Asteroids main belt

Anhydrous planetesimals formed within the inner solar system, unlike the ice-bearing bodies discussed in the next chapter. These objects, composed of rock and metal, were the primary building blocks of the terrestrial planets. Relics of that population may survive today as asteroids that dominate the inner portions of the main belt. 

The orbits of asteroids in the main belt between Mars and Jupiter show gaps in both heliocentric distance and inclination (degrees from the ecliptic, the plane in which the planets... 

Heliocentric distribution of asteroid spectral types in the main belt, (a) (modified from Gradie and Tedesco, 1982) and (b) (modified from Mothe-Diniz et al., 2003) show distributions using two different asteroid taxonomies, explained in the text. Petrologic interpretations of the various spectral classes are shown in (a). 

The heliocentric pattern of asteroid types, with thermally processed objects closer to the Sun (as inferred from spectra), persists despite subsequent dynamical stirring of asteroid orbits and ejection of bodies from the main belt. Differentiated objects appear to have formed earlier than chondritic bodies, and dynamical modeling suggests they may have accreted... 

Mothe-Diniz, T, Carvano, J. M. and Lazzaro, D. (2003) Distribution of taxonomic classes in the main belt of asteroids. Icarus, 162, 10-21. 

Several classes of asteroids are also thought to contain ices presently, or contained them at some earlier time. The D- and F-class asteroids occur in the outmost main belt, and the C-, G-, B-, and F-class asteroids are concentrated within the central part of the belt. These asteroids probably formed near their present locations, in which case they represent icebearing planetesimals that accreted inside the orbit of Jupiter. A few asteroids exhibiting cometary activity also occur within the asteroid belt. 

The low-albedo C-, B-, G-, and F-class asteroids dominate the middle part of the main belt. The sharp 3 pm feature in the spectra of these objects indicates that hydrous... 

The D- and P-class asteroids dominate the outer main belt and Trojan asteroids located in Jupiter s orbit. With only a few exceptions, the spectra of these asteroids show no 3 pm absorption bands (Jones et al., 1990). The D and P asteroids are thought to contain ice that has never been melted. However, it is also possible that D and P asteroids could contain hydrated silicates, and that the 3 pm feature is masked by an increasing abundance of elemental carbon with heliocentric distance. The unique carbonaceous chondrite Tagish Lake has a reflectance spectrum quite similar to D-class asteroids, and it has been hypothesized to be a sample of this class. However, Tagish Lake shows a significant 3 pm absorption. 

Sample returns from additional comets, outer main belt and Trojan asteroids, and Kuiper Belt objects representing distinct regions of the early Solar System. 

Chambers Cassen 2002 O Brien etal. 2006). If no other mechanisms are invoked to enhance ej and es, they must have once been 2x larger than at present. When ej and < s are large, the amount of material from the Asteroid Belt that is incorporated into the terrestrial planets is reduced because the main-belt resonances become stronger, and material is cleared from the Asteroid Belt more rapidly (e.g. Chambers Cassen 2002 Raymond et al. 2004 O Brien et al. 2006). 

Figure 10.8 Observational estimate of the main-belt size distribution from Bottke et al (2005a), which is based on the debiasing of the cataloged asteroid population and Sloan Digital Sky Survey data from Jedicke et al (2002).

Disk temperatures would have decreased rapidly with distance from the Sun as accretional energy release, optical depth, and solar radiation all declined. For example, some meteorite samples from main-belt asteroids contain hydrated silicates, formed by reactions between anhydrous rock and water ice. This implies that temperatures at 2-3 AU became low enough for ice to condense while the asteroids were forming. 

The primary feature of the main asteroid belt is its great depletion in mass relative to other regions of the planetary system. The present mass of the main belt is 5 X 10 m , which represents 0.1 -0.01 % of the solid mass that existed at the time planetesimals were forming. There are several ways the main asteroid belt could have lost most of its primordial mass. Substantial loss by collisional erosion appears to be mled out by the preservation of asteroid Vesta s basaltic cmst, which formed in the first few million years of the solar system (Davis et al., 1994). More plausible models are based on the existence of orbital resonances associated with the giant planets. 

Belt (Bell et al., 1989) and are presumed to be the origin of the radial dependence of asteroid reflectance classes with increasing distance from the Sun. Once started, the internal heating process would have been enhanced by the exothermic heat from hydration processes. Some of the water must have escaped from the interiors of internally heated asteroids and perhaps a significant proportion of main Belt asteroids showed cometary activity before they turned into what are now considered to be asteroids. Had they been observed early in the history of the solar system all of the hydrated silicate-bearing asteroids would have been considered to be comets. 

Carbon-rich CP IDPs are spectrally red with a redness comparable to the comet-like outer asteroid Pholus (Binzel, 1992). Hydrated CS IDPs that contain layer lattice silicates exhibit spectral characteristics similar to carbonaceous chondrites and main-belt C-type asteroids (Bradley et al., 1996b). 

The main-belt region is not evenly populated with asteroids, and several zones have been found in which virtually no asteroids reside (Fig. 1). The American astronomer Daniel Kirkwood (1814-1895) first noticed these empty regions, or gaps, in 1866. Now called Kirkwood gaps, these asteroid-devoid zones are located near orbits for which the time to complete one circuit around the Sun is a simple fraction (e.g., 1/2, 2/3, 3/4) of Jupiter s orbital period. For example, given that Jupiter orbits the Sun once every 11.86 years, an asteroid belt gap is expected at a distance of 3.3 astronomical units (AU) from the Sun, where any orbiting body would have a period of 5.93 years, one-half that of Jupiter. Such a gap does indeed exist. These Kirkwood gaps are produced by... 

The C-type asteroids have a bluish color, and their reflection spectra indicate the presence of carbonaceous material at their surfaces. The S-type asteroids, on the other hand, are more reddish in color and their reflection spectra indicate that presence of surface silicate material. Other classifications include M-type, indicating the presence of surface metals, and R-type, indicating a deep, dark red color. Observations have revealed that the S-type asteroids tend to reside in the inner main belt, near the orbit of Mars. The C-type asteroids, in contrast, tend to reside toward the outer edge of the main belt nearer to Jupiter s orbit. The S-type asteroids are thought to be the primary source of stony and stony-iron meteorites, while the M-type asteroids are the most likely source of the iron meteorites. 

Not all asteroids reside in the main belt. The asteroid Hidalgo, discovered by American astronomer Walter... 

In February 2000, asteroid studies took a remarkable jump forward when the Near Earth Asteroid Rendezvous (NEAR)-Shoemaker spacecraft, first of NASA s Discovery series of relatively low-cost space probes, went into orbit around the asteroid 433 Eros, an S-class asteroid about 8 X 8 X 21 mi (13 X 13 X 33 km) in size. NEAR-Shoemaker, the first spacecraft ever to orbit an asteroid, had already flown by the C-class, main-belt asteroid 253 Mathilde in 1997, taking high-resolution... 

Knezevic, Z. and Milani, A. (2000) Synthetic Proper Elements for Outer Main Belt Asteroids. Celest. Mech. Dyn. Astron., vol. 78, 17-46. 

Kirkwood gap Any of several spaces in the distribution of asteroids in the main belt that correspond to locations of orbital resonance with Jupiter. Consequently any asteroids found there have long ago b n perturbed by Jupiter s enormous gravitational influence into more eccentric orbits. The gaps were discovered by the American astronomer Daniel Kirkwood (1814-95). 

The first echoes from a near-Earth asteroid (1566 Icarus) were detected in 1968 it would be nearly another decade before the first radar detection of a main-belt asteroid (1 Ceres in 1977), to be followed in 1980 by the first detection of echoes from a comet (Encke). During 1972 and 1973, detection of 13-cm-wavelength radar echoes... 

Echoes from 37 main-belt asteroids (MBAs) and 58 near-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 metal concentration. During the past decade, radar has been established as the most powerful Earth-based technique for determining the physical properties of asteroids that come close enough to yield strong echoes. 

The large number of asteroids in the main belt make it likely that many near encounters and collisions have occurred there over the lifetime of the belt. Orbits of some asteroids were redirected towards the inner as well as the outer regions of the Solar System and many were fragmented into smaller bodies. Smaller particles may have reattached themselves to larger objects, explaining the dust cover of many... 

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