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Design and Implications of a Research Mission to Planet Earth INTRODUCTION As we have learned about the other planets of the solar sys- tem, it has become more and more evident that Earth, our own planetary home, differs from other planets in several remarkable ways. The first astronauts described Earth as "the blue planet," because of the blue oceans that cover so much of its surface. It might better be called the blue and white planet in recognition of the white clouds that obscure large areas. Our blue and white Earth contrasts sharply with the red of dusty Mars, the dazzling whiteness of Venus, and the complex swirls of paste! colors that characterize Jupiter. Continued exploration has shown other fundamental differ- ences between planet Earth and all other planets of the solar system. From the human point of view, the most striking of these is that living creatures have existed on Earth for more than 3.5 billion years and have continuously evolved over these eons of time from the simplest one-celled organism to the marvelous diversity of complex life forms that exist today. In contrast, it is almost certain that life is not present today on any of our sister planets and probably never was present during the lifetime of the solar system. Because liquid water is essential for the metabolism and

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2 reproduction of living things, the survival and evolution of life on Earth is convincing evidence that our planet has always had a temperature in which water on the surface could remain mostly in liquid form. Equally fundamental has been the very existence of large quantities of water on the outer surface of Earth, quite unlike her sister planets, Mars, Venus, or Mercury. If the liquid oceans were not present, several other of Earth's unique characteristics that make life and its evolution possible could not exist. That we have relatively modest amounts of carbon dioxide in our atmo- sphere, and hence have avoided the runaway greenhouse effect that makes Venus uninhabitable, results from the fact that nearly all the carbon dioxide that has flowed out of Earth's interior during her lifetime has been buried in ocean sediments as limestone or as organic carbon produced from atmospheric carbon dioxide by photosynthesis. The presence of free oxygen would not be possible without the photodissociation of water and the consequent escape of hydrogen. Without the presence of oxygen, ultraviolet-shielding ozone would not exist in the stratosphere and life on land would be impossible. Finally, most animals could not exist, either on land or in the sea, without the energetic metabolism made possible by free oxygen. Liquid water and carbon dioxide, acting together, transform rocks by weathering into clays, which were perhaps the template for life, and into soluble substances that are among the essen- tial inorganic nutrients for plants. These include phosphorus and potassium, and many trace substances. As we think more deeply, we realize that these processes must be limited by other, more subtle effects, or else life could not per- sist. Oxygen in moderate amounts is a necessity for animal life, but in higher concentrations is a poison. If oxygen continued to accumulate in the atmosphere, fires and other kinds of rapid oxida- tion would destroy all living things. If organic matter continued to accumulate in the deposes sediments, all the nutrients released by weathering would eventually return to insoluble forms and Earth's plants would starve. Similarly, if limestone sediments continued to accumulate in the ocean without a compensating inflow of carbon dioxide from the deep-sea ridges and other volcanic eruptions, the concentration of atmospheric carbon dioxide could become so Tow that photosynthesis would be impossible. We thus come to one of the most remarkable phenomena

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3 on Earth and in the solar system- plate tectonics the contin- ual recycling of Earth's surface materials deep into the interior, and their reappearance at mid-ocean ridges and volcanoes. This process of tectonic renewal, apparently unique to Earth, may be essential to the persistence of the benign environment that has al- lowed life to exist and evolve in all its diversity for over 3.5 billion years. Motions in the deep interior drive the plates and generate the magnetic field that partially shields the Earth from the harsh environment of space. Astronomical chance forms the framework of this benign en- vironment. If the Earth were much smaller, it could not retain an atmosphere. If it were much closer to or much further from the Sun, the oceans would boil or freeze. If its orbit and axis of rotation did not fluctuate, the cyclical variations in climate that have spurred evolution would not exist. If the Sun were a binary, a subtle orbit of uniform conditions for the Earth would be very unlikely. But it is Earth's own inner life the convective processes deep within its interior, perhaps largely controlled by the flux of heat from radioactive decay and to an unknown extent by the primordial heat of agglomeration that has determined its history and our own. Why does the phenomenon of plate tectonics exist on Earth but not on Venus, a near twin of Earth? What are the charac- teristics of Earth that make plate tectonic convection possible? Is it entirely the low surface temperature that makes it possible to recycle water as well as rock? What is the nature of the convective process; how have the rates of convection varied with the general decline in energy of the system; what are the rates of interchange between parts of the mantle? What are the effects of changing rates of convection on the Earth's surface, for example on atmo- spheric carbon dioxide concentration, and hence on climate and life itself? What insights can we gain from studies of the variable magnetic field generated by Earth's core-mantle dynamo? Even the question of the origin of life may be related to plate tectonic processes. One of the most remarkable findings of recent years has been that of the existence of complex ecosystems of fishes, invertebrates, and bacteria around the deep-sea vents in the mid-ocean ridges. In these vents, water at temperatures of several hundred degrees centigrade exists in liquid form because of the high hydrostatic pressure. In this hot liquid water there is evidence that there are living anaerobic sulfide-oxidizing bacteria,

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4 which provide (together with their relatives living symbiotically within the animals) the energy and organic compounds for the large variety of animal inhabitants of the vent communities. Here is an environment that may well have been the seat of life's ori- gin on Earth, despite its inaccessibility to photosynthesis. High temperatures would have allowed rapid chemical reactions, and reduced sulfur compounds could have provided a rich source of en- ergy and, eventually, the mineral resources upon which mankind is dependent. The great mass of overlying water would have given complete protection from destructive ultraviolet radiation. Even the composition and concentration of the salts in seawater may be determined by plate tectonic processes, specifically the hydrother- mal circulation that occurs in a wide zone between the ocean and the upper lithosphere on each side of the m-ocean ridges. Another unanswered question is the nature of the Earth's re- sponse to the asteroid and comet collisions that strike the Earth at intervals. What has been their effect on the evolution of life? Some have suggested that Great extinctions" due to these colli- sions have stimulated the rapid evolution of new living forms. The search for evidence of such collisions during the geologic past may throw a new light on evolutionary processes. Our present global view of the Earth has been synthesized from decades of painfully collected regional and local data, op- erational weather satellites, and a few tantalizing pilot programs that have mapped some properties of the ocean and land surface from space. This is in contrast to our missions to other planets, which, from the beginning, provided global, integrated, and simul- taneous measurements. The Earth Is the only planet on which we can simultaneously make global satellite observations and de- ploy adequate instrumentation to image the interior, including the mantle and core, in order to address such fundamental questions as the origin of the magnetic field and the nature of the convective overturning of the Earth's mantle and crust. In this report the task group proposes a Mission to Planet Earth as an essential part of our country's program of space ex- ploration. This proposal is not as paradoxical as it sounds, for it concerns the integrity and unity of Earth as a planet, and it emphasizes the necessity for studying Earth as a whole. Only by a mission to Earth can we obtain a satisfactory degree of resolution of the changes in the earth environment over time, and therefore

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s of history. Moreover, only by studying Earth can we begin to understand the relations among several unique phenomena. The task group therefore proposes a Mission to Planet Earth to include all the program elements required to understand a planet with an atmosphere, hydrosphere, biosphere, solid) crust, mantle, and solid-liquic! core. The proposal sets forth a concerted and integrated research program on the origin, evolution, and nature of our planet and its place in the solar system. GRAND THEMES The primary research objectives are addressed by four "grand themes that are developed at length in this report: 1. To determine the composition, structure, and dynamics of the Earth's interior and crust, and to understand the processes by which the Earth evolved to its present state. The Earth's crustal surface is the home of man and the in- terface between the rapid variation in the fluid envelopes and the usually slow, sometimes catastrophic motions of the interior. The crust contains the record of past events on Earth, which is the main object of the study of geology. The Earth's mantle is undoubtedly in a state of thermal con- vection, but such important properties as its composition, the spectrum of convective scales, the degree of interaction between upper and lower mantle, and its relationship to volcanism and tectonics are imperfectly known. Considerable improvement in understanding its effects on the crust and lithosphere (e.g., earth- quakes, igneous differentiation) are attainable. Loosely coupled to the mantle is the fluid outer core, the source of the time-varying magnetic field. Closer measurement of this variation, with seisrn~c imaging of the interior and computer modeling, should constrain the nature of the geodynamo. This entire system of the Earth's interior is evolving in a general trend of decline in energy and compositional stratification, but the rate of this decline and oscil- lations about the trend are poorly known. The starting conditions for this evolution depend on the formation of the Earth and the other planets from the solar nebula and thus constitute a signifi- cantly different problem. 2. To establish and understand the structure, dynamics, and chemistry of the oceans, atmosphere, and cryosphere and their

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6 interactions with the solid Earth, including the global hydrological cycle, weather, and climate. The factors underlying the Earth's energy budget tempera- ture, precipitation patterns, sea level rise, and other properties- are not well enough known to predict climate confidently. Mea- surements of carbon dioxide and other atmospheric gases, dust and aerosols, and related phenomena, coupled with improved model- ing, are needed. Improved understanding of oceanic circulation and its effects on climate should be achievable. The land surface needs to be more systematically monitored to map and establish trends in surface composition, tectonics, soil erosion and saliniza- tion, geomorphology, vegetation (state, as well as distribution of types), hydrologic phenomena (snow cover, ground water), and other properties. Atmospheric dynamical processes, including air- sea interaction, can be better measured and modeled. The effects of biological processes on the hydrological cycle, climate dynarn~cs, and geochemistry are major problems as discussed below. 3. To characterize the interactions of living organisms among themselves and with the physical environment, including their ef- fects on the composition, dynamics, and evolution of the ocean, atmosphere, and crust. The biosphere is an important part of the fluid outer layers of the Earth, controlling the oxygen content and other factors. Waxing and waning of the biota in the ocean are major factors in its changes; this ocean ecosystem, as well as that of land, needs to be better understood, both globally and locally. Conversely, biological evolution is influenced by the physical environment in a variety of ways, including climate, continental drift, and asteroid impacts. Thus, this theme is closely tied to the one above. 4. To monitor and understand the interaction of human activ- ities with the natural environment. The impacts of population increase, agricultural and industrial development, and energy consumption on the natural environment are subjects of great scientific interest, as well as practical concern. Human activity is clearly affecting gases such as carbon dioxide and methane as well as the dust content of the atmosphere. Pop- ulation increases in developing countries are contributing to the rate of desertification and urbanization. Tropical deforestation has important implications as to climate and genetic diversity. In- dustrial effluents are blighting temperate zone forests. Conversely,

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7 many developments have made mankind more vulnerable to natu- ral hazards such as storms and earthquakes. Most of these trends are best monitored from space. Considering the themes as a whole, the task group notes that the complexity of the Earth on a global scale makes any division, such as these grand themes, to some extent arbitrary. The atmo- sphere has a time scale of hours to days for evolution of weather systems, but its climatic conditions are obviously buffered by the ocean, which has inherent time scales of months to centuries. Meanwhile, both systems are greatly influenced by the biosphere, which can undergo changes measurable from space on time scales ranging from days to decades and longer. The influences of the Sun have an established 11-year cycle, but underlying it are vari- ations and trends on century and millennium time scales. The cryosphere underwent a great decline 8,000 years ago, but the geologic record suggests a 100,000 year time scale for its waxings and wanings. There appear also to be oscillations on the scale of a few hundred years (e.g., the "little ice age" of the seventeenth century) and some thousands of years, the latter associated with orbital dynamics. The interaction of the mantle with the crust and lithosphere is believed to undergo changes in character on a tune scale of 10 million years. In tectonically active zones a given fault may have an earthquake every 200 ~ 100 years. A volcano may outburst on a similar interval, sometimes with global climatic consequences. This great welter of causative effects with di~er- ent time scales requires measurements by a variety of means, all requiring completeness, simultaneity, and continuity. MEASUREMENT STRATEGY The issues of overall measurement strategy for an earth oh serving system have been considered in detail in a number of pre- vious reports, including the two volumes of A Strategy for Earth Science from Space in the 1980's and 1990's by the Committee on Earth Sciences (CES) of the Space Science Board (1982, 1985) and the report Oceanography from Space A Research Strategy for the Decade 1985-1995 by Joint Oceanographic Institutions (1984~. These reports note, and this task group agrees, that three basic themes are fundamental to advances in our understanding of the causes and effects of global change. The measurements must be

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8 global and synoptic, they must be carried out over the long term, and different processes such as solar output, winds, currents, and geological and biological activity must be measured simultaneously (to a degree dependent on the rates involved). The first theme Is the global and synoptic nature of measurements. We have learned that advances in earth science derive from the synthesis of new ideas that come from global synoptic observations. For example, we owe our new understanding of plate tectonics or large-scale at- mospheric circulation to global observations and models. In each case, the global observations are a synoptic view, that is, a snap- shot that gives a picture of the system over a period that Is short compared to the time over which the system changes. For exam- ple, the relevant time scales of the ocean months to decades fall between geologic periods and atmospheric weather events. The second theme of the measurement strategy is long-term continuity. The Earth as a system is energetic on many scales, from microseisms to interannual E} Ninos to ice ages. However, even if we restrict ourselves to decadal time changes the human time scale statistics dictate that measurements over many years will be required before we can make accurate statements about the energetics of the system. As we look into the future, we see the need for geodynamic, climate, and biosphere measurements for decades, and we can see that the addition of gravity and magnetic fields, continental drift, and solar-terrestrial interactions to these processes extends the necessary observational time periods to centuries. However, to understand the system we need more than long-term measure- ments. We must at the same time make clear the fundamental processes at work. The results from these process studies will be used to help put the whole picture together. Unfortunately, the time periods for the required observations are much longer than the time scales for political decision-making. Thus, there is a need for a national commitment to carrying measurements through the necessary time periods. The third theme is simultaneity that is, observations of dif- ferent kinds of processes at the same time. This includes study of the land, ocean, atmosphere, and biota as an interactive system on a global scale. Earth sciences have tended to treat these compo- nents as separate disciplines. Much is known about, for example, the processes that govern winds and temperature in the atmo- sphere, geological processes, or the chemistry of trace substances

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9 in the ocean. In the last few decades, however, increasing atten- tion has become focused on questions that transcend traditional disciplinary boundaries and require, in addition, an understanding of the complex linkages and feedbacks between these components. As is noted in the CES reports, the changes to be expected from the worldwide deforestation and consumption of fossil fuels, from increased erosion of continents, from the sensitivity of the strato- spheric ozone to trace gases such as chIorofluorocarbons, and from the causes of past and present extinctions of whole classes of liv- ing species are all questions of fundamental importance. They can only be addressed from a global, interdisciplinary perspective, drawing on a wide spectrum of observations and skills beyond the range of any one individual, institution, or agency. Observing and understanding such a complex system is a basic intellectual challenge. Yet these questions and others like them are also issues that have to be resolved if we are to predict the future, or even diagnose correctly the changes that are under way around us. Some of them are results of activities of humankind, others are natural fluctuations for which precedents are undocumented or simply unrecognized. The planet Earth is our environment and, for better or for worse, we are part of that environment, reacting to it and acting upon it in ways that are far-reaching but as yet barely perceived. It behooves us to pay attention. Obtaining such an understanding requires long-term study. In particular, the interactions between the parts are crucial. Bound- ary layers and boundary phenomena are of special interest, since this is where such interactions take place. This implies building an information base including sustained global observations, and evolving quantitative models to be used for examining responses and feedbacks as well as predicting the behavior of the whole. The required information base is large and diverse. Some aspects, such as long records of surface temperature at land stations in northern temperate latitudes, already exist, though not necessarily in a form that is readily accessible; for others, modifications of existing data sources are appropriate. However, for some aspects, particularly those providing global coverage, the deployment of new observa- tional systems is required. The coverage and uniform data quality obtainable imply a critical role for satellite-based remote sensing, but extensive in situ measurements are also needed, distributed in key locations around the globe.

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10 SYSTEMS OVERVIEW The task group's specific recommendations are for implemen- tation of a system for observing the Earth that builds on and expands the Earth Observing System (EOS) that is currently planned to fly as part of the Space Station complex in the m~- 1990s. EOS in turn will build on predecessor missions, both do- mestic and foreign, which will help define the specific parameters and orbits needed for adequate long-term monitoring of Earth. EOS will be the first phase in the development of long-term satel- lite measurement systems. Here the task group looks beyond the initial deployment of EOS to lay out a series of specific recom- mendations as to the overall structure and programmatic content of a long-term global mapping and monitoring system for Earth, including satellite and in situ systems. 1. A Satellite-based Observing System The task group recommends that the centerpiece of the global observing system be a network of satellites and platforms in the following arrangement: ~ A set of five geostationary satellites, designed to carry a wide variety of instruments to cover the entire Earth for long-term measurements. Five are required to cover the Earth completely to 60 latitude north and south. These satellites would be large, high-power spacecraft, designed to provide continuous measure- ments of every part of the Earth visible from geostationary orbit. They would carry improved versions of instruments currently used or being developed, plus new technology that would expand our view in space, time, and the energy spectrum (e.g., side-Iooking imagery) and would be used together with: . A set of two to six polar-orbiting platforms to cover the polar areas, above latitude 60, and to provide platforms for a variety of instruments that must be closer to the Earth. These polar-orbiting platforms would operate continuously at altitudes of about 824 km, being replaced as necessary, and would carry a wide spectrum of instruments together with common power sum plies, data handling, and communications. The number of polar- orbiting platforms is based on a compromise between temporal and spatial coverage and cost; two is the minimum to achieve biweekly coverage of the processes believed significant to cause

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11 global change (e.g., 200-km ocean eddies); six would be required for daily coverage. The data from the instruments on the polar platforms and the geostationary satellites would thus yield the re- quired global synoptic data, which when appropriately processed would provide the fundamental long-term data set required for monitoring global change. However, these two sets of spacecraft must be augmented by: A series of special missions, which require other orbits. Some would be short-term duration (4 days (Shuttle) to 1 to 3 years (Explorer-type missions)~; others would be essentially permanent, such as a second-generation Global Positioning Sys- tem (GPS) constellation. Some of the short-term missions would test instruments and concepts for incorporation into the long- term satellite network discussed above; others would undertake one-time mappings such as improved geological spectrometry and high-resolution terrain mapping. 2. Complementary In Situ Observing Systems The task group recommends the development, deployment, and [ong-teTm operation of a system of in situ measuring devices the Permanent Large Array of Terrestrial Observatories (PLATO}- to prov']e-complementary data to the space network. Wherever applicable, the data should be transmitted in real time and integrated with observations from space. In situ measurements represent an essential element of any oh serving system designed to investigate the Earth as a planet. There is a need to measure effects that cannot be detected through re- mote sensing from space, to provide increased resolution in regional studies, and to supply calibration and verification of space obser- vations. In situ measurements from PLATO would include, for example, detailed studies of terrestrial and oceanic biomes, ocean bottom stations designed to monitor pressure, seismic and acoustic signals, a variety of probes ranging from balloons to boreholes, and GPS receivers and laser corner reflectors for monitoring tectonic deformation. The number of sites for these instruments would range from 100s to 10,000s; their distribution typically would be nonuniform in accordance with the problem under study.

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12 3. Modeling The task group recommends that state-of-the-art computing technology be utilized for data analyms and theoretical modeling of earth processes. We are dealing with a complex, turbulent system of living and nonliving material on many scales, involving a wide variety of substances and properties, and ranging from the core to the outer atmosphere. Modeling the system requires the best data sets possible, the fastest computers, and ~rnaginative ideas from researchers. IN turn, this modeling can give a context and direction for future observations. The task group sees modeling as an integral part of this enterprise, and the need for the state-of- the-art technology in modeling as critical. 4. Data System The task group recommends that a full and coordinated data system, which both archives and disseminates data, be established. The task group does not see the necessity for a central archiving of all data, but does see the need for a central data authority to establish formats and other conventions, to identify data location, and to arrange for access to all data as required. We can expect that data rates will be very high, on the order of 10~3 to 10~4 bits per day. This will require much selective averaging and heavy use of new technology, possibly beyond video disks to new storage and retrieval technologies. Automation of some phases of the selection and averaging process will be necessary. In short, state- of-the-art technology should be made available for all phases of data handling. A more detailed description of the full Mission to Planet Earth system is provided in Chapter 5. STRATEGY OVERVIEW To be effective, an earth research system must be conceived and evaluated as an integrated whole, including contributions from other nations. Consequently, a major effort to achieve these objec- tives is timely. As world population and economic aspirations press harder on finite resources, the need for improved understanding grows year by year, and environmental problems are transformed into ever more prominent social and political problems, while be- coming less amenable to successful solutions.

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13 During the next decade a number of new programmatic initia- tives will pace the progress in earth sciences. The World Climate Research Program (WCRP), currently in operation, addresses the physical basis for regional and global climate variability from a few weeks to several years. For the study of the solid Earth, the International Lithosphere Project (ALP) is a coordinated, ongoing international activity to improve our understanding of the struc- ture and dynamics of the outer layers of the Earth. In addition, the Ocean Drilling Program probes the Earth's crust beneath the sea. Plans are being developed for a Global Seismic Net- work, which will investigate the three-dimensional structure of the crust, mantle, and core. The International Geosphere-Biosphere Program (IGBP), now in the planning stages, will study global change with a focus on the interactive ocean-atmosphere-land- biota system on a time scale of decades to centuries. The success of these ongoing and planned programs is keyed to our ability to obtain accurate global and repetitive data on certain geophysical parameters from satellites, and to complement the space data with reliable surface-based observations, as well as to develop realistic, interactive models that can be tested by diagnostic measurements. Towards 1995, as the results from these identified programs begin to reach fruition, we will be getting ready to address one of the most fundamental questions in earth sciences; namely, how does the planet Earth as a whole evolve and what path has it followed to reach its present state of evolution? It is clear that in order to succeed we will have to assure that research programs for understanding the evolution of atmosphere, oceans, land, and interior of the Earth are all proceeding in conjunction. We must also assure that satellite and earth-based observing systems put in place for one discipline of earth sciences are coordinated with those developed for the other, so that common observatories for interdisciplinary measurements are the rule rather than the excel tion. It is for this reason that the task group is proposing a Mission to Planet Earth that will: address the questions that concern the integrated function- ing of Earth as a system; . carry out measurements from space in concert with data from ground-based techniques;

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14 ~ be capable of observing the composition of the atmosphere; the composition, structure, and texture of land surfaces; the biol- ogy of the land and oceans; the distribution of land and sea ice; and the structure and dynamics of the interior of the Earth; provide lon~-term. consistent data records of the dynarn~cs of climate, lithospheric plates, biosphere, and the oceans over periods of decades or more; be compatible with our needs for monitoring certain phe- nomena with long time scales (e.g., ocean temperatures), and performing repeat measurements over certain areas for assessing changes because of catastrophic phenomena (e.g., earthquakes), and at the same time be flexible enough to be able to observe targets of opportunity (e.g., a sudden volcanic explosion); ~ assure the compatibility and continuity of the current oh erational and research satellite observing systems; . develop new instruments in response to measurement needs for parameters that are not currently measurable from space or from the ground; and . guarantee the uniformity of the data acquisition and archiv- ing system so as to facilitate their use by the research community, and ensure that future improvements in data processing technol- ogy can be applied with a minimum impact on the continuity of the data streams. From this description it should be evident that the solution of the major problems in earth science requires an integrated ap- proach. The key to progress on interdisciplinary issues in earth science during the decade of the 1990s and beyond will be ad- dressing questions that concern the functioning of the Earth as a system. The overall strategy therefore contains important prerequi- sites for U.S. science policy from both domestic and international perspectives. These are: A lone-term commitment to a vigorous. systematic ex~to- ration of the Earth. , ~ Maintenance of an open-skies policy. ~ International cooperation and coordination regarding at! rel- evant systems, beginning with the early stages of program planning. ~ Full coordination of the U.S. federal agencies involved in the civil earth science effort at the programmatic and budgetary levels.

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15 . Development and support of a more comprehensive program in the solid earth sciences within NASA. These science policy issues are addressed in more detail in Chapter 6. Finally, the strategies of the Space Science Board's Committee on Earth Sciences (CES) have emphasizer} that global earth science investigations from space will naturally be followed by global exploration for resources ant] by development of natural- hazard warning systems. Of course, all such applications must be based on thorough scientific understanding. The task group hopes that the scientific return derived from the strategy contained in the CES reports and in this report will provide a sufficient foundation for all applications.