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2 Small Bodies: Background and Considerations
Pages 8-26

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From page 8...
... 2 Many asteroid families may have dispersed since formation and are thus difficult to identify in the current era. 3 1 IAU Minor Planet Center, 2022, "Running Tallies," http://www.minorplanetcenter.net.
From page 9...
... Orange: Centaurs; red: "classical" Kuiper Belt objects; light-blue: comets. SOURCE: Minor Planet Center, The Center for Astrophysics | Harvard & Smithsonian.
From page 10...
... are asteroids at the outer part of the Main Belt (~3.7–4.2 AU) in 3:2 mean motion resonance with Jupiter; these objects are predicted to have originated in the same region as the Trojans and Kuiper Belt objects (KBOs)
From page 11...
... Ceres is now known to be the most water-rich body in the inner solar system after Earth. Ceres's visible-near infrared spectrum is similar to some C-complex asteroids, but its bulk density of 2,162 kg m–3,a intermediate between water ice and silicates, is more similar to icy moons of the outer solar system, and even early measurements showed the presence of hydrated minerals on the surface.1 Early thermal evolution models of Ceres indicate that it may have differentiated into a silicate core and a water-rich outer layer, which could suggest that Ceres harbored a global subsurface ocean for several hundred million years after its formation.b Measurements of hydrogen by Dawn indicate the presence of a global, subsurface water-ice table at depths less than a few decimeters at latitudes greater than 45°.c Water ice is also associated with some impact craters on Ceres.
From page 12...
... 9 These objects originate primarily in the main asteroid belt and are ejected into near-Earth space via dynamical "escape hatches," whereby asteroid fragments are constantly created by both collisions and mass shedding events. A fraction of this population of fragments, namely those of diameters ≲30 km, can escape the Main Belt via the gravitational resonances, thereby creating a quasi-steady-state population of near-Earth asteroids, 10 with a dynamical duration in the inner solar system of ~10 Myr.
From page 13...
... In this model, Jupiter's primordial Trojan population was lost and the Lagrange regions were repopulated with this scattered Kuiper Belt material. 15 The Jupiter Trojans thus may represent KBOs currently orbiting the Sun at 5.2 AU.
From page 14...
... Additional taxonomic types are discussed in the literature but are not included here. These various taxonomic types are distributed throughout the Main Belt, though important trends are observed (Figure 2-5)
From page 15...
... FIGURE 2-4 Representation of the visible-near infrared spectral shapes of the major taxonomic types of asteroids discussed in this report. The x axis represents wavelengths of 0.45–2.45 microns while the y axis represents normalized reflectance of values 1–1.5.
From page 16...
... The distribution shows significant overlap, but melted and metamorphosed asteroids tend to be concentrated in the inner belt, aqueously altered bodies in the middle, and bodies in which ice never melted in the outer belt.
From page 17...
... , which puts what we have learned about the composition of comets into the wider context of solar system evolution. In addition to the Rosetta mission, significant understanding of comets, particularly their organic matter, has been gained from samples returned by the Stardust mission and interplanetary dust particles (IDPs)
From page 18...
... Furthermore, interpretations of spectra are rendered difficult by effects such as space weathering, temperatures, and particle sizes. LINKS TO METEORITES In addition to spectroscopic surveys, comparisons between spectra of meteorites and asteroids can help illuminate the likely composition of parent body asteroids, particularly because space weathering processes (e.g., solar wind bombardment)
From page 19...
... More than 30 active asteroids have been discovered since 1996. The hypothesized causes of activity in these bodies include impact ejection and disruption, rotational instabilities, and dehydration stresses and thermal fracture, in addition to the sublimation of asteroidal ice.
From page 20...
... 47 As described in Box 2-2 and in Figure 2-6, spectroscopic measurements of the D/H ratios in cometary comae indicate that water ice in comets is more D-rich than the water at the surface of Earth, constraining the amount of volatile material that could be delivered from cometary impacts. Furthermore, dynamical simulations of the formation of terrestrial planets suggest that the outer asteroid belt was the primary source of impactors on the early Earth.
From page 21...
... 2028 617 Patroclus and Jupiter Trojan Lucy (flyby) 2033 Menoetius NOTE: KBO, Kuiper Belt object; MBA, Main Belt asteroid; NEO, near-Earth object.
From page 22...
... 55 Galileo cameras also provided critical insights to space weathering processes on S-type asteroids, by noting the spectrally blue nature of craters on Ida, compared to the relatively spectrally reddish overall nature of the space-weathered surface. 56 The visits by the Hayabusa2 and OSIRIS-REx spacecraft to Ryugu and Bennu, respectively, have provided insights into the nature and evolution of these "spinning top" shaped asteroids, 57 along with the remarkable particle ejection processes seen at Bennu.
From page 23...
... Understanding the origin of organic compounds in early solar system materials is central to astrobiology. Individual asteroids are "astrobiological time capsules" 60 that preserve a record of the evolution of volatiles and organics starting in the interstellar medium, through the birth and early evolution of the solar system, to present-day space weathering at asteroid surfaces.
From page 24...
... Finding 1: The primary astrobiological value of small solar system bodies is that some of these bodies contain prebiotic organic compounds that are relevant to the study of the origin of life in the solar system. As a consequence, most small-body missions are either planetary protection Category I or II and thus do not require provisions to ensure spacecraft cleanliness.
From page 25...
... These findings are exemplified by recent small-body missions: Stardust captured organic matter samples from Comet 81P/Wild2. 70 Rosetta's close proximity to the coma of comet 67P/C-G allowed it to detect numerous organic species including aromatic hydrocarbons, oxygenated hydrocarbons, and a diverse population of sulfur-bearing molecules in addition to many inorganic species.
From page 26...
... , 53rd Lunar and Planetary Science Conference, https://www.hou.usra.edu/meetings/lpsc2022/pdf/1831.pdf. 73 With regard to planetary protection, the great radiation-survivability of desiccated D


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