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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
