsurface heterogeneities (come across Section III.Grand) and single scattering from complex structure on the surface.

The MBAs 951 Gaspra and 243 Ida, imaged by the Galileo spacecraft, probably are marginally detectable with the upgraded Arecibo. Both Goldstone and Arecibo have investigated the Gaspra-sized Martian moon Phobos, whose radar properties differ from those of virtually small, World-budgeted objects simply resemble those of large (∼100-km), C-form, main-chugalug asteroids. Phobos's surface characteristics may exist more representative of Ceres and Pallas than near NEAs. The upper limit on the radar cross section of Deimos, which has defied radar detection, argues for a surface bulk density no greater than about i g cm⁻³.

During the past decade, delay-Doppler imaging of asteroids has produced spatial resolutions as fine as a few decameters. The images more often than not can be "north–south" cryptic; that is, they constitute a two-to-one (or even many-to-one) mapping from the surface to the image. However, if the radar is not in the target's equatorial plane, then the delay-Doppler trajectory of any surface bespeak is unique. Hence images that provide acceptable orientational coverage can be inverted, and in principle one can reconstruct the target's three-dimensional shape besides equally its spin state, the radar-scattering properties of the surface, and the move of the center of mass through the delay-Doppler ephemerides.

The beginning asteroid radar data ready suitable for reconstruction of the target's shape was a ii.5-hr sequence of 64 delay-Doppler images of 4769 Castalia (1989PB) (Fig. 21a), obtained two weeks after its August 1989 discovery. The images, which were taken at a subradar latitude of about 35°, evidence a bimodal distribution of echo ability over the full range of sampled rotation phases, and to the lowest degree-squares interpretation of Castalia's three-dimensional shape (Fig. 21b) reveals it to consist of two kilometer-sized lobes in contact. Castalia manifestly is a contact-binary asteroid formed from a gentle collision of the 2 lobes.

If the radar view is equatorial, unique reconstruction of the asteroid's 3-dimensional shape is ruled out, but a sequence of images that thoroughly samples rotation stage tin can allow unambiguous reconstruction of the asteroid'south pole-on silhouette. For example, observations of 1620 Geographos yield ∼400 images with ∼100-thou resolution. The pole-on silhouette'due south extreme dimensions are in a ratio, two.76 ± 0.21, that establishes Geographos as the most elongated solar system object imaged so far (see Fig. 8). The images show craters as well as indications of other sorts of big-scale topographic relief, including a prominent central indentation. Protuberances at the asteroid's ends may exist related to the pattern of ejecta removal and deposition caused past the asteroid's gravity field.

Delay-Doppler imaging of 4179 Toutatis in 1992 and 1996 achieved resolutions as fine as 125 nsec (19 m in range) and 8.three mHz (0.15 mm sec^−one in radial velocity), placing hundreds to thousands of pixels on the asteroid. This information fix provides concrete and dynamical information that is unprecedented for an Earth-crossing object. The images (Fig. 22) reveal this asteroid to be in a highly unusual, non-primary-axis (NPA) spin state with several-mean solar day characteristic time scales. Extraction of the data in this imaging data set required inversion with a much more comprehensive concrete model than in the assay of Castalia images; free parameters included the asteroid'southward shape and inertia matrix, initial conditions for the asteroid's spin and orientation, the radar-scattering properties of the surface, and the delay-Doppler trajectory of the center of mass. The shape (Fig. 23) reconstructed from the low-resolution images of Toutatis has shallow craters, linear ridges, and a deep topographic "neck" whose geologic origin is not known. It may take been sculpted by impacts into a single, coherent torso, or Toutatis might actually consist of 2 separate objects that came together in a gentle collision. Toutatis is rotating in a long-axis mode (encounter Fig. 23) characterized by periods of 5.4 days (rotation virtually the long axis) and 7.4 days (boilerplate for long-axis precession about the angular momentum vector). The asteroid's principal moments of inertia are in ratios inside 1% of three.22 and 3.09, and the inertia matrix is indistinguishable from that of a homogeneous body. Such data has yet to be determined for any other asteroid or comet, and probably is impossible to learn in a fast spacecraft flyby. Higher resolution images (e.g., Fig. 22b) from the 1992 and 1996 experiments are at present being used to refine the Toutatis model.

Accurate shape models of near-Earth asteroids open up the door to a wide variety of theoretical investigations that previously take been impossible or take used simplistic models (spheres or ellipsoids). For example, the Castalia and Toutatis models are beingness used to explore the stability and evolution of close orbits, with direct application to the design of robotic and piloted spacecraft missions, to studies of retention and redistribution of impact ejecta and to questions about plausible origins and lifetimes of asteroidal satellites. Accurate models also allow realistic investigations of the effects of collisions in various energy regimes on the object's rotation state, surface topography, regolith, and internal structure. Simulations of impacts into Castalia using smooth-particle hydrodynamics code take begun to suggest how surface and interior damage depends on impact energy, bear on location, and the equation of country of the asteroidal fabric. These figurer investigations accept clear ramifications for our understanding of asteroid collisional history, for exploitation of asteroid resources, and eventually for deflection/destruction of objects found to be on a collision course with Globe.

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Meteorites, Comets, and Planets

J.Eastward. Chambers , in Treatise on Geochemistry, 2007

1.17.5.1 Formation and Mass Depletion

The most obvious characteristic of the primary asteroid belt is its bang-up depletion in mass relative to other parts of the planetary system. The current mass is simply one/two,000th that of the Earth. Models for the collisional evolution of the asteroid chugalug advise at that place were originally 200 times as many bodies smaller than 1,000 km present in this region (Bottke et al., 2005). Bodies larger than 100 km have collisional lifetimes longer than the historic period of the solar system, and then near of this excess mass was removed dynamically rather than by collisions (Bottke et al., 2005). A smoothen interpolation of the corporeality of mass present in the terrestrial and giant-planet regions suggests the asteroid chugalug once contained several Earth masses of solid textile. This makes it likely that planetary embryos formed here. The unusual properties and composition of the CB chondrite class of primitive meteorites can be understood if they formed from the collisional breakdown of one such embryo (Krot et al., 2005).

The asteroid chugalug probably lost most of its primordial mass due to the presence of several dynamical "resonances" between 2 and 4 AU from the Sun. An asteroid in one of these resonances quickly develops an orbit that is highly eccentric. It typically collides with the Sun or is ejected from the solar system in about a million years (Gladman et al., 1997). The resonances currently occupy a minor fraction of the asteroid belt, simply the "secular resonances" swept beyond the chugalug at the time the protoplanetary nebula was dispersing (Nagasawa et al., 2000). A combination of "resonance sweeping" and gas drag, which is highly effective for bodies moving on eccentric orbits, may have transferred many asteroid-sized bodies into the region occupied by the terrestrial planets (Franklin and Lecar, 2000).

Planetary embryos were largely immune to gas drag, but shut encounters between embryos would have caused frequent changes in their orbits until they entered a resonance and were removed. Numerical simulations show the virtually likely upshot is that all planetary embryos were lost from the asteroid region (Chambers and Wetherill, 2001) along with 99% of the smaller bodies (Petit et al., 2001). Embryos caught in sweeping secular resonances may also have migrated into the terrestrial-planet region due to tidal torques from the nebula gas (Nagasawa et al., 2005).

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Primary-Belt Asteroids

Daniel T. Britt , ... Larry Lebofsky , in Encyclopedia of the Solar Organisation (Second Edition), 2007

2.2 Special Orbital Classes

Even though most asteroids are found in the Master Asteroid Belt betwixt Jupiter and Mars, there are a number of other asteroid groups. The "asteroids" beyond the orbit of Jupiter are probably volatile-rich and would become cometary if they were moved to the inner solar organisation, just for the purposes of this discussion we will list these groups of small-scale bodies as asteroids hither. The asteroids that circle the Sun at the same orbital altitude as Jupiter are called Trojan asteroids. They reside in dynamically stable zones 60° alee and backside Jupiter. These positions are the concluding 2 of the five Lagrangian points, "named by the 19th-century mathematician J. 50. Lagrange. He first described the orbital beliefs of small bodies affected by the gravitation pull of two large objects such as the Sun and Jupiter. He found that along with three unstable equilibrium points (Fifty₁ through L₃ ), a pocket-size body similar an asteroid could share Jupiter's orbit then long as both formed an equilateral triangle with the Sunday. At that place are 2 such points; the L_iv indicate lies ahead of Jupiter, while L₅ trails behind it.

The Trojans derive their name from the first such asteroid discovered, named Achilles afterward the hero of the Trojan War. The L ₄ region asteroids are named for Greek heroes of the Iliad, while Trojan heroes populate the L₅ region. (The exceptions, named earlier this rule was adopted, include two of the largest Trojans: 617 Patroclus, named for the Greek hero, orbits among the Trojans at L₅ , while 624 Hektor, the largest Trojan and a hero of Troy, orbits at L_iv with the Greeks.) Nearly 2000 Jupiter Trojans have been discovered to date; oddly, the L_iv region is nearly twice as populated as the L₅ region.

Some other major group of small planets is the Centaurs. Named as a class after the discovery of Chiron, a small-scale body orbiting between Saturn and Uranus, the term has somewhen grown to include whatever noncometary body beyond Saturn whose orbit crosses the orbit of a major planet; even the noncomet office must be relaxed, equally Chiron itself has been seen on occasion to have a comet-like coma. These "asteroids" are well-nigh likely big, volatile-rich objects (i.eastward., comets) perturbed inward from the Kuiper Belt (Fig. 3b). But considering the Centaurs orbit deep in the outer solar arrangement, they cannot warm sufficiently to allow volatiles to sublimate off and show cometary activity, and so they are considered asteroids until proven otherwise. In terms of their orbits, this group includes the classical Centaurs (some 2 dozen objects known to orbit like Chiron between Saturn and Uranus), roughly 50 objects whose orbits cantankerous Uranus' or Neptune's orbit, and the 75 objects (discovered to date) that prevarication in highly eccentric orbits ranging out across the Kuiper Belt. All are considered scattered disk objects, which have been dynamically scattered by Neptune'southward gravity out of the disk of the Kuiper Chugalug. [See Kuiper Belt: Dynamics.]

The Kuiper Chugalug itself is the outermost set of minor bodies. It is made up of objects populating infinite beyond the orbit of Neptune but inside nearly 1000 AU. The first object was discovered in 1992 (1992 QB1) with a semimajor axis of 44 AU and an estimated diameter of several hundred kilometers. Too the scattered disk objects noted earlier, other dynamical classes of Kuiper Belt objects include others similar 1992 QB1 in low-inclination, low-eccentricity orbits (sometimes chosen "cubewanos" afterwards their kickoff example) and others orbiting like Pluto (and so called "plutinos") in a 2:3 resonance with Neptune. Once more, all these objects are probably cometary. In fact, the existence of the Kuiper Belt was beginning suggested in 1949 every bit a source expanse for curt-catamenia comets. Given the nearly 1000 Kuiper Belt "asteroids" discovered so far, at that place are probably hundreds of thousands of objects larger than a kilometer populating this belt. [See Kuiper Belt: Dynamics.]

Inward from the main asteroid belt are the asteroids that cross the orbits of the inner planets: the Amor, Apollo, and Aten asteroids. Amor asteroids are asteroids whose eccentric orbits dip in from the Asteroid Belt to cross the orbit of Mars, just without reaching the orbit of the Earth. Apollos are those that practise cross Globe'southward orbit, simply whose semimajor centrality is e'er >1 AU. This differentiates them from Atens, which also cantankerous the Earth's orbit just that have semimajor axes inside of Earth'southward orbit. The Apollo and Amor objects are collectively chosen near-Globe objects or NEOs. They are relatively modest objects; the largest known NEO is the Amor object 1036 Ganymed, with a diameter of 38.v km. NEOs are also subject to a ability law distribution; as the population increases, their sizes drib chop-chop. As of September 2006, there are 830 NEOs with diameters >ane km out of a population of approximately 3800 known NEOs. Information technology is estimated that in that location are approximately 1200 full NEOs that are larger than 1 km. These are the objects that can and (in the course of geologic time) do oftentimes collide with Globe. Indeed, reckoner calculations indicate that well-nigh NEOs could but survive in their present orbits for roughly 10 million years before falling into the Lord's day, colliding with a planet, or existence ejected. Thus, the NEO population must exist continually replenished from the Asteroid Chugalug. Compositional data indicates that NEOs are drawn from every zone of the Asteroid Belt and take been perturbed into the inner solar system past a diversity of mechanisms including the Yarkovsky effect described in the adjacent section. [See Near-Earth Objects.]

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SOLAR SYSTEM | Meteorites

Grand.J.H. McCall , in Encyclopedia of Geology, 2005

Asteroidal

Meteorites are nowadays accustomed as fragments of strays from the asteroid belt betwixt Mars and Jupiter. Prior to the machinery being established of producing (due to collisions) eccentric elongated orbits for asteroids – replacing their quasi-round orbits beyond Mars – the nucleii of comets, impoverished in volatiles past repeated passage round the Sun, were long favoured as their source, but petrological and mineralogical testify is against this. The Farmington autumn in Kansas in 1890 seems to accept heralded the firm establishment of asteroidal source. Sixty reports of visual observations of this fireball, at 12.l pm on a midsummer mean solar day and reportedly rivalling the Sun, were selected by scientists who deduced an orbit indicating that the parent asteroid was 1862 Apollo, Hermes, or 1865 Cerberus. Direct ascertainment of fireballs by astronomers of the Sikhot-Alin, Siberia, 1949 and Pribram, Czechoslovakia, 1959 fireballs once again strongly supported asteroidal sources and there accept been many further supporting observations since (Figure 10). In recent years in that location take been numerous attempts to utilise optical and spectrographic methods to equate the reflectance and chemistry of asteroids with unlike classes of meteorites, only results seem to be inconclusive, possibly because of the operation of footling understood space-weathering processes which touch on the regolith surface of asteroids. Even a directly exploration mission to Eros in 2000–2001 (Figure eleven) yielded no correlation and it must be borne in listen that there must exist asteroids of classes never sampled by meteorites falling on the Earth. Several thousand asteroids are at present known and it is estimated that there may be equally many as x 000 out there.

Fifty-fifty in these small parent bodies, though some did non achieve 100°C, others heated to more than than 1200°C, the temperature needed to form a basaltic-textured eucrite. The heat sources in these small bodies are not known for sure, but a source in extreme early heating of the Sun or internal short-lived radioactive isotopes such as ²⁶Al is favoured.

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Primitive Solar System Objects: Asteroids and Comets

Lucy-Ann McFadden , Daniel T. Britt , in Encyclopedia of Physical Science and Engineering science (3rd Edition), 2003

II.A.1 Asteroid Belts and Planet-Crossing Asteroids

The vast majority of known asteroids orbit the sun in the chief asteroid belt between Mars and Jupiter. Nonetheless, this is non the simply asteroid belt and asteroids exercise non all stay within their belts. A minor fraction of principal-belt asteroids accept orbits that cantankerous the orbits of the inner planets. These are called near-earth asteroids, and are a major source of the meteorites that occasionally autumn to globe. Asteroids with orbits that approach the earth, passing within the perihelion of Mars, are called Amors. Those that cantankerous the orbit of the earth are called Apollo asteroids. A third grouping of near-globe asteroids (NEAs) that have orbits crossing that of the world's but that do non travel further from the sun than the globe's aphelion are called Aten asteroids, subsequently the name of the first asteroid discovered with this type of orbit. There are currently over 1200 known NEAs.

An estimated 100–grand tons of extraterrestrial material, ranging from submicrometer-sized grit to boulders weighing many tons, accomplish the world's surface every day. Some of this cloth is derived from the well-nigh-earth asteroids. Larger asteroids also collide with earth. It is estimated that an asteroid that is 1 km in diameter or larger will collide with the globe on the average about once every 1.5–2 meg years. Most about-world asteroids are not on collision courses with the earth, merely their orbits may be near enough to the globe to exist used for edifice and manufacturing in space in the hereafter. Calculations of the orbits of virtually-world asteroids indicate that they take lifetimes in their present orbits for ten–100 million years. They are "culled" from near-earth space either a collision with the inner planets, collisions with other asteroids, or ejection from the solar system by gravitational perturbations due to about misses with Mars, globe, or Venus. Since the age of the solar organisation as determined from radioactive dating of meteorites is at least iv.5 billion years, at that place must be a source of these asteroids to proceed replenishing that part of the population that is lost. Some of the almost-globe asteroids are probably the solid remains of comets that have exhausted all their gases and at present orbit the sun in planet-crossing orbits. The major source of near-globe asteroids are fragments from large asteroids in the primary asteroid belt that are perturbed into planet-crossing orbits by gravitational interactions with Jupiter (run into Section II.A.2, on Kirkwood gaps). 1 of the major goals of near-earth asteroid studies is to utilize these objects as a source of information on the composition and processes of the larger asteroids in the master chugalug. Other every bit important goals are to determine their importance every bit economic resource as well as hazards from near-globe asteroid impacts on earth.

The outer solar system as well has its belts of asteroids and its planet-crossing objects. Asteroids with semimajor centrality between Jupiter and Neptune are called Centaurs and these objects regularly interact gravitationally and collisionally with the massive gas behemothic planets. An additional belt of asteroids was first observed in the 1990s outside the orbit of Neptune and is called "trans-Neptunian." This is the inner edge of the Edgeworth–Kuiper belt that is thought to extend out as far as yard AU. These outer solar organisation "asteroids" probably have cometary compositions and are almost certainly the sources of comets, but every bit long as they stay in the cold outer solar system, they show no outflow of gas and dust and thus await like primitive "asteroids." New members of these groups are being discovered at a rapid charge per unit and as of November 2000 there are 63 Centaurs and 345 trans-Neptunians.

The asteroids of the outer solar arrangement, the Centaurs and trans-Neptunian objects, also interact with the planets. Orbits with semimajor axis greater than 45 AU are thought to be stable, but within 45 AU, studies show a number of chaotic zones equally well as some stable orbits. Perturbations on trans-Neptunian asteroids are thought move them into the solar system, supplying the Centaurs with fresh objects and acting as the reservoir for short-period comets (come across Department Two).

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Planetary Radar

Steven J. Ostro , in Encyclopedia of the Solar Organization (2nd Edition), 2007

3.12.i DISK-INTEGRATED Backdrop

The depression polarization ratios and broad spectral shapes of some of the largest MBAs (east.1000., 1 Ceres and 2 Pallas) reveal surfaces that are smoother than that of the Moon at decimeter scales but much rougher at some much larger scale. For some asteroids in the 200-km-diameter range (including 7 Iris, 9 Metis, and 654 Zelinda), effulgence spikes within narrow ranges of the rotation phase propose large, flat regions.

At that place is a 10-fold variation in asteroid radar albedos, implying substantial variations in these objects' surface porosities or metal concentrations, or both. The everyman MBA albedo approximate, 0.04 for Ceres, indicates a lower surface bulk density than that on the Moon. The highest MBA albedo estimates, 0.31 for 16 Psyche and 0.44 for Kleopatra, are consistent with metal concentrations nigh unity and lunar porosities. These objects might be the collisionally exposed interiors of differentiated asteroids and past far the largest pieces of refined metal in the solar organisation.

The radar albedo of the 2-km NEA 6178 (1986DA), 0.58, strongly suggests that information technology is a regolith-free metallic fragment, presumably derived from the interior of a much larger object that melted, differentiated, cooled, and later on was disrupted in a catastrophic collision. 1986 DA might be (or accept been a part of) the parent body of some iron meteorites. At the other extreme, the range for 1986 JK's radar albedo (0.005 to 0.07) suggests a surface bulk density within a cistron of two of 0.ix m cm⁻³. Similarly, the distribution of NEA circular polarization ratios runs from near zilch to near unity. The highest values, for 2101 Adonis, 1992QN, 3103 Eger, 3980 1980PA, 2000 EE104, and 2004 XP14, indicate farthermost nearly-surface structural complexity, but we cannot distinguish betwixt multiple scattering from subsurface heterogeneities (see Department 3.7) and unmarried scattering from complex structure on the surface.

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Kuiper Belt Objects: Physical Studies

Stephen C. Tegler , in Encyclopedia of the Solar Organisation (2d Edition), 2007

10.3 Origin of KBO Binaries

Ii of the almost unusual features of KBO binaries, compared to main belt asteroid and near-Globe asteroid binaries, are the wide separation and like diameter of each pair of components. These unusual features make information technology unlikely that collisions between 2 KBOs created each binary system, as in the case of the Globe and the Moon. Similarly, it isn't likely that ane KBO gravitationally captured another KBO to form a binary system. A mechanism put forth by Stuart Weidenschilling suggests that information technology is possible to create a loosely bound KBO binary past collision and capture in the presence of a third body. His mechanism requires many more than KBOs than are seen today; perhaps such a mechanism operated long ago in a more than densely populated Kuiper Belt (see the adjacent section). Peter Goldreich put forth a mechanism wherein capture takes identify during a close run into as a result of the dynamical friction with the many surrounding pocket-sized bodies. Each of these mechanisms produces its signature on the population of binaries we see today. For example, Weidenschilling's mechanism favors the production of wide binary pairs, and Goldreich's mechanism favors the product of closer pairs. Only the discovery of many more binaries will allow us to determine whether either of these mechanisms or some other machinery is responsible for the formation of KBO binaries.

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Near-Earth Objects

Lucy A. McFadden , Richard P. Binzel , in Encyclopedia of the Solar System (Second Edition), 2007

3.5 Dynamical History

Dynamicists have faux the pathways that objects might take from unstable regions of the Asteroid Belt using computations of dynamical forces interim in the solar system. In some cases, fragments from asteroid collisions may be violently cast into these regions of instability. Still, a softer touch may play an fifty-fifty bigger role. Abiding warming by the Sun causes asteroids of all sizes to reradiate their heat back into space. Considering the asteroids are rotating, the reradiation does non occur in the same direction every bit the incoming sunlight, resulting in a small force acting on the asteroid. This force acts every bit a very gentle push on the asteroid, which over many millions of years can cause the asteroid to slowly drift inwards or outward from its original main-belt location. This is called Yarkovsky migrate and is especially effective on small objects; it may be particularly important for supplying meteoroids to Earth. Cast away fragments or drifting bodies that enter regions where resonances with Jupiter's orbit are peculiarly strong, such as the 3:1 Kirkwood gap, find that small changes in the semimajor axis can result in large, exponential changes in other orbital elements, in particular eccentricity, changing the orbit significantly on a brusk timescale. Thus, the effects of chaotic regions are more than the sum of small changes in motion over long periods of time. These regions of chaotic motility are associated with resonances with both Jupiter and Saturn (Fig. 8). The ii gas behemothic planets are believed to play a significant part in directing meteoroids to Earth, and presumably too many of the near-Earth objects.

Other objects evolve from Jupiter-family comets or Halley-type short-flow comets. Life in the Jupiter family is non long-lived, as Jupiter imparts changes to the orbits on timescales of x⁴–10⁶ years. Leaving Jupiter'southward gravitational sphere of influence, the before long-to-be near-Globe objects may sometimes be perturbed by Mars and other terrestrial planets and also affected past the influences of nongravitational forces, such every bit volatile outgassing or splitting of the cometary nucleus. These phenomena too contribute to orbital changes that outcome in planet-crossing orbits.

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Solar System Dynamics: Regular and Cluttered Movement

Jack J. Lissauer , Carl D. Murray , in Encyclopedia of the Solar Organization (Second Edition), 2007

5.ii Meteorites

About meteorites are thought to be the fragments of material produced from collisions in the asteroid belt, and the reflectance properties of sure meteorites are known to exist similar to those of mutual types of asteroids. Since near collisions take place in the asteroid belt, the fragments have to evolve into World-crossing orbits before they can striking Earth and be nerveless as samples.

An estimate of the fourth dimension taken for a given meteorite to reach Earth after the collisional issue that produced it can be obtained from a measure of its cosmic ray exposure historic period. Prior to the collisions, the fragment may accept been well below the surface of a much larger trunk, and as such it would have been shielded from all only the nigh energetic cosmic rays. However, after a collision the exposed fragment would be subjected to cosmic ray battery in interplanetary space. A detailed analysis of meteorite samples allows these exposure ages to be measured.

In the case of 1 mutual class of meteorites called the ordinary chondrites, the cosmic ray exposure ages are typically less than 20 million years and the samples evidence fiddling evidence of having been exposed to high pressure, or "shocking." Prior to the application of chaos theory to the origin of the Kirkwood gaps, there was no plausible mechanism that could explicate delivery to World inside the exposure age constraints and without shocking. Withal, pocket-sized increments in the velocity of the fragments equally a result of the initial collision could easily cause them to enter a chaotic zone near a given resonance. Numerical integrations of such orbits about the 3:ane resonance showed that it was possible for them to attain eccentricities large enough for them to cross the orbit of Globe. This result complemented previous research that had established that this office of the asteroid belt was a source region for the ordinary chondrites. Another effect that must be considered to obtain understanding between theory and observations is the Yarkovski upshot which is discussed below. [See Meteorites.]

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Source: https://www.sciencedirect.com/topics/physics-and-astronomy/asteroid-belts

So the Asteroid Belts Are All Stripped Again

Asteroid Belts

The Origin of the Solar System

7. The Asteroid Belt

Planetary Radar Astronomy

Iii.J Asteroids

Meteorites, Comets, and Planets

1.17.5.1 Formation and Mass Depletion

Primary-Belt Asteroids

2.2 Special Orbital Classes

SOLAR SYSTEM | Meteorites

Asteroidal

Primitive Solar System Objects: Asteroids and Comets

II.A.1 Asteroid Belts and Planet-Crossing Asteroids

Planetary Radar

3.12.i DISK-INTEGRATED Backdrop

Kuiper Belt Objects: Physical Studies

10.3 Origin of KBO Binaries

Near-Earth Objects

3.5 Dynamical History

Solar System Dynamics: Regular and Cluttered Movement

5.ii Meteorites

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