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4 The Primitive Bodies: Building Blocks of the Solar System Studies of primitive bodies encompass asteroids, comets, Kuiper belt objects (KBOs), the moons of Mars, and samples—meteorites and interplanetary dust particles—derived from them. These objects provide unique information on the solar system’s origin and early history and help researchers to interpret observations of debris disks around other stars. Over the past decade the planetary science community has made remarkable progress in understanding primitive bodies (Table 4.1), but important questions remain unanswered. The study of primitive bodies over the past decade has been accomplished as a result of a number of space missions such as Deep Impact, Stardust, EPOXI, Cassini, and the Japan Aerospace Exploration Agency’s (JAXA’s) Hayabusa spacecraft. Discovery-class missions are ideally suited to research on primitive bodies, although larger missions also play a vital role, particularly for objects in the outer solar system that cannot be reached without radioisotope power systems. In the coming decade, several other missions currently underway, such as Dawn and New Horizons, will add substantially to knowledge of these objects. The study of primitive bodies is also aided by ground-based telescopes and radar, which are highly useful in this field because the number of objects is so great that only a tiny fraction can be visited by spacecraft, and space missions are aided substantially by prior observation. Indeed, ground-based telescopes continue to discover unusual and puzzling objects in the Kuiper belt and elsewhere, and those objects might serve as the targets for future missions. Comet sample return is a major goal of the study of primitive bodies, and one of the ultimate goals is a mission to return cryogenic samples. Although a flagship-class primitive bodies mission is not proposed for this decade, the initiation of a technology development program is necessary so that such a mission will be possible in the decade after 2022. New Frontiers-class missions can produce valuable science, and the most important missions for addressing goals related to primitive bodies in the decade 2013-2022 (in priority order) are Comet Surface Sample Return and Trojan Tour and Rendezvous. Discovery-class missions have already produced and will continue to produce important science on these objects. However, a regular, and preferably short, cadence for such missions is important. Technology develop- ment, laboratory research, and data archiving are all vital to continued success in the study of primitive bodies. And finally, assured access to large ground-based telescopes is required for observing samples of the large number of primitive bodies in our solar system. All three of the crosscutting science themes for the exploration of the solar system include the primitive bodies, and studying the primitive bodies is vital to answering a number of the priority questions in each of the themes. For example, in the theme building new worlds, What were the initial stages and conditions and processes of solar 87
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88 VISION AND VOYAGES FOR PLANETARY SCIENCE TABLE 4.1 Major Accomplishments by Telescopes and Space-Based Studies of Primitive Bodies in the Past Decade Major Accomplishment Mission and/or Technique Detailed orbital characterization of an asteroid, including successful landing Near-Earth Asteroid Rendezvous Sampling of a near-Earth asteroid and return of the sample to Earth; determination Hayabusa that small asteroids can be rubble piles Determination of the density of a comet nucleus via the first controlled cratering Deep Impact experiment on a primitive body Return of comet dust for analysis in terrestrial laboratories Stardust First reconnaissance of a possible former trans-Neptune object in the form of Saturn’s Cassini distant satellite, Phoebe Discovery that binary objects are common among near-Earth and main belt asteroids Ground- and space-based telescopes and and Kuiper belt objects, and that comets occur within the main asteroid belt radar studies system formation and the nature of the interstellar matter that was incorporated? is largely a question that can be answered only by the study of primitive bodies. The planetary habitats theme also includes the question, What were the primordial sources of organic matter, and where does organic synthesis continue today?—which is also relevant to the study of primitive bodies, because comets are believed to be a primary source of primordial organic materials. In the workings of solar systems theme, two of the questions, in particular, directly involve the primitive bodies: First, primitive bodies are central to the question, What solar system bodies endanger Earth’s biosphere, and what mechanisms shield it? because of the role that asteroid and comet impacts on Earth have played in mass extinction events, and because such impacts still pose a hazard today. How have the myriad chemical and physical properties that shaped the solar system operated, interacted, and evolved over time? is a question that is directly addressed by the study of their role in accretion and subsequent bombardment through time, in particular because the primitive bodies are believed to have served an important role in delivering organic materials and water to the inner planets, particularly Earth. SCIENCE GOALS FOR THE STUDY OF PRIMITIVE BODIES The goals for research on primitive bodies for the next decade are twofold: • Decipher the record in primitive bodies of epochs and processes not obtainable elsewhere, and • Understand the role of primitive bodies as building blocks for planets and life. DECIPHER THE RECORD IN PRIMITIVE BODIES OF EPOCHS AND PROCESSES NOT OBTAINABLE ELSEWHERE Primitive bodies, be they asteroids, comets, KBOs, possibly the martian moons, meteorites, or interplanetary dust particles—are thought to have formed earlier than the planets in the hierarchical assembly of solar system bodies. Because they witnessed, or participated in, many of the formative processes in the early solar nebula, they can provide unique constraints on physical conditions and cosmochemical abundances. Such constraints come, in part, from observations and remote-sensing measurements made from nearby spacecraft. Because it is possible to visit only a small number of the myriad, highly diverse primitive bodies, researchers must also observe them using Earth-based telescopes (Figure 4.1). Constraints on nebular processes, as well as absolute dating of events in the early solar system, also come from laboratory analyses of samples of these bodies, whether collected and
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89 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM FIGURE 4.1 Asteroids and comet nuclei visited by spacecraft as of 2010. SOURCE: Montage by Emily Lakdawalla. Ida, Dactyl, Braille, Annefrank, Gaspra, Borrelly: NASA/JPL/Ted Stryk. Steins: European Space Agency/OSIRIS team. Eros: NASA/ Johns Hopkins University Applied Physics Laboratory. Itokawa: Institute of Space and Astronautical Science/Japan Aerospace Exploration Agency/Emily Lakdawalla. Mathilde: NASA/Johns Hopkins University Applied Physics Laboratory/Ted Stryk. Lutetia: ESA/OSIRIS team/Emily Lakdawalla. Halley: Russian Academy of Sciences/Ted Stryk. Tempel 1, Hartley 2: NASA/ JPL/University of Maryland. Wild 2: NASA/JPL. returned to Earth by spacecraft missions or obtained as a result of the vagaries of celestial mechanics. The least- processed of these samples contain small amounts of tiny presolar grains, whose properties and compositions constrain astrophysical processes that predate the solar system. These processes encompass the inner workings of stars and the formation and modification of materials in the cold reaches of interstellar space. Specific objectives for continued advancement of studies of primitive bodies in the coming decade include the following: • Understand presolar processes recorded in the materials of primitive bodies; • Study condensation, accretion, and other formative processes in the solar nebula; • Determine the effects and timing of secondary processes on the evolution of primitive bodies; and • Assess the nature and chronology of planetesimal differentiation.
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90 VISION AND VOYAGES FOR PLANETARY SCIENCE Subsequent sections examine each of these objectives in turn, identify critical questions to be addressed, and suggest future investigations and measurements that could provide answers. Presolar Processes Recorded in the Materials of Primitive Bodies Traditionally, the only avenue for understanding processes that predated our own solar system has been astro- nomical observations. Now, analyses of materials from primitive bodies are revolutionizing this field of research. Studies of microscopic presolar grains in chondritic meteorites, interplanetary dust particles, and comet samples returned by the Stardust mission provide critical constraints for models of the synthesis of elements and isotopes within stars and supernovae.1 Studies to characterize the organic matter in these materials are proceeding apace; they reveal how simple carbon-based molecules formed in interstellar space have been processed into more com- plex molecules in the solar nebula and in planetesimals. Isotopic and structural fingerprints in these molecules are allowing researchers to learn how and where these molecules formed. Remarkable progress, enabled by significant advances in micro-analytical technology, has been made in documenting the compositions of presolar grains incorporated into chondrites, and in linking the various kinds of grains to the specific stellar environments in which they formed (Figure 4.2). 2,3,4 Further technological advances will continue to revolutionize understanding of stellar nucleosynthesis. Somewhat surprisingly, comet samples returned to Earth by the Stardust mission were not dominated by presolar grains, suggesting that current under- FIGURE 4.2 Transmission electron microscope image of a presolar graphite grain containing a central core (dark) of titanium- vanadium carbide. This grain formed by condensation around another star. SOURCE: H.Y. McSween and G.R. Huss, Cosmo- chemistry, Cambridge University Press, 2010, p. 131. Copyright 2010 Cambridge University Press, reprinted with permission.
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91 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM standing of how presolar grains were incorporated into the solar nebula is incomplete.5 Advances in the challenging measurements of the stable isotopic compositions of specific organic molecules in meteorites and Stardust grains have been made during this decade. Important Questions Important questions for the understanding of presolar processes recorded in the materials of primitive bodies include the following: • How do the presolar solids found in chondrites relate to astronomical observations of solids disposed around young stars? • How abundant are presolar silicates and oxides? Most of the presolar grains recognized so far are carbon (diamond, graphite) phases or carbides. • How do the compositions of presolar grains and organic molecules vary among different comets? Future Directions for Investigations and Measurements While previous laboratory work has focused on presolar grains extracted from meteorites by harsh chemical treatments, future efforts will exploit new technologies to locate and analyze presolar grains in situ in the host meteorites, so as to identify less refractory materials. Obtaining presolar grains and organic matter from additional comet sampling missions and from interplanetary dust particles will allow researchers to understand how these materials were distributed in the solar nebula and preserved in solar system solids. Condensation, Accretion, and Other Formative Processes in the Solar Nebula Primitive asteroids and the meteorites derived from them witnessed events and processes in the inner solar nebula, whereas comets formed in the outer reaches of the solar system and thus record a broader array of nebular environments. The innermost portions of the nebula were hot, causing interstellar solids to melt, evaporate, and recondense as refractory minerals. The outer portions of the nebula were cold enough to condense ices, profoundly changing the bulk compositions of accreted planetesimals and planets. The thermal and chemical conditions of vari- ous nebular regions, the processes that occurred in those regions, the timing of such processes, and the subsequent transport of materials between nebular regions can all be constrained from studies of primitive body materials. The nature of the accretion process is also revealed by the size distributions of planetesimals whose assembly was arrested before their mass reached that of a planet. Significant progress has been made in constraining the nature and mixing during formation, and the formation chronology, of primitive bodies using short-lived radioisotopes.6,7 Dynamic nebular mixing is indicated by the diverse components in comet samples returned by the Stardust spacecraft, and the composition of the Sun is now constrained by analyses of solar wind samples from the Genesis spacecraft. 8 The 16O-rich isotopic composition of oxygen in the Sun is one of the biggest surprises in cosmochemistry. 9 Another startling revelation is that melted and differentiated asteroids formed earlier than chondritic asteroids, 10 implying that the early building blocks for the terrestrial planets were already differentiated.11 The size distributions of KBOs, now moderately well known for bodies greater than 100 km, are serving as the best tracer of the accretion phase in the outer solar nebula. 12 Important Questions Some important questions concerning condensation, accretion, and other formative processes in the solar nebula are as follows: • How much time elapsed between the formation of the various chondrite components, and what do those differences mean?
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92 VISION AND VOYAGES FOR PLANETARY SCIENCE • Did evaporation and condensation of solids from hot gas occur only in localized areas of the nebula, or was that process widespread? • What are the isotopic compositions of the important elements in the Sun? • Which classes of meteorites come from which classes of asteroids, and how diverse were the components from which asteroids were assembled? • How variable are comet compositions, and how heterogeneous are individual comets? • What are the abundances and distributions of different classes of asteroids, comets, and KBOs? • How do the compositions of Oort cloud comets differ from those derived from the Kuiper belt? Future Directions for Investigations and Measurements Although progress has been made in assessing whether various kinds of interplanetary dust are derived from comets or asteroids, there remains some uncertainty about the parent objects of some kinds of primitive meteor- ites. Determining the ages of chondrite components that record specific nebular processes is required to produce a timeline for major events in the solar nebula. Further refinements in analyzing solar wind samples are needed to define isotopic ratios in the Sun. Sampling additional comets is necessary to understand the diversity within this large population of poorly studied primitive bodies. Obtaining comet samples from the surface, as opposed to dust ejected from a comet nucleus, is a high priority. Increasing the number of known KBOs may reveal the environments in which different classes of objects formed. Effects and Timing of Secondary Processes on the Evolution of Primitive Bodies The asteroidal parent bodies of most meteorites have been altered by internal heating, reactions with aqueous fluids (produced by melting accreted ices), and impacts. Telescopic spectral measurements of asteroids indicate that the nature and intensity of alteration differ with orbital position. Secondary processes on primitive bodies control the mineralogy, nature of organic compounds, and abundances of volatile elements. Abundant meteorite samples have allowed researchers to quantify the conditions under which these secondary processes occurred and to understand their timing in asteroids. However, the extent of such secondary processes in comets and KBOs is not understood at all. The conditions and timescales for metamorphism and aqueous alteration in asteroids have been quantified, 13 and considerable progress has been made in modeling asteroid thermal histories. 14 A consensus has apparently been reached that decay of the short-lived radionuclide aluminum-26 was the primary heat source active during the formation of the asteroids, based on new isotopic analyses of meteorites and thermal evolution models. Recent spacecraft missions to asteroids and comets have documented secondary processes, including extensive impact cratering on asteroid surfaces and smooth flows of erupted materials on comet nuclei. 15,16 Important Questions Some important questions concerning the effects and timing of secondary processes on the evolution of primi- tive bodies include the following: • To what degree have comets been affected by thermal and aqueous alteration processes? • How well can we read the nebular record in extraterrestrial samples through the haze of secondary processes? • What is the relationship between large and small KBOs? Is the population of small KBOs derived by impact disruption of the large KBOs? • How do the impact histories of asteroids compare to those of comets and KBOs? • How do physical secondary processes such as spin-up result from non-gravitational forces, the creation and destruction of binary objects, and space weathering?
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93 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM Future Directions for Investigations and Measurements Comets and distant KBOs likely record secondary processing under a vast range of conditions; studying such processes will require a combination of telescopic observations and challenging spacecraft missions. Formulation of more realistic thermal models for asteroids and comets is needed. Nature and Chronology of Planetesimal Differentiation Melted silicate-metal asteroids and the meteorites derived from them provide information on the formation of crusts, mantles, and cores on bodies with compositions different from that of Earth and under conditions not encountered on our planet. They allow researchers to test the generality of hypotheses about the differentiation of planets. From them, we learn how elements are partitioned between molten and solid phases. The radiogenic isotope systems in differentiated meteorites provide the information required to date differentiation processes recorded in samples of Earth, the Moon, and Mars. At the other end of the compositional scale, the Kuiper belt is home to the largest number of silicate-ice objects, some of which might have undergone internal heating and differentiation. New age dates have revolutionized the chronology of differentiated silicate bodies, 17,18 while new meteorite recoveries have broadened the range of differentiation styles and conditions experienced by these bodies. 19 The rapid cooling rates determined from nickel diffusion profiles in some iron meteorite groups suggest that glancing impacts may have stripped off their thermally insulating silicate mantles, exposing hot, naked iron cores.20 Thermal models to explain metal-silicate differentiation in asteroids and silicate-ice differentiation in KBOs have become more sophisticated as more rigorous constraints have been placed on them. 21,22 Important Questions Some important questions concerning the nature and chronology of planetesimal differentiation include the following: • Did asteroid differentiation involve near-complete melting to form magma oceans, or modest partial melting? • How did differentiation vary on bodies with large proportions of metal or ices? • Were there radial or planetesimal-size limits on differentiation, and were KBOs and comets formed too late to have included significant amounts of live aluminum-26 as a heat source? • What are the internal structures of Trojans and KBOs? Future Directions for Investigations and Measurements The Dawn spacecraft will arrive at asteroids 4 Vesta in July 2011 and 1 Ceres in February 2015. Mapping and spectroscopy of 4 Vesta and 1 Ceres will provide new insights into differing styles of asteroid differentiation and set the stage for geophysical exploration of asteroids by spacecraft to determine their interior structures and compositions (Figure 4.3). Such spacecraft missions can provide ground truth for systematic studies of KBOs with large ground-based telescopes, which might probe the state of differentiation on bodies with a broader range of sizes and dynamical locations. UNDERSTAND THE ROLE OF PRIMITIVE BODIES AS BUILDING BLOCKS FOR PLANETS AND LIFE The planets have experienced significant geologic processing, by differentiation to form crusts, mantles, and cores of varying composition, and by subsequent reworking through massive impacts, continued tectonic and igne- ous activity, and in some cases interactions of the surface with an atmosphere or hydrosphere. No planetary samples now have the chemical composition of the whole, and melting, crystallization, and metamorphism have conspired
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H.Y. McSween Jr. FORal. VISION AND VOYAGES et PLANETARY SCIENCE 94 FIGURE 4.3 Photomicrograph of the EET 90020 basaltic eucrite, a differentiated meteorite thought to be derived from asteroid 4 Vesta. The scale bar is 2.5 mm. SOURCE: With kind permission from Springer Science+Business Media: H.Y. McSween, D.W. Mittlefehldt, A.W. Beck, R.G. Mayne, and T.J. McCoy, HED me- teorites and their relationship to the geology of Vesta and the Dawn mission, Space Science Reviews, Online First, doi10.1007/ s11214-010-9637-z, Copyright Springer Science+Business Media B.V. 2010. to modify planetary matter so that its precursor materials are nearly unrecognizable. However, certain chemical and isotopic fingerprints persist, and these can be compared with measurements of meteorites to constrain the kinds and proportions of primitive body planetesimals that accreted to form the planets now dominating the solar system. Although it is possible to infer the interior structures of differentiated planets from geophysical data, dif- ferentiated meteorites provide real samples of core and mantle materials for direct analysis. Radiogenic isotopes in meteorites provide the necessary baseline for reconstructing the chronology of planet formation and differentiation. Documenting the orbital distributions and understanding the orbital evolutions of primitive bodies also constrain the dynamics and timing of planetary accretion. Astrobiology is the study of the origin, evolution, and distribution of life in the universe. Although it is unrealistic to look for life on primitive bodies, scientists know from the study of meteorites that many of them contain the organic ingredients for life. These organic compounds were formed as mantles on dust grains in cold interstellar clouds and in the outer reaches of the solar nebula, and in rocks within the interiors of planetesimals warmed as the ices in these bodies melted. The compounds are surprisingly complex and include amino acids and other molecules that are central to life on Earth. The accretion of such materials, late in the assembly of planets, is thought to have been a possible first step in the poorly understood path from organic matter to organisms. Studies of the molecular forms and isotopic compositions of the organic matter in meteorites and samples returned by spacecraft provide a prebiotic starting point for the origin and evolution of life or an independent channel of abiologic organic chemistry. Specific objectives for continued advancement of this field in the coming decade include the following: • Determine the composition, origin, and primordial distribution of volatiles and organic matter in the solar system; HED meteorites; all photomicrographs are taken with crossed nicols. a QUE 97053 • Understand how and when planetesimals were assembled to form planets; and • of plagioclase grains (gray planets partly enclosed di pyroxene; b subophitic texture Constrain the dynamical evolution of laths) by their effects on thebystribution of primitive bodies. 90020 basaltic eucrite, with recrystallized texture; c Moore County cumulate eucrite; e diogenite; e QUE 99050 diogenite, with brecciated texture; f PCA 02019 howardite, a to be addressed Subsequent sections examine each of these objectives in turn, and identify critical questions and future investigations and measurements that could provide answers. eucrite and diogenite clasts. All scale bars are 2.5 mm
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95 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM Composition, Origin, and Primordial Distribution of Volatiles and Organic Matter in the Solar System Meteorites and interplanetary dust particles are readily available sources of extraterrestrial organic matter from asteroids and comets, although the volatile species and organic components of comets and KBOs remain poorly understood. Systematic depletions in highly volatile elements in meteorites testify to an important elemental frac- tionation in the early solar system, but it is not well understood. Some organic matter in chondrites, interstellar dust particles, and Stardust comet samples has been structurally and isotopically characterized, although the insoluble organic fractions, consisting of huge complex molecules, are extremely difficult to analyze. Understanding the formation and evolution of organic molecules in space and in planetesimals is essential to astrobiology. Telescopic spectral observations of primitive bodies provide at best a tantalizing but incomplete picture of the orbital distribu- tion of organic matter in the early solar system. The planetary science community has made progress in characterizing the insoluble (macromolecular) organic matter and in analyzing the isotopic compositions of specific organic compounds in chondrites. 23,24,25 The changes in organic matter caused by alteration of the parent body are now better understood, and the fractionation of isotopes during the evaporation of volatile elements has been modeled. Cometary organic matter and volatile elements have been analyzed in samples returned by the Stardust spacecraft, 26,27,28 and cometary volatiles are now recognized to be heterogeneous as determined by the Deep Impact spacecraft.29 Spectroscopy of recently discovered KBOs has provided some constraints on their surface compositions.30 Comparison of meteoritic and cometary organic matter inventories with the composition of young circumstellar disks has been facilitated by recent Spitzer Space Telescope observations.31 Important Questions Some important questions concerning the composition, origin, and primordial distribution of volatiles and organic matter in the solar system include the following: • What are the chemical routes leading to complex organic molecules in regions of star and planet formation? • What was the proportion of surviving presolar organic matter in the solar nebula, relative to the organic compounds produced locally? • What roles did secondary processes and mineral interactions play in the formation of organic molecules? • How stable are organic molecules in different space environments? • What caused the depletions in volatile elements, relative to chondrites, observed in differentiated asteroids and planets? • What kinds of surface evolution, radiation chemistry, and surface-atmosphere interactions occur on distant icy primitive bodies? • How is the surface composition of comets modified by thermal radiation and impact processes? Future Directions for Investigations and Measurements The New Horizons spacecraft will fly past Pluto in July 2015 and obtain remote sensing data on the dwarf planet and its satellites Charon, Nix, and Hydra. It is expected that a successful encounter with Pluto will be followed by retargeting the spacecraft to a flyby with a yet-to-be-selected Kuiper belt object. The detailed charac - terization of a single small sample of KBOs—Pluto and Charon—and maybe more if suitable candidates can be found along New Horizon’s trajectory will have to be complemented by large ground-based telescope studies in order to continue the discovery and characterization of a significant portion of KBOs. Organic matter in returned comet samples will provide critical new information on organic synthesis. The study of organic matter in extrater- restrial materials will also evolve from basic characterization of simple compounds and mixtures to understanding the origin of different molecules. Return of samples from a range of organic-rich asteroids and comets (including cryogenically preserved comets) will ultimately be required to fully address these questions.
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96 VISION AND VOYAGES FOR PLANETARY SCIENCE How and When Planetesimals Were Assembled to Form Planets Planet formation was hierarchical, as small planetesimals were assembled into ever-larger ones, eventually forming the planets. The feeding zones for accretion of planets and the timing of planetary growth remain incom- pletely understood. Swarms of asteroids, comets, and KBOs provide basic information on planetesimal sizes, compositions, and orbital parameters with which to model the assembly of planets. Studies of radiogenic isotopes in meteorites allow the timing of planet formation to be constrained. Theoretical studies, particularly complex accretion models developed during this decade, follow the orbital evolution of many thousands of objects and provide constraints on the timescales and widths of feeding zones for the terrestrial planets.32 Understanding of the timing of accretion benefits from improved determinations of the formation chronology of Earth, the Moon, and Mars, which have been made using measurements of short-lived radioisotopes in samples.33,34,35 Important Questions Some important questions concerning how and when planetesimals were assembled to form planets include the following: • Are there systematic chemical or isotopic gradients in the solar system, and if so, what do they reveal about accretion? • Do we have meteoritic samples of the objects that formed the dominant feeding zones for the innermost planets? • How did Earth get its water and other volatiles? What role did icy objects play in the accretion of various planets? • What is the mechanical process of accretion up to and through the formation of meter-size bodies? Future Directions for Investigations and Measurements Understanding the formation times of the various materials comprised by comets could constrain the chronol- ogy of kilometer-size objects beyond the orbit of Neptune. Measurements of deuterium/hydrogen ratios in different primitive objects can be used to constraint their possible contributions to the water inventory of Earth and other planets. Determining the deuterium/hydrogen ratio in multiple comets would quantify the role comets may have played in delivering water and organic matter to early Earth. Spacecraft exploration of multiple asteroids could provide information on compositional gradients in the solar system. Improvements in numerical models for accre- tion could provide a more robust understanding of feeding zones. Dynamical Evolution of Planets by Their Effects on the Distribution of Primitive Bodies The orbital distribution of the giant planets is now thought to have been much more dynamic than previously appreciated. Orbital perturbations of primitive bodies are the key to unraveling planet migrations in the early solar system. Although pathways from the main belt to account for near-Earth asteroids are now clear, the source of some asteroid populations, such as Trojans (in orbits near Jupiter) and Centaurs (in orbits between the asteroid belt and the Kuiper belt) is not understood. The Kuiper belt is an important reservoir of comets, although the precise delivery paths into the inner solar system remain unclear. Bodies exhibiting cometary activity have now been recognized within the main asteroid belt and among the Centaur asteroids.36 The structure of the Kuiper belt provides one of the best constraints on the dynamical rearrangement of the giant planets, and some recent KBO studies have revised scenarios for the early orbital history of the solar system.37 The size distribution of main belt asteroids has been matched to that of impactors during the late heavy bombardment about 4 billion years ago, suggesting that the asteroid belt was the source of these impactors.38,39 A different population of impactors is indicated for the outer solar system, judging from the cratering record preserved on Saturn’s ice satellites.40
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97 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM Important Questions Some important questions concerning the dynamical evolution of planets by their effects on the distribution of primitive bodies include the following: • Which classes of asteroids participated in the late heavy bombardment of the inner planets and the Moon, and how did the current population of asteroids evolve in time and space? • What are the sources of asteroid groups (Trojans and Centaurs) that remain to be explored by spacecraft? • How are objects delivered from the Kuiper belt to the inner solar system? Specifically, by what mechanisms are Jupiter family comets resupplied to the inner solar system? Future Directions for Investigations and Measurements Determining the orbits of vast numbers of KBOs presents an unprecedented opportunity to reconstruct the early dynamic history of the solar system. Orbital surveys coupled with determination of physical characteristics can constrain physical conditions in the nebula. Missions to Trojan or Centaur objects could provide information on their sources, as well as basic characterization, and are important goals for the future. INTERCONNECTIONS Connections with Other Parts of the Solar System Mixtures of meteorite chemical compositions are commonly used to construct models of the bulk composi- tions, volatile inventories, and oxidation states of the terrestrial planets. Radioactive isotopes in meteorites provide the necessary foundation to construct timescales for planet formation. Differentiated asteroids and iron meteorites provide insights into core formation in the terrestrial planets. Resolving the debate concerning the compositions (and likely origins) of the martian moons Phobos and Deimos may be relevant to understanding the early history of Mars. The orbital distributions of primitive bodies constrain models for the orbital migrations of the giant planets in early solar system history. Cosmic element abundances, determined in part from chondritic meteorites, provide a baseline for comparison with the atmospheric compositions of Jupiter and Saturn. Prebiotic chemistry, as under- stood from organic matter in primitive bodies, is a starting point for life on Earth and for the study of astrobiology. Connections with Extrasolar Planets Studies of the sizes, orbital distributions, and compositions of the KBO population and of interplanetary dust derived from KBOs, comets, and asteroids provide critical data for interpreting accumulating data on debris disks around stars such as Beta Pictoris and Fomalhaut. Connections with Astrophysics The isotopic compositions of presolar grains in meteorites and interplanetary dust particles provide tests of theoretical models of nucleosynthesis in stars and supernovae, which are of great interest to astrophysics. The mineralogy and physical properties of presolar grains also are useful in interpreting astronomical observations of dust in the interstellar medium. The measured abundances of short-lived radionuclides in meteorites reflect the formation of the Sun in the vicinity of high-mass stars that injected supernova materials into the nebula, in agreement with astronomical observations of star-forming regions. The discovery of chondrules and refractory inclusions in comet material returned by the Stardust mission has motivated models of large-scale mixing in dust disks. The Kuiper belt offers a model for telescopic observations of the outer parts of dust disks. The existence of the Oort cloud has motivated searches for analogous comet clouds around other stars.
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100 VISION AND VOYAGES FOR PLANETARY SCIENCE Laboratory instrumentation is a fundamental part of a healthy program for the exploration and study of primi- tive bodies. Spectral and physical data from missions can only be understood fully in the context of laboratory analog measurements. Samples returned by missions require state-of-the-art instrumentation for complete analysis. Significant progress has been made in the past decade, with the initiation of the Laboratory Analysis for Returned Samples program to support laboratory equipment development, construction, and operation. This funding was particularly critical to the success of the Genesis and Stardust missions and represents the first laboratory equip- ment funding directly linked to missions since Apollo. Technology Development Currently, the principal obstacle to conducting certain missions to primitive bodies is the absence of the neces- sary power and propulsion technologies. A rendezvous with a KBO, a Centaur, or a trans-Neptune object would be a scientifically compelling mission if the appropriate power and propulsion technologies can be developed to make such a mission possible. Mating electric propulsion to advanced power systems would permit conducting a wide range of missions to primitive bodies throughout the solar system. One KBO rendezvous mission study considered the use of NASA Evolutionary Xenon Thrusters (NEXT), powered by Advanced Stirling Radioisotope Generators (ASRGs). 43 With these technologies, an orbital rendezvous could be achieved with a KBO at 33 AU from the Sun using an existing launch vehicle with a flight time of 16 years. Another study considered a long-life Hall electric thruster that, when combined with six 150-W ASRGs, would enable a New Frontiers-class mission to place a scientifically compre- hensive payload in orbit around a Centaur object within 10 years of launch using an existing launch vehicle. 44 Sample return missions from comets and asteroids provide important information on primitive bodies. Such missions require sample return capsules that must withstand Earth-entry velocities of greater than 13 kilometers per second, beyond the capability of current lightweight thermal protection system (TPS) materials. The development and qualification of new low-density TPS materials is essential to reduce the mass of entry capsules and increase science payloads. Several white papers submitted to the committee suggested that return capsules be instrumented in an effort to understand their performance margin in order that future missions can be lower in mass without taking additional risk. Funding TPS development now would leverage the experience and expertise of people who developed the original TPS technologies before they retire. Specific technology developments necessary to enable a Cryogenic Comet Sample Return mission are outlined separately below. To enable a broad range of primitive bodies missions in the near future, technology developments are needed in the following key areas: ASRG and thruster packaging and lifetime, thermal protection systems, remote sampling and coring devices, methods of determining that a sample contains ices and organic matter and preserving it at low temperatures, and electric thrusters mated to advanced power systems. ADVANCING STUDIES OF THE PRIMITIVE BODIES To date there have been no flagship missions to primitive bodies, and none was identified in the 2003 planetary science decadal survey.45 However, in March 2004 the European Space Agency launched Rosetta, a flagship-class mission in which there is modest participation by NASA-sponsored investigators. In addition, two of the Rosetta instruments (Alice and MIRO) were provided by NASA and have U.S. principal investigators. Rosetta is operating successfully and is scheduled to begin its comprehensive investigation of Comet Churyumov-Gerasimenko in 2014. Addressing some of the key goals for primitive bodies will require flagship-class missions, for example, a Cryogenic Comet Sample Return mission, which would return materials sampled from different depths—up to perhaps 1 meter—from a comet nucleus and preserve those samples at the required cold temperatures to prevent alteration of the sample in transit to Earth. New Frontiers missions can nevertheless address most (but not all) major goals for exploration of primitive bodies. The first mission of this program—New Horizons—is now on its way to Pluto, having completed a highly successful flyby of Jupiter in 2007. New Horizons is scheduled to fly past Pluto and its satellites—Charon, Nix, and
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101 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM Hydra—in June 2015 and then proceed to an encounter with a yet-to-be-determined KBO. The 2003 planetary sci- ence decadal survey and a subsequent NRC report46 identified several high-priority primitive-body missions within the New Frontiers envelope, the highest priority being a Comet Surface Sample Return mission. Also identified were a Trojan/Centaur Flyby and an asteroid surface rover/sample return. None of these missions have flown or been approved for flight to date.47 New Missions: 2013-2022 Although flagship missions are important to planetary exploration, it is essential to maintain balance among mission size, complexity, and targets. To date, most flagship missions have cost $2 billion or more. A planetary mission at this scale is not being proposed for the current decade. A more appropriate use of limited resources is the development of technology for a flagship mission in the 2020s. See below the subsection “Future Flagship Mission Candidate.” Priority New Frontiers-Class Missions Competitively selected missions provide the optimum avenue for fostering innovation and new ideas and for making flight opportunities available to a wider spectrum of investigators. Successful New Frontiers concepts will have focused objectives and well-integrated science and flight teams, aspects that lead to reduced cost and a lower risk of cost growth. An experienced principal investigator can ensure that these goals are achieved while maximiz- ing science return. It is important to note that the cost and technical evaluation (CATE) analyses of missions to primitive bodies (Appendix C) indicated that the current cost cap for New Frontiers is insufficient for missions of the highest interest. This suggests that the cost cap should be raised. Priority New Frontiers missions are the Comet Surface Sample Return and the Trojan Tour and Rendezvous. Comet Surface Sample Return It is widely believed that active comets contain the best-preserved samples of the initial rocky, icy, and organic materials that led to the formation of planets. A Comet Surface Sample Return (CSSR) mission is of the highest priority to the primitive-bodies community. A study of this mission, commissioned by NASA and published in 2008,48 served as a concept study for this decadal survey. The objective of the CSSR mission is to collect at least 100 grams of surface material and return it to Earth for analysis. The Stardust mission returned the first samples from a known primitive body, and the analysis of those samples has profoundly changed researchers’ understanding of the formation of comets. The materials collected by Stardust indicate that comets contain significant amounts of inner solar system materials, including chondrules and refractory inclusions. It appears that comets are made of materials that formed across the full expanse of the solar nebula and thus are bodies that are far more important as preservers of early solar system history than previ- ously believed. Stardust collected hundreds of particles, but most of them were small, and the high-speed capture process degraded organics, submicron grains, and the surface layers of larger grains. A CSSR mission’s collection of material could greatly improve on Stardust’s by returning from a second comet a well-preserved total sample mass 100,000 times larger that would not be altered during collection, except for compounds that are unstable above room temperature. The sample will include large numbers of 0.1- to 1-millimeter solid components that are critically important because they can be compared in exacting detail with their meteorite/ asteroid counterparts in regard to elemental composition, mineralogy, isotopic composition, and even age. Analysis of the samples will provide an unprecedented look at the formation, distribution, and timing of planetary building blocks in the solar nebula. CSSR will provide a large sample of non-volatile cometary organic material that can be compared with the organic material from primitive meteorites that formed in the inner solar system. The CSSR mission represents a quantum leap beyond Stardust, and it would expand in significant ways the data on composition expected from the surface science conducted during ESA’s flagship mission Rosetta. Deep Impact demonstrated that the nucleus of comet Tempel 1 provides suitable areas for sampling of the sort envisioned for a CSSR mission: kilometer-scale areas that are smooth at decimeter scale and have mechanical properties that
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102 VISION AND VOYAGES FOR PLANETARY SCIENCE might be similar to those of loose sand. Moreover, the CSSR is an important precursor for the flagship Cryogenic Comet Sample Return mission, because it would provide information on the state of fine- and coarse-grained aggregates and organic matter that, along with ices, control the bulk physical properties of materials to be collected. Trojan Tour and Rendezvous Trojan asteroids, at the boundary between the inner and outer solar system, are one of the keys to under- standing solar system formation. Originally thought to have been captured from the outer parts of the asteroid belt, Trojan asteroids are proposed in new theories to have been captured instead from the Kuiper belt during a phase of extreme mixing of the small bodies of the solar system. In-depth study of these objects will provide the opportunity to understand the degree of mixing in the solar system and to determine the composition and physical characteristics of bodies that are among the most primitive in the solar system. A mission to rendezvous with a Trojan asteroid after flying by several of them would provide information on the elemental and mineralogical composition of surface materials, the physical state of the surface regolith, and the geology of the surface, including surface structures. Information on the interior structure of a Trojan could be obtained from shape determination and radiometric tracking. The rendezvousing spacecraft would be equipped with instruments to study reflectance spectral properties over a wavelength range of 0.25 to 5.0 microns; gamma-ray and neutron spectroscopy to elucidate elemental composition; multispectral imaging; an ultraviolet spectrometer to search for outgassing; a thermal mapper; and possibly a LIDAR. Most of these instruments would collect important data during the flybys on the way to the rendezvous. Potential Candidate Missions Beyond 2022 and Related Technology Requirements Future Flagship Mission Candidate The return of cryogenic comet samples is viewed as an essential goal (which would be enabled by a precursor New Frontiers Comet Surface Sample Return mission), as documented by the community’s white papers submitted to the survey. To make a flagship Cryogenic Comet Sample Return (CCSR) mission feasible in the 2020s requires the development and demonstration of several key technologies, including the following: • A capability for sampling the subsurface of a comet to a depth of 0.2 to 1 meter while preserving stratigraphy; • A reliable in situ method (preferably simple) for determining that the sample contains at least 20 percent by volume of volatile ices and some fraction of organic matter; and • A method of preserving the sample at temperatures no higher than 125 K during transfer from the comet to a terrestrial laboratory. In the 2003 decadal survey, a cryogenic comet sample return mission was not advocated because of the imma- turity of critical technology. To enable a CCSR mission that can be carried out at acceptable cost and risk, certain critical technologies must be perfected. A study conducted at the committee’s request by the Johns Hopkins University Applied Physics Laboratory to identify the technological issues that must be addressed to enable a CCSR mission (Cryogenic Comet Nucleus Sample Return (CNSR) Mission Technology Study, Appendix G) concluded that it should be possible to obtain a stratigraphy-preserving core sample at least 25 cm deep and 3 cm across using a touch-and-go approach that would not require an actual landing on, or anchoring to, the comet’s surface. The study also examined potential approaches to verifying that a sample contains at least 20 percent ice and accompanying volatile organics, and it considered methods of encapsulating a cryogenic sample and assessed the relative difficulty and cost of maintaining the sample at 90 K, 125 K, or 200 K from collection to delivery to a terrestrial laboratory. The study concluded that a practical thermal design is feasible for ensuring a storage temperature of about 125 K required to preserve water ice during the expected long cruise phase back to Earth. The integrity of more-volatile ices such as hydrogen cyanide and carbon dioxide would be compromised unless a temperature of no more than 90 K was maintained,
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103 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM and achieving this lower temperature would, according to the study, have significant impacts on the complexity and cost of a CCSR mission. Relaxing the minimum temperature limit from 125 K to 200 K, however, appears to be unnecessary to reduce cost. Additional Missions The committee commissioned several studies of potential primitive-body missions that, although they were not selected for prioritization for the 2013-2022 decade, could form the basis for future missions. These missions included the following: • Asteroid Interior Composition Mission, and • Chiron Orbiter. Asteroid Interior Composition Mission The study requested by the committee of a mission focusing on an asteroid’s interior composition began by considering the use of a spacecraft to perform geophysical investigations of a main belt asteroid. The mis- sion’s primary goal would be to understand the internal structure of a differentiated asteroid (either ice-rock or metal-silicate differentiation), and a secondary objective would be to measure surface chemistry and mineralogy. Asteroids 4 Vesta and 6 Hebe were both considered as potential targets. A solar-electric propulsion mission to 4 Vesta was favored, with delivery of seismometers and explosive charges as activators via penetrators. Although the mission design was promising with respect to achieving the primary objective, significant risks remained both in the penetrator design and in incorporation of mineralogic and/or chemical instrumentation in the penetrators, and the committee chose to have the study terminated prior to its completion. Technology development of integrated penetrator systems would enable science beyond 2022 (1) by reducing risk arising from operational uncertainties owing to, for example, penetrators being buried in material of unknown strength and cohesion; and (2) by critically evaluating the ability of complementary instruments to withstand the forces inherent in the deployment of a penetrator. Chiron Orbiter The committee found that an orbiter of the Centaur Chiron would provide a rich science return and yield important information about this class of primitive objects. A study carried out by Goddard Space Flight Center (Chiron Orbiter Mission: Mission Concept Study Report to the NRC Planetary Science Decadal Survey, Primitive Bodies Panel; see Appendixes D and G) found that with current propulsion technology it is not feasible, within the New Frontiers program, to place in orbit an adequate science payload (e.g., an imager, ultraviolet and infrared spectrometers, a magnetometer, and a radio science experiment). Possible options were described for missions to Okryhoe and Echeclus, but these Centaurs have not yet been studied enough to make them compelling targets. The Goddard study also looked for technological advances that could enable a mission to Chiron that would be sufficiently comprehensive to answer the most basic questions about the nature and behavior of Centaurs. The best option requires more efficient packaging of ASRGs for higher-power systems and ion thrusters rated for long mission lifetimes and larger integrated thrust. More-capable propulsion systems will have to be developed before scientifically rewarding rendezvous investigations of Chiron and other targets of high scientific interest in the outer solar system are possible. Given the growing number of known Centaurs and KBOs, the committee concluded that it is scientifically desirable that missions directed to the outer solar system take advantage of opportunities to fly by such objects (at ranges less than 10,000 km) en route to their ultimate targets. During the next decade there will be a growing desire to investigate some large trans-Neptune objects beyond the orbit of Pluto. The New Horizons mission already en route to Pluto (Figure 4.4) has the potential to fly by a small KBO. This extended mission opportunity will be a first chance for a close-up view of this class of object and should not be missed if a suitable target is available. From the perspective of primitive bodies, New Frontiers and Discovery (Box 4.1) missions are both critically important, and a reasonable cadence of such missions must be maintained. Flagship missions are also vital, but
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104 VISION AND VOYAGES FOR PLANETARY SCIENCE FIGURE 4.4 The current best map and images of Pluto based on data from the Hubble Space Telescope. This will be the best view possible until the arrival of the New Horizons mission in 2015. NOTE: This figure was part of a NASA-sponsored press release in March 2010. It is based on results published in Astronomical Journal 139:1128-1143, 2010, and Astronomical Journal 139:1117-1127, 2010. SOURCE: NASA, ESA, and M. Buie. they cannot be allowed to consume all the resources of the planetary exploration program, especially when smaller missions are still capable of returning valuable science. Missions of Interest to NASA’s Exploration Systems Mission Directorate Although NASA’s plans for human exploration activities beyond low Earth orbit are in flux, there is consider- able interest in missions to near-Earth objects. Thus, precursor robotic missions to small bodies can accommodate both human exploration and science goals. Potentially significant areas of overlapping interest between NASA’s Science Mission Directorate (SMD) and Exploration Systems Mission Directorate (ESMD) include the following: • Identification of hazards that requires an understanding of the geophysical behavior of NEOs, a science goal; • For human-precursor missions, development of technologies, especially advanced power systems, that are similar to those required for science missions; and • Resource identification encompassing scientific measurements of objects’ composition. The Lunar Reconnaissance Orbiter provides a recent demonstration of synergy between the interests of NASA’s SMD and ESMD. Proximity operations around small bodies might allow some science observations, and eventual human landings on small bodies would presumably involve sample returns. Such interaction might present a spec- trum of opportunities, including providing inputs into mission design, furnishing flight instruments, characterizing objects through data analysis, and sharing newly developed technologies. Because of their proximity, NEOs are obvious targets for low-cost scientific reconnaissance, rendezvous, and sample return. Notional ESMD plans include several missions to NEOs. Many of these objects, with diameters ranging from ~100 meters to a few tens of kilometers, have been well characterized by ground-based astronomy. Those that come very close to Earth (the so-called Potentially Hazardous Asteroids) have occasionally been extremely well characterized by optical instruments and by radar observations from Goldstone and Arecibo, and
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105 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM BOX 4.1 The Discovery Program, Vital to Exploring Primitive Bodies The Discovery program continues to be an essential part of the exploration and scientific study of primitive bodies. Significant breakthroughs in the understanding of comets and asteroids can be attributed directly to Discovery-class investigations. This trend will continue in part because the extreme diversity of primitive bodies has only begun to be explored, and also because the discovery of primitive bodies con- tinues at ever-increasing rates, thus opening new opportunities (e.g., the discovery of “main belt” comets or asteroids displaying comet-like outgassing). During the past decade the science of primitive bodies has benefited greatly from the Discovery pro- gram. Past and ongoing successes include: • Near Earth Asteroid Rendezvous—The first mission to orbit and land on an asteroid; • Stardust—The first mission to return samples of comet dust to Earth; • Deep Impact—The first mission to investigate the subsurface of a comet and determine the density of a comet nucleus; and • Dawn—A solar-electric propulsion mission on its way to explore two of the largest asteroids in the main belt, 4 Vesta and 1 Ceres. Investigations of primitive bodies are ideally suited for the Discovery program. The vast number and diversity of targets provide opportunities to benefit from the frequent launch schedule envisioned by this program. The proximity of some targets allows for important missions that can be carried out at costs below the Discovery cap. Potentially the need to study diverse targets within a population provides opportunities to re-fly proven technology to new targets, thus reducing mission risk and cost (Figure 4.1.1). There remain many important investigations of primitive bodies that can be carried out within the scope of the Discovery program. Given that the Discovery program is founded on the enterprise and initiative of individuals and is a principal-investigator-led endeavor, this decadal survey does not attempt to define a set of candidate missions or priorities, but the committee gives some examples of important Discovery-class investigations that could be carried out in the coming decade. The population of scientifically compelling targets is not static, but is continually increasing as a consequence of discoveries. In no priority order, Discovery missions of significance to primitive bodies may include, but are not limited to, the following: • Multiple flybys of asteroids and comets to further investigate the great diversity of these bodies— Such missions may benefit from using already proven flight systems and instrument technology. A study of NEO target accessibility (Appendix G) performed by the Jet Propulsion Laboratory at the committee’s request identified asteroids of at least five different taxonomic types, including several not previously visited by spacecraft, that were sufficiently large, required mission durations of moderate to short length, and had delta-V low enough to be accessible with current resources. A flyby visit of several members of one dynamical family would help provide an understanding of the interior structure and composition of their parent asteroids, and the process of collisional evolution. Telescopic surveys reveal diverse organic compo- sitions in comets, the exploration of which would constrain processes in the protoplanetary disk. • Orbital/rendezvous missions to selected comets or asteroids of high scientific interest—While Dawn’s exploration of 4 Vesta represents a first spacecraft study of a differentiated asteroid, a logical follow-on would be an orbital mission to explore an M-class asteroid with high radar reflectivity that could reasonably be the stripped core of a differentiated asteroid. Differentiation was a fundamental process in shaping many asteroids and all terrestrial planets, and direct exploration of a core could greatly enhance understanding of this process. Detailed studies of comets that have naturally broken apart provide oppor- tunities to study their pristine interiors. continued
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106 VISION AND VOYAGES FOR PLANETARY SCIENCE BOX 4.1 Continued • Sample return or geophysical reconnaissance missions to easily accessible NEOs—Although meteorites provide a rich sampling of NEOs, that sampling is certainly incomplete. As an example, recent spectroscopic studies suggest that, compared to known meteorites, some asteroids may be enriched in solid materials from the earliest stage of the solar system’s accretion. The microgravity and geophysical properties of primitive bodies are not understood. Landed missions to study seismological, radar, or rheological responses of comets and asteroids will help to answer questions about their formation, accre- tion, and evolution and will set the stage for advanced missions such as sample return. • Landed investigations of Phobos and Deimos—A major goal of in situ surface science on the martian moons is to determine their compositions in order to constrain their origins. • Stardust-like sample return missions to other Jupiter-family comets to investigate mineralogical and chemical diversity—The results of multiple missions would provide fundamental insights into the origin of crystalline materials around stars and the processes of radial transport in circumstellar disks. • Flyby intercepts of “new” Oort cloud comets to investigate possible chemical differences between these comets and the Jupiter-family comet population—Such a mission would identify possible chemical and isotopic differences between comets that formed inside Neptune’s orbit and the Jupiter-family comet population that formed beyond the planets. • Near-Earth space observatory to study primitive-body populations—Observations from near-Earth space enable the discovery and characterization of primitive-body populations that are not observable from ground-based observations, including NEOs with orbits largely inside Earth’s orbit, kilometer-size KBOs, and extremely distant bodies out to the inner parts of the Oort cloud. During the last half of the past decade the Discovery program was expanded to include Missions of Opportunity. This program has been highly successful both in providing flight opportunities for U.S.-built instruments on foreign spacecraft and in enabling re-use of NASA-built spacecraft that have completed primary missions by retargeting them to new destinations. Additional important exploration of primitive bodies can be achieved by taking advantage of opportunistic flybys as was done on Galileo (asteroids Gaspra and Ida) and on Rosetta (asteroids Steins and Lutetia). Although many important future missions to primitive bodies can be accomplished with existing tech- nology, the general availability of ASRGs will increase the range of accessible targets and facilitate opera- tions in the close proximity of asteroid and comet surfaces. are of great societal interest on account of their potential hazard. Asteroid 433 Eros, one of the largest NEOs, has been visited by a low-cost Discovery rendezvous mission (Near-Earth Asteroid Rendezvous), and one of the asteroids most accessible by spacecraft (25143 Itokawa) was visited by JAXA’s Hayabusa sample return mission. Delivered to near-Earth space from the main belt and more distant reservoirs, NEOs encompass a stunning variety of taxonomic types, sizes, and histories representative of the solar system at large, including a fraction of extinct comets. Future missions may take advantage of low-delta-V trajectories to visit bodies of less common and more primitive spectroscopic type, both to study them geologically and to return samples whose compositions are unlikely to be represented by meteorites. Notional ESMD robotic precursor mission to Mars have also been discussed. There has been little mention of missions to the martian moons, Phobos and Deimos. It is clear that these two moons could play important roles in the future exploration of Mars, especially if they turn out to be related to volatile-rich asteroids, a possibility that has not been excluded by existing data and observations. If so, they may be the surviving representatives of
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107 THE PRIMITIVE BODIES: BUILDING BLOCKS OF THE SOLAR SYSTEM Figure 2-2. Sample Return Trajectories FIGURE 4.1.1 Spacecraft time of flight versus returned mass for sample return missions to a variety of potential near- Earth-object targets. This chart assumes launch on an Atlas V401-class rocket and a spacecraft with chemical propulsion. The size of the “bubble” for each asteroid is based on its estimated diameter. More spacecraft mass is possible with the use of solar-electric propulsion (SEP). (A Dawn-like SEP system could increase spacecraft mass by 40 to 50 percent.) Each point has been optimized for spacecraft mass and not for flight time. SOURCE: NASA/JPL/Caltech. a family of bodies that originated in the outer asteroid belt or at a farther distance, and reached the inner solar system to deliver volatiles and organics to the accreting terrestrial planets. Investigation of Phobos and Deimos crosscuts disciplines of planetary science, including the nature of primi- tive asteroids, formation of the terrestrial planets, and astrobiology. Key science questions concern the moons’ compositions, origins, and relationship to other solar system materials. Are the moons possibly re-accreted Mars ejecta? Or are they possibly related to primitive, D-type bodies? These questions can be investigated by a Discovery-class mission that includes measurements of bulk proper- ties and internal structure, high-resolution imaging of surface morphology and spectral properties, and measure- ments of element and mineral composition. A possible follow-up New Frontiers-class sample return mission could provide more detailed information on composition. Because Phobos and Deimos are potential staging areas and sources of resources for future human exploration of Mars, missions to the martian satellites would contribute uniquely to human exploration goals.
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108 VISION AND VOYAGES FOR PLANETARY SCIENCE Summary The scientific study of primitive bodies can be advanced during the next decade if the following activities are addressed: • Flagship missions—A mission at this scale is not proposed for the current decade. A more appropriate use of limited resources is the initiation of a technology program focused on ensuring that a Cryogenic Comet Sample Return mission can be carried out in the decade of the 2020s at acceptable cost and risk. • New Frontiers missions—Raise the cost cap of the New Frontiers program and keep New Frontiers-class as principal-investigator-led missions. The most important missions for addressing goals related to primitive bodies during the decade 2013-2022 are, in priority order: 1. Comet Surface Sample Return and 2. Trojan Tour and Rendezvous. • Discovery missions—Ensure an appropriate cadence of future Discovery missions. This is critical to the exploration of primitive bodies (see Box 4.1) because of the large number of interesting targets. A regular, prefer- ably short, cadence is more important than increasing the cost cap for Discovery missions. • Technology development—Expand technology developments in the following areas that affect the highest- ranked missions to primitive bodies: ASRG and thruster packaging and lifetime, thermal protection systems, remote sampling and coring devices, methods for determining that a sample contains ices and organic matter and for preserving it at low temperatures, and electric thrusters mated to advanced power systems. Develop a program to bridge the TRL 4-6 development gap for flight instruments to ensure that state-of-the-art instrumentation is available for future missions to primitive bodies. • Ground-based telescopes—Ensure access to large telescopes and to LSST for planetary science observations and maintain the capabilities of the Goldstone and Arecibo radar systems. The large number of primitive bodies in the solar system requires that sufficient telescope time be available for observations of statistically significant samples of these populations to expand scientific knowledge and to plan future missions. Characterization of this multitude of bodies requires large ground-based facilities. • Laboratory research—Continue funding of programs to analyze samples of primitive bodies in hand and to develop next-generation instruments for laboratory analyses of samples returned from comets and asteroids. • Data archiving—Support the ongoing effort to evolve the Planetary Data System from an archiving facility to an effective online resource for the NASA and international communities. • Data curation—Maintain current sample curation facilities and expand their capabilities to handle comet nucleus samples anticipated from the CSSR and CCSR missions. These mission priorities, research activities, and technology development initiatives—discussed and referred to above—are assessed and prioritized in Chapters 9, 10, and 11, respectively. NOTES AND REFERENCES 1 . D. Brownlee , P. Tsou, J. Aléon, C.M.O’D. Alexander, T. Araki, S. Bajt, G.A. Baratta, R. Bastien, P. Bland, P. Bleuet, J. Borg, et al. 2006. Comet 81P/Wild2 under a microscope. Science 314:1711-1716. 2 . L.R. Nittler. 2003. Presolar stardust in meteorites: Recent advances and scientific frontiers. Earth and Planetary Science Letters 209:259-273. 3 . E. Zinner. 2004. Presolar grains. Pp. 17-39 in Treatise on Geochemistry, Volume 1: Meteorites, Comets, and Planets (A.M. Davis, ed.), Elsevier, Oxford. 4 . T.J. Bernatowicz, T.K. Croat, and T.L. Daulton. 2006. Origin and evolution of carbonaceous presolar grains in stellar environments. Pp. 109-126 in Meteorites and the Early Solar System II (D.S. Lauretta and H.Y. McSween, eds.). Uni- versity of Arizona Press, Tucson, Ariz.
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