The cumulative number of known asteroids and the yearly discovery rate are plotted above. The surveys responsible for spikes in detection are marked, the Palomar- Leiden (P-L), Trojan (T-1, T-2, T-3), Spacewatch, and LINEAR. Recent years have seen an explosion in asteroid discoveries due to automated telescopic surveys with advanced detection algorithms. By the year 2015, nearly 700,000 asteroids have been discovered. 

The cumulative number of known asteroids and the yearly discovery rate are plotted above. The surveys responsible for spikes in detection are marked, the Palomar- Leiden (P-L), Trojan (T-1, T-2, T-3), Spacewatch, and LINEAR. Recent years have seen an explosion in asteroid discoveries due to automated telescopic surveys with advanced detection algorithms. By the year 2015, nearly 700,000 asteroids have been discovered. 

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The past decade has brought major improvements in large-scale asteroid discovery and characterization with over half a million known asteroids and over 100,000 with some measurement of physical characterization. This explosion of data has allowed us to create a new global picture of the Main Asteroid Belt. Put in context with meteorite measurements...

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Context 1
... past decade has brought major improvements in large-scale asteroid discovery and characterization with over half a million known asteroids and over 100,000 with some measurement of physical characterization. This explosion of data has allowed us to create a new global picture of the Main Asteroid Belt. Put in context with meteorite measurements and dynamical models, a new and more complete picture of Solar System evolution has emerged. The question has changed from “What was the original compositional gradient of the Asteroid Belt?” to “What was the original compositional gradient of small bodies across the entire Solar System?” No longer is the leading theory that two belts of planetesimals are primordial, but instead those belts were formed and sculpted through evolutionary processes after Solar System formation. This article reviews the advancements on the fronts of asteroid compositional characterization, meteorite measurements, and dynamical theories in the context of the heliocentric distribution of asteroid compositions seen in the Main Belt today. This chapter also reviews the major outstanding questions relating to asteroid compositions and distributions and summarizes the progress and current state of understanding of these questions to form the big picture of the formation and evolution of asteroids in the Main Belt. Finally, we briefly review the relevance of asteroids and their compositions in their greater context within our Solar System and beyond. By the early 1990s in the era of Asteroids II , roughly 10,000 asteroids had been discovered, only a fraction of the total number of asteroids that are now known to exist. At that time, asteroids were discovered by visually inspecting photographic plates for light trails. In the 1990s, many of the major automated discovery surveys came online. By the year 2000 around the time of Asteroids III , ~20,000 asteroids were known (see Figure 1). Today (in 2015, Asteroids IV ) there are roughly 700,000 asteroids with known orbits, revealing much new information about the Asteroid Belt’s dynamic past. About 100,000 asteroids have measurements that tell us about their surface compositions ( Ivezic et al. , 2001, Szabo et al. , 2004, Nesvorny et al. , 2005, Carvano et al., 2010), providing a broader view of the Asteroid Belt than ever before. With the explosion of asteroid discoveries over the past decade (Fig. 1) and the conclusion of two of the largest asteroid physical measurement surveys, the Sloan Digital Sky Survey (SDSS; Ivezic et al., 2001) and the Wide-field Infrared Survey Explorer (WISE; Mainzer et al., 2011), it is timely to reflect upon recent advances, leaving us well poised for the next generation of major surveys including Gaia ( Mignard et al. , 2007) and the Large Synoptic Survey Telescope (LSST; Ivezic et al., 2007, Jones et al., 2009). This Chapter reviews our current understanding of the compositions of asteroids in the Main Belt from asteroidal, meteoritic, and dynamical perspectives. Our view of the Asteroid Belt has changed dramatically since the “big picture” chapters from Asteroids I and II ( Chapman 1979, Gradie et al., 1989, Bell 1989). In the context of this chapter, the use of the term "compositional trends" is more akin to taxonomic rather than mineralogical trends and the "compositions" discussed here refer more to broad trends of observational data than any derived mineralogical information. In Section 2, we review the current tools used to compositionally (taxonomically) characterize and classify asteroids and meteorites, and the dynamical tools that help us interpret the current orbital distribution of the asteroids. We also review the current distribution of asteroid classes in the main Asteroid Belt. Section 3 focuses on how the current observational constraints strengthen or weaken leading dynamical theories. In Section 4, we summarize the meteoritic evidence for asteroid compositions, as well as when and how asteroids formed. In Section 5, we compile the major compositional questions in asteroid science and review the progress made toward answering many of them to provide a broad view of asteroid compositions and their locations in the Belt. Section 6 looks at asteroids in their greater context for Earth, the Solar System, and beyond. The Appendix lists the major outstanding questions relevant to asteroid compositions and distributions. The Appendix also briefly summarizes the current state of knowledge and suggests future work to solve these problems. Many of these questions are noted specifically in this chapter and are addressed, and most are covered in more detail in other chapters. The surface properties (grain size, mineralogy, degree of space weathering, etc.) of an asteroid can be inferred through spectral and photometric measurements at wavelengths from the ultraviolet (UV) to the infrared. Thermal emission at longer wavelengths is used to calculate surface albedos that are related to surface compositions (see chapter by Mainzer et al. ). Absorption features in reflectance spectra from UV to mid-infrared wavelengths and emission features in the mid-infrared range can be used to identify minerals and other compounds on the surface of an asteroid or meteorite (see chapter by Reddy et al. ). For example, olivine and pyroxene have readily identifiable absorption features located at one and, for pyroxene, two microns. While mineralogical analysis is most appropriate for detailed studies of bodies with distinct features, relatively few asteroids have the high quality spectra that are required for this. On the other hand, taxonomic classifications can be made using lower quality or lower resolution spectra, providing a rapid characterization of asteroid spectra and a common language for comparing them. Hence, taxonomic information is available for orders of magnitude more than the number of asteroids for which detailed spectra have been measured. At the time of Asteroids III, the majority of reflectance spectra were taken at visible wavelengths to one micron, and were classified according to the Bus or Tholen taxonomies ( Bus & Binzel 2002b, Tholen & Barucci, 1989 ). The mafic silicate- rich asteroids with available near-infrared spectra at the time were classified mineralogically by the Gaffey system ( Gaffey et al. , 1993). By the early 2000s, near- infrared spectrometers became available, such as SpeX on the NASA IRTF ( Rayner et al., 2003). An extension of the Bus taxonomy, the so-called Bus-DeMeo taxonomy, was created to classify both the visible and near-infrared data in such a way as to be as consistent with the Bus taxonomy as possible ( DeMeo et al., 2009). A comparison of each of these major taxonomies is presented in Table 1. Table 1 Near Here Asteroid spectra are traditionally divided into three major complexes and each of the complexes is divided into individual classes (also called “types”). The S-complex, originally named for its expected silicaceous composition ( Chapman et al., 1975 ), is characterized by spectra with moderate silicate absorption features at 1 and 2 microns. The C-complex, historically named in connection with carbonaceous chondrite meteorites, have low albedo surfaces with spectra that have flat or low slopes and are subtly featured to featureless. Subtle features have absorptions of only a few percent, one of the most notable ...
Context 2
... past decade has brought major improvements in large-scale asteroid discovery and characterization with over half a million known asteroids and over 100,000 with some measurement of physical characterization. This explosion of data has allowed us to create a new global picture of the Main Asteroid Belt. Put in context with meteorite measurements and dynamical models, a new and more complete picture of Solar System evolution has emerged. The question has changed from “What was the original compositional gradient of the Asteroid Belt?” to “What was the original compositional gradient of small bodies across the entire Solar System?” No longer is the leading theory that two belts of planetesimals are primordial, but instead those belts were formed and sculpted through evolutionary processes after Solar System formation. This article reviews the advancements on the fronts of asteroid compositional characterization, meteorite measurements, and dynamical theories in the context of the heliocentric distribution of asteroid compositions seen in the Main Belt today. This chapter also reviews the major outstanding questions relating to asteroid compositions and distributions and summarizes the progress and current state of understanding of these questions to form the big picture of the formation and evolution of asteroids in the Main Belt. Finally, we briefly review the relevance of asteroids and their compositions in their greater context within our Solar System and beyond. By the early 1990s in the era of Asteroids II , roughly 10,000 asteroids had been discovered, only a fraction of the total number of asteroids that are now known to exist. At that time, asteroids were discovered by visually inspecting photographic plates for light trails. In the 1990s, many of the major automated discovery surveys came online. By the year 2000 around the time of Asteroids III , ~20,000 asteroids were known (see Figure 1). Today (in 2015, Asteroids IV ) there are roughly 700,000 asteroids with known orbits, revealing much new information about the Asteroid Belt’s dynamic past. About 100,000 asteroids have measurements that tell us about their surface compositions ( Ivezic et al. , 2001, Szabo et al. , 2004, Nesvorny et al. , 2005, Carvano et al., 2010), providing a broader view of the Asteroid Belt than ever before. With the explosion of asteroid discoveries over the past decade (Fig. 1) and the conclusion of two of the largest asteroid physical measurement surveys, the Sloan Digital Sky Survey (SDSS; Ivezic et al., 2001) and the Wide-field Infrared Survey Explorer (WISE; Mainzer et al., 2011), it is timely to reflect upon recent advances, leaving us well poised for the next generation of major surveys including Gaia ( Mignard et al. , 2007) and the Large Synoptic Survey Telescope (LSST; Ivezic et al., 2007, Jones et al., 2009). This Chapter reviews our current understanding of the compositions of asteroids in the Main Belt from asteroidal, meteoritic, and dynamical perspectives. Our view of the Asteroid Belt has changed dramatically since the “big picture” chapters from Asteroids I and II ( Chapman 1979, Gradie et al., 1989, Bell 1989). In the context of this chapter, the use of the term "compositional trends" is more akin to taxonomic rather than mineralogical trends and the "compositions" discussed here refer more to broad trends of observational data than any derived mineralogical information. In Section 2, we review the current tools used to compositionally (taxonomically) characterize and classify asteroids and meteorites, and the dynamical tools that help us interpret the current orbital distribution of the asteroids. We also review the current distribution of asteroid classes in the main Asteroid Belt. Section 3 focuses on how the current observational constraints strengthen or weaken leading dynamical theories. In Section 4, we summarize the meteoritic evidence for asteroid compositions, as well as when and how asteroids formed. In Section 5, we compile the major compositional questions in asteroid science and review the progress made toward answering many of them to provide a broad view of asteroid compositions and their locations in the Belt. Section 6 looks at asteroids in their greater context for Earth, the Solar System, and beyond. The Appendix lists the major outstanding questions relevant to asteroid compositions and distributions. The Appendix also briefly summarizes the current state of knowledge and suggests future work to solve these problems. Many of these questions are noted specifically in this chapter and are addressed, and most are covered in more detail in other chapters. The surface properties (grain size, mineralogy, degree of space weathering, etc.) of an asteroid can be inferred through spectral and photometric measurements at wavelengths from the ultraviolet (UV) to the infrared. Thermal emission at longer wavelengths is used to calculate surface albedos that are related to surface compositions (see chapter by Mainzer et al. ). Absorption features in reflectance spectra from UV to mid-infrared wavelengths and emission features in the mid-infrared range can be used to identify minerals and other compounds on the surface of an asteroid or meteorite (see chapter by Reddy et al. ). For example, olivine and pyroxene have readily identifiable absorption features located at one and, for pyroxene, two microns. While mineralogical analysis is most appropriate for detailed studies of bodies with distinct features, relatively few asteroids have the high quality spectra that are required for this. On the other hand, taxonomic classifications can be made using lower quality or lower resolution spectra, providing a rapid characterization of asteroid spectra and a common language for comparing them. Hence, taxonomic information is available for orders of magnitude more than the number of asteroids for which detailed spectra have been measured. At the time of Asteroids III, the majority of reflectance spectra were taken at visible wavelengths to one micron, and were classified according to the Bus or Tholen taxonomies ( Bus & Binzel 2002b, Tholen & Barucci, 1989 ). The mafic silicate- rich asteroids with available near-infrared spectra at the time were classified mineralogically by the Gaffey system ( Gaffey et al. , 1993). By the early 2000s, near- infrared spectrometers became available, such as SpeX on the NASA IRTF ( Rayner et al., 2003). An extension of the Bus taxonomy, the so-called Bus-DeMeo taxonomy, was created to classify both the visible and near-infrared data in such a way as to be as consistent with the Bus taxonomy as possible ( DeMeo et al., 2009). A comparison of each of these major taxonomies is presented in Table 1. Table 1 Near Here Asteroid spectra are traditionally divided into three major complexes and each of the complexes is divided into individual classes (also called ...
Context 3
... 3. The distribution of asteroid classes by mass in distinct size ranges and distances from the sun. Asteroid mass is grouped according to objects within four size ranges, with diameters of 100–1,000 km, 50–100 km, 20–50 km and 5–20 km. Seven zones are defined as in Fig. 1: Hungaria, inner belt, middle belt, outer belt, Cybele, Hilda and Trojan. The total mass of each zone at each size is labeled and the pie charts mark the fractional mass contribution of each unique spectral class of asteroid. The total mass of Hildas and Trojans are underestimated because of discovery incompleteness. The top row is consistent with results from Gradie & Tedesco 1982 and Gradie et al ., 1989. The figure is from DeMeo & Carry 2014.  ...

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