2

Small Bodies: Background and Considerations

Since the 1801 discovery of Ceres, more than 1.2 million small bodies have been discovered in the solar system.¹ These small bodies can be sorted according to their orbital characteristics into the following groups: Main Belt, near-Earth, and Trojan asteroids; comets sourced from the Kuiper Belt and the Oort Cloud; Centaurs; and Trans-Neptunian objects. These groups are further subdivided based on additional dynamical and also spectroscopic/color characteristics.

A relatively small number of small bodies have been visited by spacecraft, greatly enhancing scientific understanding of these widely varying objects, but most information is gleaned from multi-wavelength telescopic imaging and spectroscopy from Earth- and space-based facilities. Laboratory studies of meteorites and returned samples have added further knowledge. Studies over the last 200+ years have revealed sizes, densities, compositions, space weathering effects, and dynamical and cratering histories of these worlds, delivering a rich catalogue that provides evidence of the history of the solar system, including pointers to the ultimate sources of water and organics on Earth. Compositional gradients among small bodies across the solar system are linked to early solar system processes and provide clues about the building blocks of life and their distribution throughout the solar system.

DYNAMICAL GROUPINGS OF SMALL BODIES

Small solar system bodies comprise a wide variety of types of objects. Here, the committee initially discusses these objects as grouped by their dynamical configurations.

Main Belt Asteroids

Main Belt asteroids (MBAs) (see Figures 2-1 and 2-2) are those bodies in heliocentric orbits with semi-major axes between the orbits of Mars and Jupiter. Ceres is particularly notable as the largest object in the Main Belt (940 km in diameter) and was classed as a dwarf planet in 2006. (See Box 2-1 on Ceres for more information.) Rare, giant collisions among asteroids have resulted in dynamical asteroid families, wherein gravitational re-accumulation of material after a large collision leads to the formation of an entire family of large and small objects with dynamical properties similar to those of the original body.² Many asteroid families may have dispersed since formation and are thus difficult to identify in the current era.³

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¹ IAU Minor Planet Center, 2022, “Running Tallies,” http://www.minorplanetcenter.net.

² P. Michel, 2001, “Collisions and Gravitational Reaccumulation: Forming Asteroid Families and Satellites,” Science 294(5547):1696–1700, https://doi.org/10.1126/science.1065189.

³ D. Nesvorný, M. Broz, and V. Carruba, 2015, “Identification and Dynamical Properties of Asteroid Families,” Pp. 297–321 in Asteroids IV, W.F. Bottke, F.E. DeMeo, and P. Michel, eds., Tucson: University of Arizona Press, https://doi.org/10.2458/azu_uapress_9780816532131-ch016.

Collisions among asteroids can also result in the formation of natural satellites,^4,5 observations of which can help constrain the density of the parent asteroid.⁶ Models of the collisional history within the asteroid belt, using the current size frequency distribution as a constraint, suggest that asteroids of diameter ≳100 km are primordial, with their physical properties likely determined during the accretion epoch⁷ (Vesta is an exception here given its differentiated nature).⁸ Most smaller asteroids (≲40 km diameter) are byproducts of fragmentation events. The Hildas (shown in Figure 2-2) 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) (beyond the primordial orbits of the ice giants).

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⁴ D.D. Durda, W.F. Bottke, B.L. Enke, et al., 2004,” The Formation of Asteroid Satellites in Large Impacts: Results from Numerical Simulations,” Icarus 170(1):243–257, https://doi.org/10.1016/j.icarus.2004.04.003.

⁵ The first asteroidal satellite discovered was Dactyl, the moon of (243) Ida, the target of a flyby of the Galileo spacecraft in 1993. M.J.S. Belton, C.R. Chapman, P.C. Thomas, et al., 1995, “Bulk Density of Asteroid 243 Ida from the Orbit of Its Satellite Dactyl,” Nature 374(6525):785–788, https://doi.org/10.1038/374785a0.

⁶ W.J. Merline, L.M. Close, C. Dumas, et al., 1999, “Discovery of a moon orbiting the asteroid 45 Eugenia,” Nature 401(6753):565–568, https://doi.org/10.1038/44089.

⁷ W.F. Bottke, D.D. Durda, D. Nesvorný, et al., 2005, “The Fossilized Size Distribution of the Main Asteroid Belt,” Icarus 175(1):111–140, https://doi.org/10.1016/j.icarus.2004.10.026.

⁸ A. Ruzicka, G.A. Snyder, and L.A. Taylor, 1997, “Vesta as the Howardite, Eucrite and Diogenite Parent Body: Implications for the Size of a Core and for Large-Scale Differentiation,” Meteoritics and Planetary Science 32:825–840, https://doi.org/10.1111/j.1945-5100.1997.tb01573.x.

Near-Earth Objects

Near-Earth objects (NEOs, or near-Earth asteroids, NEAs) are those objects with perihelia within 1.3 AU.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. NEOs, as fragments of MBAs, thus represent a sample of some compositional types of MBAs that are more dynamically accessible from Earth than many MBAs, making them attractive targets for sample-return missions such as Hayabusa, Hayabusa2, and OSIRIS-Rex [Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer]. With orbits in the inner solar system, NEOs experience greater amounts of processing (e.g., extreme heating, solar wind exposure) and consequently, their surfaces are less pristine than their parent bodies in the Main Belt. As evidenced by Ryugu and Bennu (targets of Hayabusa2 and OSIRIS-REx, respectively), however, carbonaceous NEOs do retain some level of volatiles.

Kuiper Belt Objects and Centaurs

KBOs (a subclass of Trans-Neptunian objects, TNOs) are small bodies inhabiting the outer reaches of the solar system, extending from roughly 30 to 50 AU; the region is named for Gerard Kuiper, who speculated some seven decades ago about objects beyond Pluto. The so-called cold classical KBOs have orbits with relatively low inclinations and eccentricities, while the orbits of hot classical KBOs are more highly inclined and less circular, having been more influenced and perturbed by Neptune’s gravity. Dwarf planet Pluto is known as one of the largest KBOs, and the contact binary KBO Arrokoth¹¹ was the target

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⁹ Center for Near Earth Object Studies, “NEO Basics,” https://cneos.jpl.nasa.gov/about/neo_groups.html.

¹⁰ W.F. Bottke, M. Brož, D.P. O’Brien, A.C. Bagatin, A. Morbidelli, and S. Marchi, 2015, “The Collisional Evolution of the Main Asteroid Belt,” Pp. 701–724 in Asteroids IV, P. Michel, F.E. DeMeo, and W.F. Bottke, eds., Tucson: University of Arizona Press.

¹¹ W.M. Grundy, M.K. Bird, D.T. Britt, et al., 2020, “Color, Composition, and Thermal Environment of Kuiper Belt Object (486958) Arrokoth,” Science 367(6481), https://doi.org/10.1126/science.aay3705.

of the New Horizons flyby in 2019. The Nice model¹² describes perturbations of the primordial belt as including resonant capture and scattering of KBOs by an outward migrating Neptune; even after its migration ended, Neptune has continued to erode the Kuiper Belt by gravitational scattering, by sending objects outward (to the “scattered disk”) or inward to become Centaurs and precursors to the Jupiter family comets discussed below. The orbits of Centaurs occupy the space between the orbits of Jupiter and Neptune, interacting strongly with the gravity of these giant planets and as a result are either ejected from the solar system or pushed into the inner solar system where they become comets.

Jupiter Trojans

Jupiter Trojans are those bodies that orbit the Sun near the stable Jupiter Lagrangian points L4 and L5, leading and trailing Jupiter by 60°. The number of asteroids in the leading group is larger than that of the trailing group, by a factor of ~1.4±0.2 for Trojans larger than 10 km.¹³ Orbital inclinations of these objects are up to 40°.¹⁴ In the Nice model, it is proposed that resonant interactions between Jupiter and Saturn temporarily destabilized the orbits of Uranus and Neptune, which moved into the primordial Kuiper Belt, scattering material widely across the solar system. 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. The Lucy mission will make the first up-close observations of Trojans during flybys of five of these objects (including one binary pair) in 2027–2033.

Comets

Comets are volatile-rich bodies that can periodically enter the inner solar system following long-term storage in the Kuiper Belt and the Oort Cloud reservoirs. Whereas asteroids largely have low-eccentricity orbits (mostly < ~0.3–0.4), most comets have higher-eccentricity orbits. The short-period comets (Jupiter Family comets, JFCs), with periods of 5–10 years, tend to have lower inclinations and are thought to originate in the Kuiper Belt (see Figure 2-3). Jan Oort first suggested that the long-period comets (P > 200 years) that enter the inner solar system come from a cloud of icy bodies as far as 2,000 to 100,000 AU from the Sun.¹⁶ The Oort cloud is estimated to hold billions or even trillions of bodies; when these objects in the cloud interact with passing stars, molecular clouds, and gravity from the galaxy, they can spiral inward toward the Sun as long-period comets.^17,18

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¹² A. Morbidelli, H.F. Levison, K. Tsiganis, and R. Gomes, 2005, “Chaotic Capture of Jupiter’s Trojan Asteroids in the Early Solar System,” Nature 435:462–465, https://doi.org/10.1038/nature03540.

¹³ T. Grav, A.K. Mainzer, J. Bauer, et al., 2011, “WISE/NEOWISE Observations of the Jovian Trojans: Preliminary Results,” The Astrophysical Journal 742(40), https://doi.org/10.1088/0004-637X/742/1/40.

¹⁴ S. Pirani, A. Johansen, and A.J. Mustill, 2019, “On the Inclinations of the Jupiter Trojans,” Astronomy and Astrophysics 631:A89, https://doi.org/10.1051/0004-6361/201936600.

¹⁵ E. Dotto, J.P. Emery, M.A. Barucci, A. Morbidelli, and D.P. Cruikshank, 2008, “De Troianis: The Trojans in the Planetary System,” Pp. 383–395 in The Solar System Beyond Neptune, M.A. Barucci, H. Boehnhardt, D.P. Cruikshank, and A. Morbidelli, eds., Tucson: University of Arizona Press.

¹⁶ J.H. Oort, 1950, “The Structure of the Cloud of Comets Surrounding the Solar System and a Hypothesis Concerning Its Origin,” Bulletin of the Astronomical Institutes of the Netherlands 11(408):91–110, https://hdl.handle.net/1887/6036.

¹⁷ R. Brasser and A. Morbidelli, 2013, “Oort Cloud and Scattered Disc Formation During a Late Dynamical Instability in the Solar System,” Icarus 225:40–49, https://doi.org/10.1016/j.icarus.2013.03.012.

¹⁸ R. Brasser and M.E. Schwamb, 2015, “Re-Assessing the Formation of the Inner Oort Cloud in an Embedded Star Cluster—II. Probing the Inner Edge,” Monthly Notices of the Royal Astronomical Society 446:3788–3796, https://doi.org/10.1093/mnras/stu2374.

COMPOSITIONAL, TAXONOMIC, AND SPECTRAL DISTINCTIONS

Much of what is known about small bodies’ compositions comes from Earth-based spectroscopic observations. MBAs and NEOs are grouped taxonomically by their spectral properties at visible and near-infrared (NIR) wavelengths (~0.4–2.4 microns) (Figure 2-4). The first asteroid taxonomy was assembled by Chapman, Morrison, and Zellner¹⁹ in 1975. Since then, various additional taxonomic systems have been published based on broadband colors covering the wavelength range 0.337–1.055 microns coupled with visible albedo to delineate asteroids,²⁰ as well as using narrowband spectroscopy in the 0.44–0.92 micron spectral range²¹ and through longer NIR wavelengths.²² In all of these systems, S-types are rocky (melted/metamorphosed); C-complex (including B, C, Cg, Cb, Ch, and Cgh) are low-albedo and presumably more carbonaceous and volatile-rich (aqueously altered); and P- and D-types are low-albedo and spectrally redder, likely consistent with an organic-rich composition. M-type asteroids are thought to be metal-rich. 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). In particular, the majority of the mass of the inner Main Belt is dominated by S-type, rocky material, while the mass of the outer belt is dominated by more volatile-rich, carbonaceous asteroids.

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¹⁹ C.R. Chapman, D. Morrison, and B. Zellner, 1975, “Surface Properties of Asteroids: A Synthesis of Polarimetry, Radiometry, and Spectrophotometry,” Icarus 25:104–130, https://doi.org/10.1016/0019-1035(75)90191-8.

²⁰ D.J. Tholen and M.A. Barucci, 1989, “Asteroid Taxonomy,” Pp. 298–315 in Asteroids II, R.P. Binzel, T. Gehrels, M.S. Matthews, eds., Tucson: University of Arizona Press.

²¹ S.J. Bus and R.P. Binzel, 2002, “Phase II of the Small Main-Belt Asteroid Spectroscopic Survey: A Feature-Based Taxonomy,” Icarus 158:146–177, https://doi.org/10.1006/icar.2002.6856.

²² F.E. DeMeo, R.P. Binzel, S.M. Slivan, and S.J. Bus, 2009, “An Extension of the Bus Asteroid Taxonomy into the Near-Infrared,” Icarus 202:160–180, https://doi.org/10.1016/j.icarus.2009.02.005.

Based on Earth-based observations, the Jupiter Trojans are comprised of largely P- and D-type asteroids. A long-standing paradigm is that the low-albedo and red spectral slopes are due to the presence of complex organic molecules.²³ Based on reported signatures of fine-grained silicates²⁴ on some Trojans, a bulk-density measurement,²⁵ and their locations at 5.2 AU, Trojans have generally been inferred to contain a large fraction of H₂O ice, though covered by a refractory mantle, and a higher abundance of complex organic molecules than most MBAs. The Hildas at the outermost part of the Main Belt, exhibit the same bimodal color distribution as the Trojans.²⁶ KBOs are generally classed into “red” and “less-red” groupings based on their spectral slopes at visible wavelengths.

As of October 2021, a total of 27,196 NEOs have been discovered, and more than 1.1 million Main Belt asteroids (>1 km in diameter) are known; 4,429 comets have been discovered (as of December 2021; about 3,000 of these belong to the Kreutz family of Sun-grazing comets), and there are ~9,800 known Jupiter Trojans. Given these numbers, even a body with a rare taxonomic type is likely represented by innumerable smaller, similar bodies. However, all bodies of the same “type” cannot be assumed to be the same; as discussed later, spacecraft visits have played important roles in studying diversity among apparently taxonomically similar asteroids.

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²³ J. Gradie and J. Veverka, 1980, “The Composition of the Trojan Asteroids,” Nature 283:840–842, https://doi.org/10.1038/283840a0.

²⁴ J.P. Emery, D.P. Cruikshank, and J. Van Cleve, 2006, “Thermal Emission Spectroscopy (5.2 38 μm) of Three Trojan Asteroids with the Spitzer Space Telescope: Detection of Fine-Grained Silicates,” Icarus 182:496–512, https://doi.org/10.1016/j.icarus.2006.01.011.

²⁵ F. Marchis, D. Hestroffer, P. Descamps, et al., 2006, “A Low Density of 0.8gcm3 for the Trojan Binary Asteroid 617 Patroclus,” Nature 439:565–567, https://doi.org/10.1038/nature04350.

²⁶ I. Wong and M.E. Brown, 2016, “A Hypothesis for the Color Bimodality of Jupiter Trojans,” The Astronomical Journal 152(90), https://doi.org/10.3847/0004-6256/152/4/90.

The number of discovered NEOs is expected to increase by orders of magnitude when the Vera C. Rubin Observatory begins operations; identifications of more than 100,000 NEOs are anticipated by the Legacy Survey of Space and Time. In addition, sample returns from NEOs, and improved orbital characterizations will help fill the knowledge gaps for small-body objects of sizes smaller than 10 m. Toward this outcome, spectroscopic analyses are also being conducted to compare the asteroid spectra to the spectra of meteorites held in collections, which may provide the link to their parent families.²⁷ Over recent decades, about 1,000 NEOs have been observed by radar, and this has provided information on both their shape and size. Furthermore, space-based infrared measurements by the Spitzer and NEOWISE observatories and the ground-based IRTF/MIRSI [Infrared Telescope Facility/Mid-InfraRed Spectrometer and Imager] observation platforms have provided additional data on the morphology of such objects. The space science community has already acquired very detailed information on the composition of three NEOs from sample-return missions to Itokawa, Bennu, and Ryugu.

Comets, with sources in the most distant reaches of the solar system, represent the oldest relics of solar system formation and thus can serve as records of the formation period. Their compositions of carbons and other volatile species are the building blocks of planets (dust, ices, and organics).

Comets have long been studied by dedicated ground-based and space-based remote sensing campaigns. Spectroscopic and photometric studies of cometary comae provided a wealth of data on an increasing sample of comets.^28,29 These observations have led to many discoveries and have allowed for a detailed comparison between comets, other solar system objects, and have provided a link between objects in our solar system and the interstellar medium (see Box 2-2).

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²⁷ R.P. Binzel, F.E. DeMeo, E.V. Turtelboom, et al., 2019, “Compositional Distributions and Evolutionary Processes for the Near-Earth Object Population: Results from the MIT-Hawaii Near-Earth Object Spectroscopic Survey (MITHNEOS),” Icarus 324:41–76, https://doi.org/10.1016/j.icarus.2018.12.035.

²⁸ M.J. Mumma, M.A. Disanti, K. Magee-Sauer, et al., 2005, “Parent Volatiles in Comet 9P/Tempel 1: Before and After Impact,” Science 310:270–274, https://doi.org/10.1126/science.1119337.

²⁹ D.C. Lis, D. Bockelée-Morvan, R. Güsten, et al., 2019, “Terrestrial Deuterium-to-Hydrogen Ratio in Water in Hyperactive Comets,” Astronomy and Astrophysics 625(L5), https://doi.org/10.1051/0004-6361/201935554.

Early remote sensing observations of the apparently anomalous acceleration of comets Encke, d’Arrest, and Wolf 1 led to the conclusion³⁰ that comets are “dirty snowballs” emitting gas and dust which can either decelerate or accelerate the comet along its orbit around the Sun, known as nongravitational forces. However, only in 1986 did the European Space Agency Giotto mission and the Soviet Vega 2 mission flying by comet 1P/Halley³¹ confirm that comets possess a solid nucleus composed of volatile and refractory materials. From these investigations, it was established³² that comets have preserved the accreted and condensed materials moreso than other objects in the solar system.

In general, however, though much work has been accomplished regarding small-body taxonomies and spectral and compositional studies, neither taxonomic classes nor higher resolution spectra are highly diagnostic of unambiguous composition and they generally do not reveal composition of minor constituents. 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) can alter the spectral properties of the surfaces of asteroids, confounding the identification of the surface components.³³ Ordinary chondrite meteorites are good representatives of rocky types of asteroids larger than 10 m, and there are remarkable linkages between howardite-eucrite-diogenite meteorites and asteroid 4 Vesta.³⁴ Importantly, less than 5 percent of meteorite falls are carbonaceous chondrite meteorites, while their presumed parent bodies, C-complex asteroids, are plentiful in the Main Belt. Meteors from these more primitive types of asteroids may be more fragile³⁵ and may not survive atmospheric entry. Nevertheless, organics are present in some meteorites; they represent precursors to life, but are not indicators of life. Rather, the organic matter in comets and asteroids is derived from interstellar space,³⁶ as revealed by extreme isotopic fractionations, but has been further processed in the solar nebula and/or within small bodies after accretion.

Significant progress has been made connecting asteroids and meteorites. For instance, despite spectral differences, it is now understood that ordinary chondrites have S-type parent bodies whose surfaces are more space-weathered than the interiors of the meteorites. However, gaps exist in attempts to connect

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³⁰ F.L. Whipple, 1951, “A Comet Model. II. Physical Relations for Comets and Meteors,” The Astrophysical Journal 113(464), https://doi.org/10.1086/145416.

³¹ R. Reinhard, 1986, “The Giotto Encounter with Comet Halley,” Nature 321:313–318, https://doi.org/10.1038/321313a0.

³² J. Geiss, 1987, “Composition Measurements and the History of Cometary Matter,” Astronomy and Astrophysics 187:859–866.

³³ See, e.g., C.M. Pieters, L.A. Tayler, S.K. Noble, et al., 2000, “Space Weathering on Airless Bodies: Resolving a Mystery with Lunar Samples,” Meteoritics and Planetary Science 35:1101–1107, https://doi.org/10.1111/j.1945-5100.2000.tb01496.x.

³⁴ H.Y. McSween, Jr., R.P. Binzel, M.C. De Sanctis, et al., 2013, “Dawn; the Vesta-HED Connection; and the Geologic Context for Eucrites, Diogenites, and Howardites,” Meteorics and Planetary Science 48:2090–2104, https://doi.org/10.1111/maps.12108.

³⁵ A.L. Graps, P. Blondel, G. Bonin, et al., 2016, “ASIME 2016 White Paper: In-Space Utilisation of Asteroids: ‘Answers to Questions from the Asteroid Miners,’ ” arXiv 1612.00709v2, https://doi.org/10.48550/arXiv.1612.00709.

³⁶ H. Busemann, A.F. Young, C.M.O’D. Alexander, P. Hoppe, S. Mukhopadhyay, and L.R. Nittler, 2006, “Interstellar Chemistry Recorded in Organic Matter from Primitive Meteorites,” Science 312:727–730, https://doi.org/10.1126/science.1123878.

asteroids and meteorites.³⁷ For instance,³⁸ the asteroid 2008 TC3 (known as Almahata Sitta) lost more than 99 percent of its mass in the atmosphere. Its spectrum was flat, and yet a remarkable mixture of composition was found in the meteorite samples. Indeed, presumed primitive carbonaceous asteroids have relatively flat and featureless spectra that are difficult to link to specific chondrite groups.³⁹ Ground-based data do not provide information on possible asteroid surface and interior heterogeneities. Furthermore, dynamically distant objects (Hildas, Trojans, and Kuiper Belt) are underrepresented or absent in the meteorite collection (perhaps because these parent bodies are structurally weak as mentioned above).⁴⁰

ICE IN THE ASTEROID BELT

A subclass of MBAs is the so-called “active asteroids.”⁴¹ Active asteroids are bodies that exhibit comet-like mass loss, ejecting volatiles and dust and producing transient, comet-like comae and tails, but have asteroid-like orbits; a few are found in near-Earth space. 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. Whereas “disrupted asteroids” are those active asteroids whose activity is driven by processes such as impacts and rotational disruption, the activity of another subset of active asteroids, “Main Belt comets (MBCs),” is driven by sublimation of volatiles; these MBCs thus provide new clues regarding the abundance of asteroid ice, and the origin of terrestrial planet volatiles. The activity of MBCs, driven by sublimation of volatiles, is evidence for water in the Main Belt; there are likely many more water-rich asteroids (probably mostly in the outer Main Belt) that have not yet shown activity. (Note that none of the water present in/on small bodies is liquid.) Further evidence of water comes from spectroscopic clues, on the low-albedo classes of asteroids. The 0.7 micron feature detected in some asteroids (particularly the Ch- and Cgh-types) is an oxidized iron feature indicative of Fe-bearing phyllosilicates that is the result of aqueous alteration.⁴² The 3 micron feature⁴³ is prevalent at low-albedo asteroids and is indicative of OH or H₂O; the maximum absorption of H₂O is at 3.1 microns. Organics, with diagnostic absorptions in the 3.4–3.5 micron range, have also been found on some low-albedo asteroids.^44,45 In the Main Belt, depending on the thermal properties (e.g., composition, porosity, grain sizes, etc.) of the dust, ice can remain for a long period of time; modeling has demonstrated that

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³⁷ C.R. Chapman, D. Morrison, and B. Zellner, 1975, “Surface Properties of Asteroids: A Synthesis of Polarimetry, Radiometry, and Spectrophotometry,” Icarus 25:104–130, https://doi.org/10.1016/0019-1035(75):90191–90198.

³⁸ A.L. Graps, P. Blondel, G. Bonin, et al., 2016, “ASIME 2016 White Paper: In-Space Utilisation of Asteroids: ‘Answers to Questions from the Asteroid Miners,’ ” arXiv 1612.00709v2, https://doi.org/10.48550/arXiv.1612.00709.

³⁹ D.S. Lauretta, O. Barnouin-Jha, M.A. Barucci, et al., 2009, “Astrobiology Research Priorities for Primitive Asteroids,” Lunar and Planetary Laboratory, https://www.lpi.usra.edu/decadal/sbag/topical_wp/lauretta_etal.pdf.

⁴⁰ P. Vernazza, M. Marsset, P. Beck, et al., 2015, “Interplanetary Dust Particles as Samples of Icy Asteroids,” The Astrophysical Journal 806(204), https://doi.org/10.1088/0004-637X/806/2/204.

⁴¹ D. Jewitt, H. Hsieh, and J. Agarwal, 2015, “The Active Asteroids,” Pp. 221–241 Asteroids IV, P. Michel, F.E. DeMeo, and W.F. Bottke, eds., Tucson: University of Arizona Press.

⁴² F. Vilas and M.J. Gaffey, 1989, “Phyllosilicate Absorption Features in Main-Belt and Outer-Belt Asteroid Reflectance Spectra,” Science 246:790–792, https://doi.org/10.1126/science.246.4931.790.

⁴³ A.S. Rivkin, B.E. Clark, M. Ockert-Bell, et al., 2011, “Asteroid 21 Lutetia at 3 μm: Observations with IRTF SpeX,” Icarus 216:62–68, https://doi.org/10.1016/j.icarus.2011.08.009.

⁴⁴ A.S. Rivkin and J.P. Emery, 2010, “Detection of Ice and Organics on an Asteroidal Surface,” Nature 464:1322–1323, https://doi.org/10.1038/nature09028.

⁴⁵ H. Campins, K. Hargrove, N. Pinilla-Alonso, et al., 2010, “Water Ice and Organics on the Surface of the Asteroid 24 Themis,” Nature 464:1320–1321, https://doi.org/10.1038/nature09029.

buried ice on spherical bodies, within the top few meters of the surface, orbiting 2–3 AU from the Sun, can survive ~10⁹ years.⁴⁶

Organic- and volatile-rich asteroids provide fundamental information about the source of water and prebiotic compounds for the terrestrial planets.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. The discovery of MBCs suggests that similar bodies may have delivered water and other volatiles to the inner solar system.⁴⁸

SPACECRAFT VISITS

Relatively few small bodies have been visited by spacecraft (Table 2-1), given the large number of small bodies, which means that nearly all knowledge of small bodies throughout the solar system is based on Earth-based observations and models, along with meteorite and sample studies. All the JFCs that have been visited by spacecraft demonstrate remarkable diversity (e.g., Figure 2-2). A small number of spacecraft have visited comets in situ, including coma sample-return missions: after flying by 1P/Halley,

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⁴⁶ N. Schorghofer, 2008, “The Lifetime of Ice on Main Belt Asteroids,” The Astrophysical Journal 682:697–705, https://doi.org/10.1086/588633.

⁴⁷ D.S. Lauretta, O. Barnouin-Jha, M.A. Barucci, et al., 2009, “Astrobiology Research Priorities for Primitive Asteroids,” Lunar and Planetary Institute, https://www.lpi.usra.edu/decadal/sbag/topical_wp/lauretta_etal.pdf.

⁴⁸ H.H. Hsieh and D. Jewitt, 2006, “A Population of Comets in the Main Asteroid Belt,” Science 312:561–563, https://doi.org/10.1126/science.1125150.

TABLE 2-1 Small Bodies Visited by Spacecraft

NOTE: KBO, Kuiper Belt object; MBA, Main Belt asteroid; NEO, near-Earth object.

Giotto flew-by comet 26P/Grigg-Skjellerup;⁴⁹ Deep Space One passed by 19P/Borrelly;⁵⁰ and 81P/Wild 2 was visited by the Stardust spacecraft before its extended mission, NExT, flew by 9P/Tempel 1.⁵¹ Cometary dust, collected in the coma of 81P/Wild 2 was returned to Earth for in-depth analysis. Other comets visited were 9P/Tempel 1 by Deep Impact⁵² and 103P/Hartley 2 by the (renamed) EPOXI spacecraft,⁵³ before ESA’s Rosetta mission encountered comet 67P/C-G.⁵⁴ Rosetta was the first spacecraft to rendezvous with a comet, and it orbited the nucleus for about 2 years with the Philae lander module being deployed onto the comet’s nucleus early in the orbiting phase.

When the Galileo spacecraft flew by (243) Ida in 1994, the remarkable discovery of a natural satellite at Ida (Dactyl) was made, not observable from Earth.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.⁵⁸ Almost every small body studied close-up by spacecraft has had unique characteristics, sometimes quite unexpected.

LINKS TO THE EARLY SOLAR SYSTEM

Dynamic modeling combined with observational constraints indicates that the rocky bodies in the inner Main Belt likely formed somewhere close to their current locations and have been thermally metamorphosed or melted. Models such as the Grand Tack/Nice models suggest that planetary migration of Jupiter and Saturn produced sweeping resonance through the main asteroid belt and dislodged most of the asteroids. The resulting liberated asteroids could have been responsible for the impact cataclysms that occurred on all terrestrial planets and satellites around 4 billion years ago. Some of the low-albedo objects in the Main Belt likely formed in the outer solar system and ended up in the Main Belt as a result of giant planet migration. Some ended up in cold enough locations such that pre-existing ice never melted, and some were aqueously altered due to melting of ice (Figure 2-7). The role of ²⁶Al in heating and melting is

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⁴⁹ M.G. Grensemann and G. Schwehm, 1993, “Giotto’s Second Encounter: The Mission to Comet P/Grigg-Skjellerup,” Journal of Geophysical Research 98:20907–20910, https://doi.org/10.1029/93JA02528.

⁵⁰ L.A. Soderblom, T.L. Becker, G. Bennett, et al., 2002, “Observations of Comet 19P/Borrelly by the Miniature Integrated Camera and Spectrometer Aboard Deep Space 1,” Science 296:1087–1091, https://doi.org/10.1126/science.1069527.

⁵¹ D. Brownlee, P. Tsou, J. Aléon, et al., 2006, “Comet 81P/Wild 2 Under a Microscope,” Science 314(1711), https://doi.org/10.1126/science.1135840.

⁵² M.F. A’Hearn, M.J.S. Belton, W.A. Delamere, et al., 2005, “Deep Impact: Excavating Comet Tempel 1,” Science 310:258–264, https://doi.org/10.1126/science.1118923.

⁵³ M.F. A’Hearn, M.J.S. Belton, W.A. Delamere, et al., 2011, “EPOXI at Comet Hartley 2,” Science 332(1396), https://doi.org/10.1126/science.1204054.

⁵⁴ K.-H. Glassmeier, H. Boehnhardt, D. Koschny, E. Kührt, and I. Richter, 2007, “The Rosetta Mission: Flying Towards the Origin of the Solar System,” Space Science Reviews 128:1–21, https://doi.org/10.1007/s11214-006-9140-8.

⁵⁵ M.J.S. Belton, C.R. Chapman, P.C. Thomas, et al., 1995, “Bulk Density of Asteroid 243 Ida from the Orbit of Its Satellite Dactyl,” Nature 374(6525):785–788, https://doi.org/10.1038/374785a0.

⁵⁶ C. Chapman, 1996, “S-Type Asteroids, Ordinary Chondrites and Space Weathering: The Evidence from Galileo’s Flybys of Gaspra and Ida,” Meteoritics and Planetary Science 31:699–725, https://doi.org/10.1111/j.1945-5100.1996.tb02107.x.

⁵⁷ P. Michel, R.-L. Ballouz, O.S. Barnouin, et al., “Collisional Formation of Top-Shaped Asteroids and Implications for the Origins of Ryugu and Bennu,” Nature Communications 11(1), https://doi.org/10.1038/s41467-020-16433-z.

⁵⁸ D.S. Lauretta, C.W. Hergenrother, S.R. Chesley, et al., 2019, “Episodes of Particle Ejection from the Surface of Active Asteroid (101955) Bennu,” Science 366(6470), https://doi.org/10.1126/science.aay3544.

also important; one model⁵⁹ suggests that bodies forming closer to the Sun accreted earlier and thus had more ²⁶Al, which would have led to silicate melting, thermal metamorphism, and ice melting. From a planetary protection perspective, the most astrobiologically relevant targets are volatile-rich carbonaceous asteroids. But, with the exception of Ceres, aqueous fluids only existed ~4.5 billion years ago.

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.

Small bodies record the radial compositional gradients of material that were present in the protosolar disk, and they represent all stages of the formation and early evolution of the solar system. Primitive small bodies are the debris left over from planet formation and they contain examples of the primordial ingredients from which the planets and life arose. Small bodies record internal processing such as aqueous

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⁵⁹ R.E. Grimm and H.Y. McSween, 1993, “Heliocentric Zoning of the Asteroid Belt by Aluminum-26 Heating,” Science 259(5095):653–655, https://doi.org/10.1126/science.259.5095.653.

⁶⁰ D.S. Lauretta, O. Barnouin-Jha, M.A. Barucci, et al., 2009, “Astrobiology Research Priorities for Primitive Asteroids,” Lunar and Planetary Institute, https://www.lpi.usra.edu/decadal/sbag/topical_wp/lauretta_etal.pdf.

alteration, thermal metamorphism, melting and differentiation. These evolved small bodies record processes that occurred during the formation and evolution of planets. Small bodies thus trace growth from primordial condensates and presolar/interstellar grains to pebbles, planetesimals, and planetary embryos, to planets. Because catastrophic collisions can completely shatter protoplanets, small bodies can be samples of core, mantle, or crustal material of once much larger bodies. Indeed, this is the only known reservoir of accessible core and, potentially, deep mantle material. Small bodies also record the history of the solar system such as the dynamical evolution of the solar system, the evolution of surface materials through time and as they approach the Sun, and the primordial cosmo-chemical gradients established within the solar nebula.

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. There are a few exceptions, most notably Ceres (see Box 2-1), and these may require special attention as will be discussed in Chapter 3.

CONDITIONS ON SMALL BODIES WITH RESPECT TO SURVIVAL AND PROLIFERATION

In the framework of planetary protection for solar system small bodies, scientific understanding of life is based on our knowledge of life on Earth. Because of the incomplete current state of knowledge about small bodies as well as the survival-limits of life, all judgments regarding biological potential are qualitative, not quantitative. On Earth, liquid water is the solvent for life, and desiccation alone will prevent cell proliferation, but can allow for survival of dormant cells in which intracellular water is lost and metabolism is undetectable, yet can resume activity upon aquation. As examples, archaea, bacteria, and fungi possess a number of strategies that allow them to survive desiccation in the form of spores or when simply dried as vegetative cells.^61,62,63 Of course, small bodies exist in the vacuum of outer space, and any foreign contaminating cells would be frozen, desiccated, and unable to repair genetic damage caused by unremitting solar and galactic cosmic radiation (GCR). In determining the likelihood of forward contamination, considerations of water content and radiation on small bodies thus allow for a conservative approach toward gauging survival of life on asteroids and comets.

The survivability of frozen and desiccated microorganisms transported to surface environments of small bodies would be governed by radiation exposure, which varies across the surface and subsurface of small bodies and with distance from the Sun. On an asteroid’s surface, ultraviolet C (UVC, light in the 200–280 nm range) will inactivate cells within days to months.^64,65 For microorganisms shielded from UVC and heat in the near subsurface (0.1 m), ionizing radiation, largely due to GCR and solar protons,

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⁶¹ K.L. Anderson, E.E. Apolinario, and K.R. Sowers, 2012, “Desiccation as a Long-Term Survival Mechanism for the Archaeon Methanosarcina Barkeri,” Applied and Environmental Microbiology 78(5):1473–1479, https://doi.org/10.1128/AEM.06964-11.

⁶² P. Setlow, 2007, “I Will Survive: DNA Protection in Bacterial Spores,” Trends in Microbiology 15(4):172–180, https://doi.org/10.1016/j.tim.2007.02.004.

⁶³ V. Mattimore and J.R. Battista, 1996, “Radioresistance of Deinococcus Radiodurans: Functions Necessary to Survive Ionizing Radiation Are Also Necessary to Survive Prolonged Desiccation,” Journal of Bacteriology 178(3):633–637, https://doi.org/10.1128/jb.178.3.633-637.1996.

⁶⁴ NASEM, 2021, Report Series: Committee on Planetary Protection: Evaluation of Bioburden Requirements for Mars Missions, Washington, DC: The National Academies Press, https://doi.org/10.17226/26336.

⁶⁵ A. Vicente-Retortillo, G.M. Martínez, N.O. Rennó, M.T. Lemmon, M. de la Torre-Juárez, and J. GómezElvira, 2020, “In Situ UV Measurements by MSL/REMS: Dust Deposition and Angular Response Corrections,” Space Science Reviews 216:97, https://doi.org/10.1007/s11214-020-00722-6.

would extinguish survival over thousands of years (~0.1 Gy/yr); but for desiccated and frozen microbes transported to the deep subsurface (10 m), only internal background radiation is expected to limit survival over millions of years, as on Earth (~0.5 mGy/yr).⁶⁶ No dormant DNA-based lifeforms would be expected to withstand billions of years of continuous background radiation on asteroids because, without growth and DNA repair, contaminating cells would inevitably be destroyed.⁶⁷ With regard to planetary protection for missions to asteroids, therefore, the committee considerations support the conclusion that any forward biological contamination will be harmless because cells would not be able to grow or proliferate and thus would be sterilized over time, leaving only macromolecular cellular debris.

No known small bodies have atmospheres (with the exception of cometary comae), and with the possible exceptions of Ceres, Pluto, and Themis, there is no known geologic activity on any small bodies, limiting the transport of material across their surfaces. An exception might be the particle ejection seen to occur on Bennu (and likely on other bodies, probably the result of thermal processing); another possible exception is electrostatic levitation⁶⁸ or redistribution of surface material (e.g., on top-shaped asteroids) via rotational forces. Moreover, comets have extensive resurfacing due to repeated passes in the inner solar system, as described in the next chapter. The lack of atmosphere also means that temperatures are extreme and there is no protection from radiation, nor is there liquid water. Implications for planetary protection are that any terrestrial microbes are extremely unlikely to survive, much less proliferate, on any small body.

The absence of geologic and weather processes both on asteroids and comets further means that their bulk materials and composition have not changed significantly since they were formed in the early solar system, 4.6 billion years ago. Indeed, the abundances of asteroidal and cometary parent molecules and those observed in the interstellar medium show a striking similarity for many of the simple chemical species. Recent discoveries, though, have changed the perception that asteroids and comets contain only simple volatile molecules like CO, CO₂, NH₃, and water, to include multiple amino acids.⁶⁹ So, small-body objects represent targets for the study of materials which not only gave rise to planets but also to the organic precursors of life on Earth. Although small bodies may have served as a delivery mechanism for the building blocks of life, the small-body objects themselves are not relevant to prebiotic biochemical evolution of macromolecular life as occurred on Earth.

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.⁷¹ Similarly, the Hayabusa2 sample-return mission showed surface-excavated materials of the rubble-pile asteroid Ryugu

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⁶⁶ See Chapter 5 and Appendix A in NASEM, 2002, The Quarantine and Certification of Martian Samples, Washington, DC: The National Academies Press, https://doi.org/10.17226/10138.

⁶⁷ D. Ghosal, M.V. Omelchenko, E.K. Gaidamakova, et al., 2005, “How Radiation Kills Cells: Survival of Deinococcus Radiodurans and Shewanella Oneidensis Under Oxidative Stress,” FEMS Microbial Reviews 29(2):361–375, https://doi.org/10.1016/j.femrre.2004.12.007.

⁶⁸ M.S. Robinson, P.C. Thomas, J. Veverka, S. Murchie, and B. Carcich, 2001, “The Nature of Ponded Deposits on Eros,” Nature 413:396–400, https://doi.org/10.1038/35096518.

⁶⁹ T. Yada, M. Abe, A. Nakato, et al., 2022, “Preliminary Analyses on Bulk and Individual Ryugu Samples Returned by Hayabusa2,” LPI Contributions 2678(1831), 53rd Lunar and Planetary Science Conference, https://www.hou.usra.edu/meetings/lpsc2022/pdf/1831.pdf.

⁷⁰ S.A. Sandford, J. Aléon, C.M.O’D. Alexander, et al., 2006, “Organics Captured from Comet 81P/Wild2 by the Stardust Spacecraft,” Science 330:468–472, https://doi.org/10.1126/science.1135841.

⁷¹ M. Schuhmann, K. Altwegg, H. Balsiger, et al., 2019, “Aliphatic and Aromatic Hydrocarbons in Comet 67P/Churyumov-Gerasimenko Seen by ROSINA,” Astronomy and Astrophysics 630(A31), https://doi.org/10.1051/0004-6361/201834666.

to contain more than 10 types of amino acids including glycine and L-alanine.⁷² In summary, with regard to planetary protection of asteroids and comets, the committee concludes that forward contamination by microorganisms would not be harmful because cells would not proliferate in vacuo.⁷³

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⁷² T. Yada, M. Abe, A. Nakato, et al., 2022, “Preliminary Analyses on Bulk and Individual Ryugu Samples Returned by Hayabusa2,” LPI Contributions 2678(1831), 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. radiodurans cells support two findings. First, it must be assumed that any forward contamination of small bodies with terrestrial microorganisms would essentially be permanent, over mission timeframes of thousands of years, and this could complicate scientific efforts in the search for life, even though terrestrial microbes would not proliferate in vacuo. Second, if whole viable D. radiodurans cells could survive the equivalent of 1.4 Myr in the near subsurface environments of asteroids, then their macromolecules will survive much, much longer.