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NASA's Plans for Post-2002 Earth Observing Missions A letter report sent by the Task Group on Assessment of NASA Plans for Post-2000 Earth Observing Missions to Dr. Ghassem Asrar, Associate Administrator for NASA's Office of Earth Science (April 8, 1999). Task Group on Assessment of NASA Plans for Post 2000 Earth Observing Missions Board on Atmospheric Sciences and Climate Commission on Geosciences, Environment, and Resources Board on Sustainable Development Policy Division Space Studies Board Commission on Physical Sciences, Mathematics, and Applications CONTENTS • Transmittal Letter to Dr. Asrar • Letter Report • Appendix A: Task Group to Review NASA's Plans for Post-2000 Earth Observing Missions [membership] • Appendix B: Request for Study • Appendix C: Earth Science Enterprise Mission Scenario for the Post-2002 Period • Appendix D: Task Group Meeting Agenda • Board Membership

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NASA's Plans for Post-2002 Earth Observing Missions April 8, 1999 Dr. Ghassem Asrar Associate Administrator for Earth Science NASA Headquarters 300 E Street, SW Washington, DC 20546 Dear Dr. Asrar: We are pleased to provide you with the report of the Task Group on Assessment of NASA Plans for Post-2002 Earth Observing Missions. The report was prepared in response to your request for a National Research Council assessment of NASA's candidate mission profile for the period 2003 to 2010 from the perspective of overall scientific priorities, program coherence, and scientific balance. In conducting this analysis, the NRC drew upon the expertise, members, and past studies of the Board on Atmospheric Sciences and Climate, the Board on Sustainable Development, and the Space Studies Board. The report is based on information provided by your office and representatives of other U.S. Global Change Research Program agencies, review of a number of recent relevant reports from the three boards, and deliberations by members of the task group during and following its meeting on February 10-11, 1999. The task group report, like the candidate mission profile, was developed under a very rapid timetable. Therefore, the report should be viewed as a starting point for a number of actions that are recommended for follow-up rather than as the final word on post-2002 plans. The Task Group is hopeful that taking the steps outlined in the report will result in Earth science research by NASA, in collaboration with other partners, that is of the highest caliber and is capable of supporting the crucial environmental decisions that face our nation and the world. On behalf of the task group and its related NRC boards, we would be pleased to assist in any way we can. Sincerely, Marvin A. Geller, Chair, Task Group on Assessment of NASA Plans for Post 2000 Earth Observing Missions Eric J. Barron and James R. Mahoney, Co-chairs, Board on Atmospheric Sciences and Climate Edwin A. Frieman, Chair, Board on Sustainable Development Claude R. Canizares, Chair, Space Studies Board Feedback

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NASA's Plans for Post-2002 Earth Observing Missions I. INTRODUCTION The Task Group on Assessment of NASA's Plans for Post-2002 Earth Observing Missions (Appendix A) was formed in response to a request from NASA's Office of Earth Science (Appendix B). The Associate Administrator for the Office of Earth Science (OES) requested a fast-track review of NASA's proposed mission scenario (Appendix C) for Earth observing missions during the period from 2003 to 2010. Within the National Research Council (NRC), the study was organized by the Board on Atmospheric Sciences and Climate, the Board on Sustainable Development, and the Space Studies Board, thereby providing a direct link through membership with the NRC units that had published reports with particular relevance to planning for post-2002 Earth observing missions. Background materials were distributed in advance to the task group; however, given NASA's deadline for completion of work, the task group could meet only once, on February 10-11, 1999. On February 10, the task group held briefings with representatives from NASA; NOAA and the NPOESS (National Polar-orbiting Operational Environmental Satellite System) Integrated Program Office; the U.S. Global Change Research Program; the Office of Management and Budget; and the White House Office of Science and Technology Policy. The task group also held discussions with the chairs of three recent NRC studies pertinent to the current assessment and with several of the authors of the Easton workshop report1 that evaluated NASA's post-2002 mission scenario (Appendix D shows the meeting agenda and lists all presenters). Task group deliberations began on February 11 and continued informally via e-mail and telephone. According to NASA officials, the rapid timetable for completion of the task group's work was necessary to provide guidance on upcoming budget submissions, technology development efforts for post- 2002 missions, and potential negotiations with international partners. The task group notes, and NASA officials acknowledged, the obvious limitations imposed by the rapid timetable for completion of the study. As a result, the task group regards the assessments in this report as preliminary and recommends a number of essential follow-up assessment activities. The task group's charge (Appendix B) included consideration of the following topics: 1. The extent to which the mission set contributes to a coherent overall program that addresses important science themes and priorities, 2. The responsiveness of the missions to scientific priorities identified in recent relevant NRC reports, 3. Broad aspects of balance between various Earth science discipline areas, 4. General technical and programmatic feasibility, 5. Identification of major scientific or technical problems implicit in the mission scenario, and 6. Evaluation of the efficacy of the process employed by NASA to solicit ideas and to distill them to frame the proposed mission set. NASA's post-2002 mission plans were an output of the Easton workshop process. As discussed in the report of the workshop, this process began with an RFI (Request for Information). Through the RFI, NASA informed potential respondents of its intention to promote a program of smaller satellite missions with shorter implementation times from inception to launch in order to respond more quickly to new research priorities and to reduce the risk to program objectives from any single mission failure. One hundred responses were received, roughly half from NASA centers. Six disciplinary panels covering complementary domains of Earth system science reviewed the submissions and integrated them into 23 mission concepts. These were

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analyzed by NASA technical staff and an industrial contractor, and estimated implementation costs were developed. NASA convened a workshop involving 150 participants in late August 1998 in Easton, Maryland, to review and amend the mission scenario. At the Easton workshop, the responses to the RFI were reviewed by both disciplinary and interdisciplinary panels. Prior to the workshop three categories of NASA Earth-observing missions were defined: EOS follow-on missions for systematic measurements of critical parameters, Earth Probe missions for exploratory research or focused process studies, and Pre-operational instrument development to provide new or more capable sensors for operational observing systems. The RFI was circulated by NASA to the community with a 6-week response deadline. The Easton workshop was held 10 weeks later, using the RFI responses as a significant input. The report of the Easton workshop was completed approximately 2 months later. To formulate complex program plans on such a short time scale (see Box 1), NASA necessarily built on the very extensive heritage of NRC and NASA studies and reports, as well as 10 years of EOS Science Team operations. BOX 1 NASA and NRC Milestones Relevant to Post-2002 Mission Planning NASA RFI Announced April 10, 1998 NRC Pathways Report Overview Volume, with May 19, 1998 Recommendations and Research Imperatives, Published NRC "NPOESS and Climate Change" Letter Report2 May 27, 1998 Published NASA Deadline for Submission of Post-2002 Era June 8, 1998 EOS Mission Concepts NASA Panel Review of Submissions Mid-June 1998 August 24-26, NASA Workshop at Easton, Maryland 1998 Early Easton Workshop Results Available for Program September Formulation 1998 NRC Report, The Atmospheric Sciences Entering the October 22, Twenty-First Century, Published 1998 Easton Workshop Report ("Kennel Report") November 12, Published 1998 November 13, NRC Pathways Full Report Prepublication Release 1998

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February 10- NRC Post-2002 Task Group Meets 11, 1999 NRC Report, Adequacy of Climate Observing February 26, Systems, Published 1999 NRC Post-2002 Letter Report Target Date for March 15, 1999 Publication NASA, the USGCRP, and the NRC Pathways Report The task group's evaluation of the process and outcomes of the Easton workshop relies heavily on the recent NRC publication, Global Environmental Change: Research Pathways for the Next Decade.3 This approach is consistent with NASA's intent to rely on Pathways for guidance during the Easton process; it also conforms to the charge for this review. It is important to recognize, however, that the sponsor and audience for the Pathways report are broader than NASA. Indeed, Pathways provides a comprehensive review and scientific framework for future directions in the U.S. Global Change Research Program (USGCRP). As discussed below, some of what the task group perceives as shortcomings in the Easton process are, in fact, reflections of larger problems within the USGCRP. The USGCRP was established in 1989 and codified by Congress "to provide for development and coordination of a comprehensive and integrated U.S. research program which will assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change."4 This effort requires planning and coordinating research and policy development interests of several U.S. government departments and agencies, including the Executive Offices of the President.5 Thus, the USGCRP provides a mechanism for obtaining the necessary scientific knowledge to document global change phenomena and enabling informed decision making on potential response strategies. These responses include such international agreements as the Montreal Protocol and the Framework Convention on Climate Change. The importance of NASA's role in the USGCRP cannot be overstated. For example, NASA's role in understanding the causes of global and polar stratospheric ozone depletion stands as one of the outstanding scientific accomplishments of the last two decades. In addition, the agency's development and implementation of satellite altimetry and scatterometry have made today's approach to global oceanography possible. Overall, NASA accounts for nearly 75 percent of the resources made available under the USGCRP, with some 60 percent devoted to space- based observation programs.6 Thus, NASA's directions in Earth science during the first part of the next century will be pivotal in determining the success of the USGCRP and in international global change programs such as the World Climate Research Programme, the International Geosphere Biosphere Programme, and the International Human Dimensions of Global Environmental Change Programme. II. SUMMARY OF KEY FINDINGS AND RECOMMEDATIONS Although the results of the initial planning phase have merit, the development of a coherent overall EOS program depends on the development of a fully integrated science plan. To ensure a balanced and coherent strategy that will elucidate the key mechanisms thought to underlie global change phenomena, the task group recommends that NASA

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develop the science plan with the participation of USGCRP agencies and the academic scientific community and in consultation with international partners. While the task group believes that NASA's plan for the post-2002 missions tries to be responsive to broad aspects of Pathways, the NASA planning effort needs to be refocused on addressing the major unanswered scientific questions specified in Pathways. The task group believes that the results of the Easton workshop gave a mission set that is balanced among the disciplines; however, it has concerns regarding other, more important aspects of balance, especially the balance between space-based and in situ observations. The vital role of research and analysis (R&A) programs in developing an effective program of research must also be acknowledged, and specific plans for linking R&A programs with post-2002 mission plans should be developed. The task group agrees with NASA that a successful transition of certain EOS observations to NPOESS would realize many benefits. It would be premature, however, to place sole reliance on this strategy for key global change time series. The task group also notes that sole reliance on NPOESS for crucial global change science time series would preclude achieving the objectives, noted in Pathways and endorsed by NASA, of developing principal investigator-led, technologically agile, missions. No federal entity is currently the "agent" for climate or longer-term observations and analyses, nor has the "virtual agency" envisioned in the USGCRP succeeded in this function. The task group endorses NASA's call for a high-level process to develop a national policy to ensure that the long-term continuity and quality of key data sets required for global change research are not compromised in the process of merging research and operational data sets. The task group recommends that NASA establish a broadly based Science Integration Team charged with developing the requirements for data integration and for reviewing NASA's plans for sensor design, data acquisition, and data management to determine if they are consistent with expected scientific uses of the data. This Science Integration Team should build upon the science plan that is to be developed (see first recommendation above). Constrained by a tight publication deadline and the absence of a detailed post-2002 science plan, the task group was unable to conduct a thorough analysis of the data set characteristics to be acquired (as opposed to the variables to be measured ) in NASA's mission scenario. III. TASK GROUP ASSESSMENTS 1. The extent to which the mission set contributes to a coherent overall program that addresses important science themes and priorities. NASA has identified five science thematic areas that reflect some of the scientific directions identified in the NRC Pathways and The Atmospheric Sciences Entering the Twenty-First Century7 reports, and that draw on the existing EOS science plan.8 The themes identified by NASA are climate change and variability, the global carbon cycle, the global water cycle, atmospheric composition and ozone, and solid Earth and natural hazards. The proposed mission set tries to address the science priorities in NASA's themes, taking into account the scientific

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imperatives in Pathways and other the recent NRC reports. The experiments proposed will make major contributions to key science and applications problems such as: • Climate change detection and attribution; • Marine and terrestrial carbon sources and sinks; • Seasonal to interannual climate variability; • Changes to atmospheric ozone; • Water resources, flood hazards, and severe storm impacts; and • Ecosystem management, deforestation, and agriculture. However, embedded in the mission set are issues such as data continuity, instrument calibration and validation, and data simultaneity. The task group could not determine if such issues had been adequately considered. The proposed mission set has a strong heritage in previous EOS missions, and several important new records will also be initiated, including global precipitation, ocean circulation and terrestrial hydrology (estimated from gravity measurements), and biomass in regrowing forests. These missions have the potential to yield entirely new information on Earth system components. Clearly, however, a more deliberate planning process is needed to identify data gaps and scientific opportunities that remain after this initial planning stage. Plans for continuation of important data records begun in the first series of EOS missions need a rigorous assessment to ensure that the strategy will meet long-term continuity requirements for Earth science. Quite possibly, major changes in the mission plans will be needed as a consequence of this next stage of planning. Furthermore, new measurements of immense scientific value may be possible in the coming decade, including, for example, additional tropospheric species, three-dimensional winds, and CO2 vertical profiles. These and other opportunities would complement the existing program. The development of a coherent overall EOS program depends on the development of a fully integrated science plan. This plan must identify or address: 1. Instrument synergism (i.e., which experiments should overlap in the same time period); 2. NASA's contributions to the major scientific questions and priorities outlined by Pathways and other recent NRC reports (those listed in Box 1); 3. The interagency partnerships and collaborations necessary to address these issues and priorities, including the balance between in situ and satellite-based measurements; 4. The role of international partners; 5. An assessment of the overall characteristics of the data sets and their suitability for addressing key scientific questions and priorities; 6. Data management; 7. The role of field studies, data assimilation, and modeling studies; 8. Balance and integration of long-term, consistent observations and exploratory efforts; and 9. Potential uses of observations and data in applications. This science plan is crucial for mission planning and for gaining strong support from the Earth sciences community. NASA expects to complete its science plan for the post-2002 missions by September 1999.9 The integration of the missions into a consistent set has to be addressed in the science plan, which then would also set conditions for the time line of the missions, data management, and modeling. Timing will be crucial for those missions where achieving maximum science benefit requires that observations from different missions be combined. The task group expects the science plan to draw from the EOS science plan, address the detailed recommendations given in Pathways and other recent NRC reports, and take note of Decision 14 at the COP4 of UNFCCC in Buenos Aires.10 The task group recommends that the science plan to

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underpin the mission set be developed in an open and deliberative process involving the full range of scientific disciplines and a diverse set of potential users. To ensure a balanced and coherent strategy that will elucidate the key mechanisms thought to underlie global change phenomena, the task group recommends that NASA develop the science plan with the participation of USGCRP agencies and the academic scientific community and in consultation with international partners. In addition to experts in the various disciplines, NASA should involve scientists who understand the human role and the socioeconomic and health impacts of the designated priority science and applications problems. Certainly, a very significant part of the science plan should address how the individual mission data streams will be merged and how modeling and assimilation systems will be applied. NASA's observation strategy must be tied to a data management strategy if the scientific goals of the EOS program are to be achieved. NASA's plan to rely on its Federation concept for data production and management coupled with the planned change toward greater emphasis on a PI (principal investigator) mode of operation raises a number of issues related to how a long-term data record, and concurrent calibration and instrument performance metadata, will be guaranteed. While this letter report focuses on the transition to the "NPOESS era," the problem of how to ensure data continuity is broader and includes, for example, the issue of how to introduce innovations into the data management system while maintaining continuous records. 2. The responsiveness of the missions to scientific priorities identified in recent relevant NRC reports. The task group reviewed NASA's plan for post-2002 missions based on provided text material, presentations by NASA and NPOESS personnel, and with reference to the recent NRC reports noted below, especially the Pathways report: • "On Climate Change Research Measurements from NPOESS," letter, May 1998; • The Atmospheric Sciences Entering the Twenty-First Century, October 1998; • Global Environmental Change: Research Pathways for the Next Decade, prepublication copy, November 1999 (the Pathways report); and • Adequacy of Climate Observing Systems, February 1999. The authors of NASA's Report of the Workshop on NASA Earth Science Enterprise Post- 2002 Missions (the Easton workshop report, also known as the Kennel report) found "the 1995 La Jolla review and this 1998 [RFI to Easton] process and workshop responsive to the National Academy of Sciences' Pathways report" (p. 25). Indeed, the task group concurs that the RFI and the outcome of the Easton workshop were consistent with important elements of Pathways. Specifically, the RFI notified respondents of NASA's intent to promote a program of smaller satellite missions with a shorter implementation time; NASA has stated that post-2002 mission development and selection will be science-driven; NASA intends to emphasize PI-led missions in its post-2002 planning; post-2002 mission scenarios have a "systematic" measurement component for acquisition of long time series; and NASA has several programs to infuse new technology into Earth observation programs. However, the task group finds that there is much more work to be done for NASA to be responsive to the full set of standards set by the Pathways report, both in planning and in implementing the Pathways recommendations. For example, the Pathways report advocates a USGCRP scientific strategy-including supporting observational, data management, and analysis activities-that is: 1. Agile-to enable timely response to technological changes or to changing research priorities; 2. Focused-to enable progress on answering specific, central scientific questions about global change phenomena; and

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3. Coherent-to enable a balanced (e.g., space-based and in situ) and integrated, interagency response to global change issues. The task group believes that the Easton process mostly addresses only the first element above (via its call for "agility, responsiveness and a PI mode of operation"). Deficiencies in the latter two elements can be traced to the rapidity of the Easton process, the absence of a completed science plan, and the need for further integration with the USGCRP. Although the full Pathways report, with detailed information on research and observations in each thematic area, was not available at the time of the Easton workshop, the published Overview volume presented the full set of Scientific Questions from which it should have been possible to elaborate a "focused" and "coherent" effort. The task group believes that the Easton process was hampered by its abbreviated timetable. NASA intended its solicitation to reach the broad scientific community; the task group fully supports this strategy. However, the rapidity of the process-especially the initial 6-week phase- may not have facilitated the desired response. The task group notes that fully 50 percent of the RFI responses were from NASA centers. Nevertheless, it also notes that some very exciting proposals emerged from the RFI. With careful structure, an earlier announcement, and a longer period for community response to the RFI, an improved solicitation and planning cycle should be achievable within an approximately 1-year period and is recommended for the future. The Pathways report outlines a research framework across the wide scope of global environmental changes in terms of the following primary topical areas: • Changes in the Biology and Biogeochemistry of Ecosystems, • Change in the Climate System on Seasonal to Interannual Timescales (S-I), • Changes in the Climate System on Decadal to Century Timescales (Dec-Cen), • Changes in the Chemistry of the Atmosphere, • Paleoclimate, and • Human Dimensions of Global Environmental Change. The discussion of each of these six primary topical areas is structured in terms of Research Imperatives-central issues posed to the corresponding scientific community by the challenge of global environmental change. Each research imperative is addressed by a set of Scientific Questions posed at a level of detail from which an observational program, space-based and in situ, can be defined, refined, and realized. The NASA themes do not directly correspond to the Pathways themes, and the specific questions discussed in Pathways were not the basis of a rigorous evaluation of the proposed missions during the Easton process. More importantly, Pathways calls for an integrated and balanced program of in situ and space-based measurements together with modeling, theory, and process studies. Noting the USGCRP's central contributions to science-driven programs, Pathways also includes recommendations related to enhancing the research and analysis (R&A) component of a restructured national strategy for Earth observations.11 The Easton process was a NASA-sponsored exercise that could not address some of these important issues and did not address others. In fact, the creation of a fully integrated program, as called for in Pathways, represents a major challenge to all of the USGCRP agencies and their scientific partners. Perhaps the most serious deficiency in NASA's post-2002 mission scenario relates to missions intended to support research on long-term processes in the Earth system. The global change program is fundamentally a research program on how Earth may change in the

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future on time scales of years to decades and longer. Historically, NASA has seen its role as an agent to develop research instrumentation that can become operational to permit agencies such as NOAA to perform such "monitoring" missions. This sets up a fundamental conflict with Pathways. The research challenges in Dec-Cen, S-I, Ecosystems, and Atmospheric Chemical Change require measurements on the time scale of the relevant processes and long-term consistency. This may require observations over several ENSO (El Niño, Southern Oscillation) cycles, over the disturbance and regrowth cycle in forests, over an extended period to examine changes in the ice caps, or over the time period for the stratosphere to evolve as ozone-depleting compounds decay. These are central science questions for global change research in frontier areas, but it is not at all clear that they can be readily transferred to operational settings without diminished standards for calibration, stability, and continuity. Nor is it clear that researchers are yet making the correct measurements. There must be room allowed for mission concepts that preserve continuity for time series while enhancing the quality of the measurement or greatly reducing its cost through technological improvement. Interagency collaboration is essential, and there must be a rigorous process for transferring responsibilities to operational missions-one that also ensures the continuity of measurements required to address critical questions whose answer is not amenable to this mode of operation. Indeed the distinction between long-term science (the study of processes that occur on long time scales) and monitoring (the routine observation of processes for operational forecasting, early warnings, or management) must be made crystal clear. The task group concurred with the conclusion of the Easton report that when considering NASA's new approach to mission planning and implementation, "the single most critical concern is the lack of a national policy to address long-term measurements to meet known national and international needs" (p. 26). The task group was made aware of NASA's call for a high-level process to develop a national policy to ensure that the long-term continuity and quality of key data sets required for global change research are not compromised in the process of merging research and operational data sets.12 Such a process is needed to address the task group's concerns regarding continuity and integrity of certain long-term measurements. Neither NASA nor any other single agency can develop such a policy on its own; it will necessarily involve examination of the missions and responsibilities of a number of federal agencies. The task group believes that a strategy of migration to NPOESS simply on the grounds of the length of measurement time is both ill-advised and in conflict with the recommendations in the Pathways report. Indeed, as discussed in item 5 below, the task group believes that far greater effort in leadership and planning is necessary to ensure continuity in the transition of research to operations. 3. Broad aspects of balance between various Earth science discipline areas. The need for discipline balance derives from the fact that many important problems in the Earth sciences involve an interplay among individual disciplines. A familiar example is understanding ENSO phenomena. ENSO involves a joint oscillation of the atmosphere and ocean; therefore, any sensible study approach must include observation and modeling of both the atmosphere and ocean. It is becoming increasingly clear that many, if not most, Earth science problems require an interdisciplinary approach for their understanding and prediction. Currently, some of the most interesting scientific problems occur at disciplinary interfaces. There also needs to be a balanced approach to assessing and integrating the diverse observations in NASA's suggested mission set. A data system is required that facilitates the merging of diverse data sets for interdisciplinary science. There needs to be a balance between mission measurement, modeling, and data analysis activities for the solution of problems. Without such balance, progress will not be possible on many of the cross-cutting scientific themes. There should also be a balance between research and applications in the design of programs. Some of the same data streams will be used for both scientific research and societal applications.

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For example, climate information on storm frequency is used to validate the output of climate models, but it is also useful for assessing logical rate structures for the insurance industry. Similarly, data on the productivity of marine and terrestrial systems, which is critical for improving our understanding of the carbon cycle, can also be used in developing early warning systems for conditions likely to give rise to public health problems, for example, cholera and famine. Without a balance between research and applications, realization of potential benefits such as these will not be possible. There also needs to be a balance between conservatism and innovation. Conservatism is needed to give confidence that a long-term data set will be acquired, but innovation is needed to design new observational systems that will obtain previously unavailable data, obtain data that may have higher quality or accuracy, and/or acquire data at less cost in the future. The task group believes that the results of the Easton workshop gave a mission set that is balanced among the disciplines; however, it has concerns regarding other, more important aspects of balance, especially the balance between space-based and in situ observations. Another critical aspect of achieving program balance is the role of research and analysis programs. The vital role of research and analysis (R&A) programs in developing an effective program of research must also be acknowledged, and specific plans for linking R&A programs with post-2002 mission plans should be developed (see Box 2). BOX 2 Supporting Research and Data Analysis in NASA's Science Programs Principles for Strategic Planning Finding: The [R&A] task group finds that R&DA is not always thoroughly and explicitly integrated into the NASA enterprise strategic plans and that not all decisions about the direction of R&DA are made with a view toward achieving the goals of the strategies. The task group examined the trend and balance of R&DA budgets and found alarming results; it questions whether these results are what NASA intends. Recommendation 1: The task group recommends that each science program office at NASA do the following: - Regularly evaluate the impact of R&DA on progress toward the goals of the strategic plans. - Link NASA research announcements (NRAs) to addressing key scientific questions that can be related to the goals of these strategic plans. - Regularly evaluate the balance between the funding allocations for flight programs and the R&DA required to support those programs (e.g., assess whether the current program can support R&DA for the International Space Station). - Regularly evaluate the balance among various subelements of the R&DA program (e.g., theoretical investigations; new instrument

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the highest potential for discovery. This scientific judgment is reflected by the scientific priority given to a number of promising measurement concepts in low earth orbit. The scientific discovery potential of global tropospheric chemistry justifies at least one and ideally two experimental missions during the period of reference. Each would a one-time mission, carrying a payload limited to a small number of sensors (to be determined by the assessment of competing research mission proposals). The instrument payload could include passive and active sensors (such as tunable differential absorption lidars) to observe ozone, CO and precursor species, or pollutant emitted by surface sources (SO2, hydrocarbons, etc.). EX-2: Aerosol Radiative Forcing Research Mission A high visibility issue in climate change research is the impact of natural and anthropogenic aerosols on the radiative balance of the planet. One possible strategy for investigating this problem is based on monitoring trends in the global distribution of stratospheric and tropospheric aerosols. Two candidate systematic observation missions listed in Appendix 1 address this objective (measurements of solar occultation by stratospheric aerosol and solar radiation backscatter by tropospheric aerosol). Nevertheless, the diversity of aerosol origin, composition and optical properties, and the complexity of radiation scattering and absorption by aerosol and ice/water particles are so overwhelming that conclusive findings can only be expected from considerably more sophisticated and penetrating observations. It is essential, in particular, to resolve the vertical layering of aerosol distribution in order to backtrack tropospheric transport and identify the source of the material. The instrument payload that could provide this information would be organized around a backscatter lidar with a range of smaller complementary sensors (polarimeter, multi-directional radiometer, etc.) that could contribute to characterizing the size, shape and optical properties of aerosol and (optically thin) cloud particles. EX-3: Cloud-Radiation Feedback Research Mission After water vapor, clouds are the next largest contributors to the planetary greenhouse effect (about 30 Watt/m2). Altogether, the net radiative impact of clouds on the planetary radiation balance is large (of order of - 20 Watt/m2) and highly variable. The cloud response to changing climatic conditions is the biggest source of uncertainty in climate model simulations, to say nothing of the essentially unknown indirect radiative forcing of aerosols through the modification of cloud particle size and optical properties. Understanding and modeling cloud processes with adequate accuracy remains the most vexing problem of climate physics, despite decades of research in cloud physics and progress toward explicitly introducing cloud micro-physical processes in specialized "cloud resolving models" and general circulation models. A principal reason is the lack of sufficient (global) observational data to reflect the diversity of weather phenomena and climatic regimes in which clouds are embedded. Understanding cloud-radiation feedback in the context of climate change is the frontier of atmospheric radiation research. Effective observing tools to resolve the diversity of cloud system geometry and the complexity of cloud optical properties are only now becoming available: backscatter lidar, cloud profiling radar operating in the millimeter wave range, precipitating cloud profiling radar operating in the centimeter wave range, visible, IR and sub-millimeter radiometers or spectrometers. Considering the complexity of the problem and the diversity of observing tools that can shed light on some aspects of the problem, no single cloud-

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radiation research mission can be singled out as uniquely effective, but several candidate concepts appear thoughtful and scientifically promising. Any such mission would be organized around a cloud profiling radar and lidar system (the only observing technique that can provide adequate vertical resolution and detect overlapping cloud layers), with complementary passive sensors focused on the same atmospheric column. (The spatial variability of cloud is such that the benefit of multiple sensor observation would be compromised if co-registration was lost.) A state-of-the-art cloud-radiation feedback research mission would be a relatively ambitious project, requiring a medium-size spacecraft and a multiple instrument payload (to be determined by selection of one among several comprehensive proposals for mission concept and implementation). This particular experimental mission concept has been studied in depth by at least two partner agencies and would therefore be a good candidate for a joint international cooperative project. EX-4: Soil Moisture and Ocean Salinity Observing Mission Soil moisture, a component of ground water storage, is the state variable that represents the terrestrial hydrologic system on time scales relevant to flooding, evapotranspiration and impacts on vegetation (water stress). Soil moisture integrates precipitation and evaporation over periods of days to weeks and introduces a significant element of memory in the atmosphere/land system. There is strong climatological and modeling evidence that the fast recycling of water through evapotranspiration and precipitation is the primary factor in the persistence of dry or wet anomalies over large continental regions during summer. On this account, soil moisture is the most significant boundary condition that controls summer precipitation over the central US and other large mid- latitude continental regions, and essential initial information for seasonal predictions. Precise in situ measurements of soil moisture are available but each value is only representative of a small area. Remote sensing, if achievable with sufficient accuracy and reliability, would provide truly meaningful wide-area soil wetness or soil moisture data for macroscale hydrological studies and precipitation anomaly prediction over large continental regions. The most mature technique, low-frequency passive microwave radiometry, would also allow the determination of Sea Surface Salinity (SSS). Global surface salinity measurement would provide invaluable information to close the planetary water budget over the oceans and understand the pre-conditioning of surface waters that controls deep water formation in the north Atlantic. The SSS measurement places a challenging requirement on the sensitivity (signal/noise ratio) of spaceborne passive microwave radiometers. The measurement of soil moisture (and ocean salinity) must still be considered experimental and, for this reason only, was ranked as the second priority of the Hydrology and Global Water Cycle discipline. Developing an effective soil moisture remote sensing system based on passive radiometry requires the deployment of very large antennas (or realization of a correspondingly large synthetic aperture) in order to achieve meaningful spatial resolution (of order ~ 10 km or less) at the relatively low microwave frequencies that can penetrate moderately dense vegetation. The objective of an experimental soil moisture/ocean surface salinity measurement mission would be a 3 to 5 year demonstration of an advanced low-frequency dual-polarization passive microwave radiometer or combined active/passive system in low earth orbit (to be determined by selection of competing mission proposals). EX-5: Time-Dependent Gravity Field Mapping Mission

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Measuring the time-varying component of the gravity field is a totally new "remote sensing" approach that provides a unique insight in mass redistribution within the earth system, including climate effects such as ground or surface water storage, and changes in oceanic circulation, as well as tectonic motions and post-glacial rebound. The concept of measuring temporal variations in the gravity field to monitor mass redistribution has already been demonstrated, using various time series of geodetic and gravimetric data. The Earth System Science Pathfinder GRACE mission will extend this proven capability to harmonics above 100. There are strong expectations from both the solid earth science community and global oceanography community that the GRACE mission (to be launched in 2001) will be a pathfinder for a powerful new method to investigate geophysical and geodynamic phenomena. If this breakthrough is achieved, further technological advances are clearly in sight that will allow at least one order of magnitude improvement in the sensitivity of the method, thus expanding the range of scientific applications. Knowledge of the geoid is a limit to the scientific utility of sea-surface topography data for dynamic oceanography at shorter length scales. Advanced satellite-to-satellite tracking in low Earth orbit would allow significant refinements of the shape of the geoid down to 50-100 km scales, comparable to the scale of ocean eddies and the exploitation of altimetric observations closer to continental margins to characterize coastal currents). In addition, directly detecting changes in total water column mass would allows computing the mean geostrophic flow or Sverdrup circulation. In view of the fundamental importance of earth gravity data, the oceanic, polar and geodynamic disciplines would place this measurement in their top two or three scientific priorities for long-term systematic observation of fluid and solid earth. On the other hand, the required technology (satellite-to-satellite laser interferometry) is definitely a technical challenge, so that the concept must still be considered experimental. An experimental mission would involve launching two essentially identical spacecraft on the same orbit with a single launch vehicle. Operational life time should be a minimum of five years. In view of a broad international interest in space geodesy, this mission would be also a likely candidate for an international cooperative project. EX-6: Vegetation Recovery Mission Understanding the carbon cycle is essential to assess future changes in the atmospheric concentration and greenhouse effect of carbon dioxide. A major component of this cycle is net ecosystem productivity in terrestrial temperate and boreal ecosystems, which integrates the regrowth of previously disturbed landscapes, carbon dioxide fertilization, and the result of nitrogen deposition. Quantifying the first of these effects is critical to understanding the response of the carbon cycle to human perturbations. For this reason, the land cover and terrestrial ecosystems discipline places high priority on a disturbance recovery mission, that could be flown in the late 2000's time frame. The main instrument would be a steerable lidar altimeter system, based on technological evolution of the ESSP Vegetation Canopy Lidar mission (to be launched in year 2000). The purpose of the mission would be to sample the evolution of specific terrestrial biosphere targets that have been subject to major disturbances, like clear-cutting or fires. The scientific objective is to characterize the recovery of above-ground biomass in those areas. A complementary visible-near IR imager could document the recovery of grasslands and semi-arid ecosystems. Altogether this experimental mission could be implemented on a small spacecraft and aim for a 3-5 year life time. X-7: Cold Land Processes Research Mission

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Over large regions (e. g. the interior of North America and Eurasia) and high altitude mountainous areas, much of the annual precipitation contributing to streamflow occurs in the form of snow during the winter months. Snow accumulation is a major storage term that strongly impacts the seasonal cycle of runoff. The freeze-thaw status of the soil surface determines the partitioning of precipitation or snowmelt between runoff and infiltration. The high albedo of snow-covered terrain results in large contrasts in net radiation during the thaw period. Important science questions that come to mind are: How does the extent of snow and frozen ground affect atmospheric climate? Can snow water equivalent be quantified from remote sensing data with sufficient accuracy to improve hydrologic forecast? Could these factors be measured accurately enough to identify meaningful climatic trends? Snow water equivalent and the extent of frozen ground have not been adequately measured from space, due to limitations in spatial resolution of passive microwave instruments and the poor sampling frequency achievable with existing spaceborne imaging radar systems. A promising, but technically challenging measurement concept is based on applying active SAR imaging techniques at relatively coarse spatial resolution (of order ~ 1 km) to detect freezing conditions on the ground, the extent & amount of snow, and probably various vegetation properties. Coarse resolution could allow a wider swath and short repeat cycle (~ 3 days). This experimental mission could be implemented on a dedicated platform in low altitude sun-synchronous orbit. The primary payload would be a 2-polarization, coarse resolution SAR system at L-band or lower frequency. The technical challenge is measuring the intensity of the backscatter signal with much higher accuracy than currently envisaged in high-resolution imaging radar systems. NASA intends to carefully examine and take advantage of potential commercial and international initiatives in this domain of global SAR observation with high revisit frequency and relatively coarse spatial resolution. APPENDIX 3 PROTOTYPE OPERATIONAL INSTRUMENT DEVELOPMENT The Step 1 review highlighted several projects to develop and demonstrate new sensors intended for operational applications as particularly meaningful for scientific research. It has been long recognized that earth system science relies heavily on information and climatological records acquired and archived by operational environmental agencies (for a variety of applications). This is especially true in the field of climatology, as most of what is currently known about the earth climate is derived from the study of weather observation records. Thus, improving the capabilities of operational observing systems (especially polar satellites that provide global coverage) is also essential for the progress of earth system science. On the other hand, there is currently no established process for identifying joint scientific and application priorities for operational sensor developments, nor for transition from scientific developments to the procurement and accommodation of new operational instruments on operational satellite systems. The development and flight demonstration of specific prototype operational instruments is not explicitly included in the nominal mission plan but could be accommodated by re-ordering flight priorities in the Enterprise's

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EOS follow-on, Earth Probe and New Millennium programs. NASA is seeking active participation of cognizant user agencies in the definition, development and transition to operational use of new advanced instruments that would meet ESE long-term science objectives as well as operational application requirements. The following is a list (no priority order implied) of instrument concepts that were discussed in the RFI process or otherwise brought to the attention of the Enterprise: The Workshop generally agreed with this new NASA approach to contributing to the development of new or improved operational observing capabilities. Although no discipline had ranked high-frequency observation from geostationary orbit as their highest scientific priority, there was general recognition of the value of developing a new geostationary sensors for a diversity of research and application objectives. NASA has focused the forthcoming announcement of opportunity for the next New Millennium Program technology demonstration mission precisely to address this objective. NASA is also holding consultations with NOAA/NESDIS on priorities for the development of improved sensors for operational GOES satellites. OP-1: Advanced Microwave Sounder The current operational microwave sounder suite, including AMSU-A and MHS, has a total mass of 160 kg. The utilization of new microwave circuit technology would permit substantial weight reduction for the same functionality and the addition or substitution of new microwave channels that would better support the retrieval of precision temperature/moisture soundings in combination with a companion IR sounder. NASA had studied the feasibility of upgrading existing microwave sounders, as part of the Integrated Multispectral Atmospheric Sounder (IMAS) project. Significant progress had been made in the development of microwave technology at the relevant (very high) frequencies and NASA plans to apply these technique to the development of an advanced operational microwave sounder for NPOESS. OP-2: Tropospheric Wind Sounder Global measurement of tropospheric wind has been widely heralded as potentially the most significant contribution of satellite remote sensing to existing global meteorological observations (World Weather Watch). Direct measurement of horizontal wind vectors in clear air has been demonstrated using lidar from the ground and from aircraft, based on determination of the wind-induced Doppler shift in the backscatter signal. Two competing techniques are envisaged: Coherent detection Doppler lidar system, which is the most sensitive and potentially most accurate technique, but works only in atmospheric layers where sufficient density of scattering particles exists (aerosols). The technique requires development of a unique laser transmitter technology. Incoherent detection Doppler lidar system, which is less sensitive but operates uniformly in clear air (works with both Mie scattering from aerosol particles and Rayleigh scattering from air molecules). The technique can utilize a widely used type of laser transmitter. NASA is preparing a demonstration of the first technique (coherent detection) on a Space Shuttle flight in 2001 (SPARCLE project). There is also private sector interest in developing alternate measurement techniques which could offer the prospect of the availability of tropospheric wind data from a commercial provider.

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OP-3: GPS Constellation for Atmospheric Sounding Measurement of the phase-delay occurring in the propagation of GPS signals near the limb of the atmosphere allows inferring dry air density, temperature and pressure as a function of geopotential height in the region where the concentration of water molecules remains negligible. Below this level, the same technique allows estimating water vapor concentration, provided reasonably accurate temperature information can be obtained from other sources. Altogether, the technique is a completely different approach to atmospheric sounding and would, in principle, provide practically drift-free temperature information throughout the upper troposphere and lower stratosphere, as well as unmatched vertical resolution. Further refinements are also conceivable to extend the domain of application of this and related microwave limb sounding methods. NASA has made substantial investments in the development of relevant spaceborne GPS receiver technology, as well as software for flight equipment operation and data processing. NASA has also begun to constitute an experimental GPS constellation by furnishing GPS equipment to scientific satellite missions of opportunity developed by international partners. It is expected that this international system will deliver a sufficient number of GPS soundings per day to carry out a meaningful test of the impact of this type of data on the quality of global weather forecast (although only in a delayed or "hindcast" mode). A further initiative, co-sponsored by UCAR and the Taiwan Academy of Sciences would launch a constellation of 8 dedicated micro-satellites, allowing real-time collection of GPS measurements and delivery of temperature/moisture profile data to weather forecasting centers in time for insertion into the operational analysis and prediction system. NASA is considering possible means to demonstrate this new observing technique. OP-4: Advanced Geostationary Sounder One of the two principal sensor on NOAA Geostationary Operational Environmental Satellites (GOES) is an IR atmospheric sounder of relatively conservative design and technology. The sensor allows repeated soundings at very short time intervals over specific regions of interest (where rapid weather development is being observed). However the lack of vertical resolution in the lower and mid-troposphere, where rapid weather development actually occurs, reduces the usefulness of frequent soundings for the purpose of numerical weather prediction. This deficiency could be overcome by a new sounder instrument using state-of-the-art technology (in particular, advanced IR detector arrays and mechanical cryogenic cooling systems). Dynamical meteorology supports the expectation that AIRS-grade temperature and moisture soundings at high spatial and temporal resolution would bring a significant improvement in the ability to forecast mesoscale weather systems and, in general, assist with severe storm warning. OP-5: Volcanic Ash and Gas Emission Mapping Mission and Advanced Geostationary Earth Imager The visible and IR imaging radiometer on the current GOES series is a new instrument design that delivers images of the earth disc with unprecedented spatial and temporal resolution. Nevertheless, several improvements are envisaged, such as augmenting the number of spectral channels and further increasing spatial resolution. These upgrades would be justified by a multiplicity of operational applications of geostationary imager

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data, from tornado warning to fire detection to tracking ash clouds from volcanic eruptions. OP-6: Special Event Imager The "Special Event Imager" concept (SEI) is a steerable high-resolution imager that could be pointed to stare at occasional or predictable regional events that vary within a time span of hours rather that days. The SEI is being promoted by the biological oceanography community as well as operational users as a desirable addition to the standard payload of GOES satellites. In addition to numerous applications from wildfire assessments to algal bloom monitoring, the SEI could provide invaluable ocean color change information to capture coastal phenomena that are dependent upon tidal effects. OP-7: Geostationary Lightning Mapper Electrical charges that cause lightning strikes are created by rapid ascending air flow associated with strong convective storms. There is evidence that instantaneous mapping of lightning strikes over the disc of the earth from geostationary orbit would enhance the ability to judge the strength of developing storm cells and forecast the likelihood of tornadoes and severe downdraft. The strike rate can also be related in a semi- quantitative manner to convective precipitation. Altogether, a geostationary lightning mapper holds considerable attraction for weather forecasters, but the scientific significance of such observations from one or two geostationary satellites does not match the scientific interest of global lightning distribution data obtained by the NASA-provided lightning detection sensor on TRMM. NOTE: Reprinted from Charles Kennel et al., Report of the Workshop on NASA Earth Science Enterprise Post-2002 Missions, NASA Headquarters, Washington, D.C., November 12, 1998, Appendix 1. Available online at http://www.earth.nasa.gov/visions/Easton/index.html.

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Appendix D Task Group Meeting Agenda WEDNESDAY, FEBRUARY 10, 1999 8:00 Continental Breakfast a.m. Closed Session 8:30 Discussion of task, initial views of the task M. Geller, Chair a.m. group Bias and conflict discussion Sherburne Abbott Open Session (A.M. session meets jointly with the Committee on Earth Studies) 9:30 NASA presentations G. Asrar and P. Morel a.m. 11:00 NOAA and NPOESS IPO presentations C. Nelson, Mike Crison, Ray Taylor a.m. Lunch and discussion of Pathways report1 12:30 B. Moore p.m. 1:30 Review of Easton meeting L. Shaffer p.m. 2:00 Discussion with Sarah Horrigan Office of Management and Budget p.m. 2:30 Discussion with Bob Corell National Science Foundation p.m. 3:00 Discussion with Rosina Bierbaum Office of Science and Technology p.m. Policy 3:30 Break p.m. Discussion of NPOESS integration report2 4:00 M. Abbott p.m. Discussion of 21st century report3 4:30 E. Barron p.m. 5:00 Open session for roundtable or splinter discussions among task group members, p.m. presenters, and invited scientists 6:00 Adjourn for reception p.m. THURSDAY, FEBRUARY 11, 1999 8:00 Continental Breakfast a.m. Closed Session

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8:30 Task group discussion and writing a.m. 5:30 Adjourn p.m. 1 National Research Council, Board on Sustainable Development, Global Environmental Change: Research Pathways for the Next Decade, prepublication copy, National Academy Press, Washington, D.C., 1998. 2 National Research Council, Space Studies Board, "On Climate Change Research Measurements from NPOESS," letter report to Dr. Ghassem Asrar, NASA, and Mr. Robert S. Winokur, NOAA, May 27, 1998. 3 National Research Council, Board on Atmospheric Sciences and Climate, The Atmospheric Sciences Entering the Twenty-First Century, National Academy Press, Washington, D.C., 1998.

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NASA's Plans for Post-2002 Earth Observing Missions BOARD ON ATMOSPHERIC SCIENCES AND CLIMATE ERIC J. BARRON, Pennsylvania State University (Co-Chair) JAMES R. MAHONEY, International Technology Corporation (Co-Chair) SUSAN K. AVERY, University of Colorado, Boulder LANCE F. BOSART, State University of New York, Albany MARVIN A. GELLER, State University of New York, Stony Brook DONALD M. HUNTEN, University of Arizona, Tucson JOHN IMBRIE, Brown University CHARLES E. KOLB, Aerodyne Research, Inc. THOMAS J. LENNON, WSI Corp. MARK R. SCHOEBERL, NASA Goddard Space Flight Center JOANNE SIMPSON, NASA Goddard Space Flight Center NIEN DAK SZE, Atmospheric and Environmental Research, Inc. ELBERT W. (JOE) FRIDAY, Jr., Director BOARD ON SUSTAINABLE DEVELOPMENT EDWARD A. FRIEMAN, University of California, La Jolla, Chairman ROBERT W. KATES, Independent Scholar, Vice Chairman LOURDES ARIZPE, UNESCO, France JOHN BONGAARTS, The Population Council RALPH J. CICERONE, University of California, Irvine WILLIAM C. CLARK, Harvard University ROBERT A. FROSCH, Harvard University MALCOM GILLIS, Rice University RICHARD R. HARWOOD, Michigan State University PHILIP J. LANDRIGAN, Mount Sinai School of Medicine KAI N. LEE, Williams College JERRY D. MAHLMAN, Princeton University RICHARD J. MAHONEY, Washington University PAMELA A. MATSON, Stanford University WILLIAM J. MERRELL, H. John Heinz III Center G. WILLIAM MILLER, G. William Miller & Co., Inc. M. GRANGER MORGAN, Carnegie-Mellon University PAUL RASKIN, Tellus Institute JOHN B. ROBINSON, University of British Columbia VERNON W. RUTTAN, University of Minnesota, St. Paul THOMAS C. SCHELLING, University of Maryland, College Park MARVALEE H. WAKE, University of California, Berkeley WARREN WASHINGTON, National Center for Atmospheric Research, Boulder M. GORDON WOLMAN, Johns Hopkins University Ex-Officio Member Chairman, Committee on Global Change Research BERRIEN MOORE III, University of New Hampshire, Durham

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Staff SHERBURNE B. ABBOTT, Executive Director SPACE STUDIES BOARD CLAUDE R. CANIZARES, Massachusetts Institute of Technology, Chair MARK R. ABBOTT, Oregon State University FRAN BAGENAL, University of Colorado DANIEL N. BAKER, University of Colorado ROBERT E. CLELAND, University of Washington GERARD W. ELVERUM, JR., TRW Space and Technology Group MARILYN L. FOGEL, Carnegie Institution of Washington BILL GREEN, former member, U.S. House of Representatives JOHN H. HOPPS, JR., Morehouse College CHRIS JOHANNSEN, Purdue University RICHARD G. KRON, University of Chicago ANDREW H. KNOLL, Harvard University JONATHAN I. LUNINE, University of Arizona ROBERTA BALSTAD MILLER, Columbia University GARY J.OLSEN, University of Illinois, Urbana-Champaign MARY JANE OSBORN, University of Connecticut Health Center THOMAS A. PRINCE, California Institute of Technology PEDRO L. RUSTAN, JR., Ellipso Inc. GEORGE L. SISCOE, Boston University EUGENE B. SKOLNIKOFF, Massachusetts Institute of Technology NORMAN E. THAGARD, Florida State University ALAN M. TITLE, Lockheed Martin Advanced Technology Center RAYMOND VISKANTA, Purdue University PETER W. VOORHEES, Northwestern University JOHN A. WOOD, Harvard-Smithsonian Center for Astrophysics JOSEPH K. ALEXANDER, Director