2
Science Priorities and NASA Mission Plans

INTRODUCTION

In Chapter 1, the ad hoc committee considered a number of principles and issues to illustrate the complexity of mission choices and the size options that flow from those principles. In this chapter, the committee examines the relationship between the goals established by the science community and the strategic mission plans developed by NASA.

The committee’s methodology for this examination was to obtain input from the Space Studies Board (SSB) discipline committees1 based on their particular perspectives, experience, and previously published analyses. Each discipline committee was asked to answer four key questions:

  • Are there arguments for having a spectrum of mission sizes to achieve near-term (10 years) and far-term (10 to 20 years) goals in your discipline?

  • What are examples of existing, planned, or proposed missions in that spectrum?

  • What criteria would you develop for evaluating the mix of missions you chose?

  • Applying the criteria you developed to NASA’s portfolio of missions in the NASA strategic plan for your discipline, what are your observations?

The request to the discipline committees for information (Appendix C) also included questions on such factors as the impact of new technology and international cooperation to ascertain the range of mission sizes that scientists consider appropriate for their discipline area and its high-priority scientific objectives.

The ad hoc committee drew on the discipline committees’ contributions (Appendix E) in assessing the relationship between NASA’s strategic plan, the scientific strategies laid out by the community, and the portfolio of missions assembled to meet the scientific objectives.2

1  

The Committee on Astronomy and Astrophysics (CAA), the Committee on Earth Studies (CES), the Committee on Planetary and Lunar Exploration (COMPLEX), the Committee on Solar and Space Physics (CSSP), and the Committee on International Space Programs (CISP).

2  

The tables in Appendix E list nonexhaustive examples of missions in the discipline areas and include the mission parameters or top-level scientific objectives for the mission, the relative size range (according to NASA definitions), the mission status of the program, and the timescale of the observations or measurements to be taken.



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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions 2 Science Priorities and NASA Mission Plans INTRODUCTION In Chapter 1, the ad hoc committee considered a number of principles and issues to illustrate the complexity of mission choices and the size options that flow from those principles. In this chapter, the committee examines the relationship between the goals established by the science community and the strategic mission plans developed by NASA. The committee’s methodology for this examination was to obtain input from the Space Studies Board (SSB) discipline committees1 based on their particular perspectives, experience, and previously published analyses. Each discipline committee was asked to answer four key questions: Are there arguments for having a spectrum of mission sizes to achieve near-term (10 years) and far-term (10 to 20 years) goals in your discipline? What are examples of existing, planned, or proposed missions in that spectrum? What criteria would you develop for evaluating the mix of missions you chose? Applying the criteria you developed to NASA’s portfolio of missions in the NASA strategic plan for your discipline, what are your observations? The request to the discipline committees for information (Appendix C) also included questions on such factors as the impact of new technology and international cooperation to ascertain the range of mission sizes that scientists consider appropriate for their discipline area and its high-priority scientific objectives. The ad hoc committee drew on the discipline committees’ contributions (Appendix E) in assessing the relationship between NASA’s strategic plan, the scientific strategies laid out by the community, and the portfolio of missions assembled to meet the scientific objectives.2 1   The Committee on Astronomy and Astrophysics (CAA), the Committee on Earth Studies (CES), the Committee on Planetary and Lunar Exploration (COMPLEX), the Committee on Solar and Space Physics (CSSP), and the Committee on International Space Programs (CISP). 2   The tables in Appendix E list nonexhaustive examples of missions in the discipline areas and include the mission parameters or top-level scientific objectives for the mission, the relative size range (according to NASA definitions), the mission status of the program, and the timescale of the observations or measurements to be taken.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Chapter 2 is organized into two parts. The first part looks at the linkage between discipline science objectives and agency strategic plans in terms of common, cross-cutting themes that affect the plans for and the development of the Earth science and space science programs. The cross-cutting themes include issues such as portfolio balance, science objectives and larger missions, spacecraft and instrument availability, long-term planning, international partnerships, and the trade-off between more frequent flights for scientific spacecraft and the risk that science objectives will not be met. The second part of the chapter addresses the history, requirements, and programmatic directions of the various disciplines as those factors influence the mission size mix in the disciplines (Earth science, planetary science, solar and space physics, and astronomy and astrophysics). CROSS-CUTTING THEMES Is There a Balance of Mission Sizes in Space Science and Earth Science Programs? Underlying the committee’s assessment of mission size trade-offs for NASA’s Earth and space science programs is the question of whether the mix of sizes among the ongoing and planned missions addresses high-priority science questions and how the mix might differ by discipline. The committee evaluated balance on two counts: (1) a nonexhaustive account of NASA’s ongoing and planned mission mix in each discipline, provided in Appendix E, and (2) the SSB discipline committees’ comments on balance in the programs and how well the mission mix responds to high-priority science objectives. Summarized below are the mission sizes3 of ongoing and planned missions in NASA’s Earth science, planetary science, solar and space physics, and astronomy and astrophysics programs. Attempts to count or quantify the mix of missions sizes raise many questions. Should the count include foreign-led missions in which NASA participates? Should the size of a NASA-led international mission be based on the total cost or just the NASA contribution? Should failed missions be included? A simple count of the nonexhaustive lists shown in Appendix E (not including foreign-led or commercial missions) shows that astronomy and astrophysics has 6 small, 5 medium, and 12 large missions; planetary sciences has 1 small, 6 medium, and 6 large; solar and space physics has 7 small, 9 medium, and 3 large; and Earth sciences has 2 small, 3 medium, and 10 large. Attempts to assess the mission-size mix on gross counts alone, however, overlook several issues, which are detailed below. The portfolio of mission sizes in astronomy and astrophysics includes large-scale missions that undertake broad areas of astronomy and astrophysics research and small missions (e.g., those in the Explorer line) that address very focused objectives. The medium missions, with the exception of the Gamma Ray Large Area Space Telescope (GLAST), are on the lower end of the range, so the portfolio is weighted to both large and small missions. The scientific objectives for which medium-class missions are appropriate will not be met adequately by the current or planned portfolio laid out by NASA. However, as noted in Chapter 1, international astronomy and astrophysics missions often fit into the medium class and give the discipline a better balance overall. Unlike NASA’s plans for astronomy and astrophysics, which favor small and large missions, its plans for the planetary science program emphasize medium and larger platforms, which can accommodate the power and fuel resources needed to travel to other planetary systems, as discussed elsewhere in this report. While small and medium missions have greater resiliency and flexibility, some high-priority science questions genuinely require large missions (e.g., those where samples must be returned). However, as noted later in this chapter, in the planetary sciences subsection “Discipline-Specific Issues and Concerns,” missions with numerous and comprehensive objectives are being planned more or less as medium-size missions (e.g., Pluto Kuiper Express). If the constraint on mission size is too severe relative to the scientific objectives, the inevitable increase in risk may threaten mission success. Thus, the balance of mission sizes for planetary science remains questionable. In solar and space physics, the SSB has recognized that “although the Explorers do an excellent job of focusing on specific scientific objectives, most of the broader top-priority objectives summarized in the NRC 3   For the purpose of this study, NASA defined mission sizes as small (less than $150 million), medium ($150 million to $350 million), and large (more than $350 million). Costs include expenses for launch and science analysis.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Science Strategy4 report can only be accomplished with larger, more scientifically capable missions.”5 The SSB also noted that “space physics thus has a critical need for an external line of Solar-Terrestrial Probes (such as TIMED), together with occasional use of larger Frontier Probes, to carry out its science program. The Explorer program can be successful only in such a context.”6 For several years, there has been a line in the NASA budget for the development of Solar-Terrestrial Probes (STPs), which account for most of the medium-size solar and space physics missions noted in Appendix E that are in the development or definition phases. The continuing need for large missions to carry out science objectives is reflected in extensions to several ongoing large missions, the development of Cluster II, and the recommendation for the Interstellar Probe mission. Solar and space physics plans do not identify any future small missions, as these will evolve from the Explorer program. Small solar and space physics missions must continue to emerge regularly from the Explorer program to sustain portfolio balance at the smaller end. The Earth science program is in transition. The three most prominent missions (Terra, Aqua, and Chem) are large, and they have been under development for many years. In general, their successors and subsequent ESE missions will be smaller, requiring much less time to define, design, and implement. Thus, a census of sizes at this time of transition does not reflect the true state of NASA’s Earth science mission portfolio. The situation is complicated by the inclusion of operational weather satellites, which are by tradition large satellites. Their evolution to smaller systems, should that occur, would reduce development costs while maintaining continuity of observations. In contrast to the quantitative assessment in Appendix E, which shows many larger missions, a qualitative assessment of new starts in NASA’s Earth science portfolio would show more medium and smaller missions. Some Important Science Objectives Require Larger Missions Inserting smaller missions into the Earth and space science programs promises greater flexibility and more timely science. Focused, well-constrained science goals tend to be good candidates for small missions. Conversely, comprehensive, wide-ranging science goals often demand medium or large missions. Critical scientific objectives that call for missions at the large end of the spectrum can be identified in all four disciplines: planetary sciences, astronomy and astrophysics, solar and space physics, and Earth sciences. In the planetary sciences, such high-priority objectives as comet nucleus sample return7 require large missions. In addition, plans to return samples from Mars, such as the Mars Sample-Return Lander 2, would necessitate a large, complex mission, including a lander and rover to collect samples and a Mars return orbiter to carry the samples back to Earth.8 Exploring for the presence of liquid water on Europa would also call for a larger 4   See Space Studies Board, National Research Council, A Science Strategy for Space Physics, National Academy Press, Washington, D.C., 1995. 5   Space Studies Board, National Research Council, Scientific Assessment of NASA’s SMEX-MIDEX Space Physics Mission Selections, National Academy Press, Washington, D.C., 1997, p. 14. 6   Ibid. 7   “… COMPLEX’s Integrated Strategy report assigns its highest priority to the study of cometary nuclei, which ultimately will require a returned sample. Any sample return is an ambitious task, and previous plans to achieve this objective have been well outside the scope of a small [or medium] mission. COMPLEX’s Integrated Strategy also identified the outer solar system (particularly, Neptune and Pluto/Charon) as the key to several questions about solar system origin and evolution … Missions to the outer solar system will, however require powerful launch vehicles and specialized power and communications systems. Therefore, unless these requirements are reduced as a result of technological innovation (e.g., development of new propulsion systems), small [and medium] missions are not likely to contribute to this area of planetary science …. Even if it does prove feasible to investigate the outer solar system through a small[/medium]-mission program, it may not be cost-effective—that is, the use of small[-medium] missions does not assure that the most science will be returned per dollar spent, especially in the outer solar system. Because of the long flight times and different mission requirements (e.g., long-lived power sources and powerful transmitters) for spacecraft sent to the outer solar system, significant overall economy frequently can be achieved by maximizing the scientific return of any such mission.” Space Studies Board (SSB), National Research Council, The Role of Small Missions in Planetary and Lunar Exploration, National Academy Press, Washington, D.C., 1995, p. 14. 8   Space Studies Board, “Assessment of NASA’s Mars Exploration Architecture,” letter from Ronald Greeley, Chair, Committee on Planetary and Lunar Exploration (COMPLEX), and Claude Canizares, Chair, Space Studies Board, to Carl Pilcher, science program director, Solar System Exploration, National Aeronautics and Space Administration, November 11, 1998, p. 16.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions spacecraft to accommodate a suite of possible mobile platforms, including a surface rover, a multifunctional arm on a rover or lander, drilling and coring devices, devices for collecting samples, and a small submarine.9 In astronomy and astrophysics many primary science questions require observations by telescopes with pointing systems that distinguish radiation from regions of the sky of small angular extent and observations of objects that are intrinsically faint. Many studies therefore impose relatively strict requirements on spacecraft pointing stability, telescope aperture, and exposure time. These requirements are very difficult or impossible to meet with a small spacecraft using present technology or even technology that may become available in the near term. For example, planet detection and the study of stellar motions with the Space Interferometry Mission (SIM) requires optical interferometry, and the study of high-redshift galaxies with the Next Generation Space Telescope (NGST) requires near-infrared capability. Astronomy and astrophysics programs could be at risk if they are caught between the marginal adequacy of small missions and the cost and time delays associated with observatory-class missions. An intermediate-scale mission range may bridge that gap. In solar and space physics, the NASA strategic plan and the new Roadmap plan10 offer mainly medium-size missions (the Solar-Terrestrial Probes, capped at $250 million) and occasional large missions (the Frontier Probes, at more than $250 million). Frontier Probes are more challenging missions and will exceed $350 million or more in total cost. They would address scientific objectives that require difficult orbits—for example, an orbit needed to make in situ measurements of interstellar space—or would involve studies of planetary environments requiring long-duration travel and significant power and fuel resources. In Earth sciences, large platforms often have been used for measurements that require instruments with large aperture sizes. This is especially true of microwave instruments such as radiometers or synthetic aperture radars (see Chapter 3), whose aperture size is set by fundamental limits (see Chapter 1). For example, several science goals identified in the NRC report Global Environmental Change: Research Pathways for the Next Decade11 require synthetic aperture radar (SAR) measurements. While technological advances may reduce the cost and size of certain electronic subsystems, SAR antenna size is determined primarily by physical limits. Thus, even when a major SAR design goal is smallness, SAR antenna size cannot be reduced to fit that objective.12 Any mission that relies on SAR to conduct science measurements will continue to require a medium-sized or large platform. Although some science goals may require large spacecraft, the science community’s efforts to focus on only the most essential goals is important. Eliminating hardware that does not contribute to those essential goals may reduce costs and optimize mission size. Spacecraft and Instrument Availability Affects the Success of FBC The availability of off-the-shelf spacecraft buses and instruments that rely on existing, flight-proven technologies can help shorten mission development times and keep mission costs from growing. To that end, NASA has started a catalog of available bus designs and is promoting its use, because it can be a cost-effective resource. For example, most of the 20 respondents to the 1998 ESSP program opportunity proposed using a catalog bus. The benefits of using flight-proven scientific instruments (as opposed to spacecraft buses and subsystems) are more variable. Within specific classes of missions the availability of off-the-shelf hardware has clearly had a large impact on both the missions that are planned and the missions that are selected. Many of the small and medium-size planetary missions, i.e., those in the Discovery and Mars Surveyor mission lines, make extensive use of off-the-shelf instruments, particularly flight spares and copies of existing instruments. For example, seven of the nine instruments on the first three Mars Surveyor orbiters are flight spares from the Mars Observer program. Similarly, 9   Space Studies Board, National Research Council, A Scientific Rationale for Mobility in Planetary Environments, National Academy Press, Washington, D.C., 1999, pp. 23-25. 10   NASA, Office of Space Science, Sun-Earth Connection Roadmap: Strategic Planning for 2000-2025, 1999. 11   National Research Council, Global Environmental Change: Research Pathways for the Next Decade, National Academy Press, Washington, D.C., 1999. 12   Space Studies Board, National Research Council, Development and Application of Small Spaceborne Synthetic Aperture Radars, National Academy Press, Washington, D.C., 1998.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions the camera system on Stardust (a Discovery mission) makes use of flight spares from Voyager. Clementine’s low cost was enabled by the availability of hardware previously developed under the Strategic Defense Initiative (SDI) program. In the solar and space physics program, the competition for a small number of Explorer mission opportunities demands that proposed science plans be robust, novel, and scientifically world class. The tight cost caps on the programs (University Class Explorer (UNEX), SMEX, and MIDEX) lead researchers to include as much off-the-shelf hardware as possible. In addition to the cost savings from using off-the-shelf hardware and technology, this practice gives proposals an advantage when they undergo a technology evaluation. Such evaluations are typically quite conservative and may rate low-risk flight heritage (existing, proven technology) above science requirements that demand new technologies. Thus, proposals for Explorer missions rely heavily on off-the-shelf equipment to meet cost constraints and to survive the selection process.13 In solar and space physics, for example, three SMEX selections—SAMPEX, Fast Auroral Snapshot Explorer (FAST), and Transition Region and Coronal Explorer (TRACE)—depended heavily on heritage, and the MIDEX Imager for Magnetopause-to-Aurora Global Exploration (IMAGE) benefited directly and significantly from the closely related mission definition studies funded over the years immediately preceding its selection.14 In the short term, instruments that rely on existing technologies may enable a principal investigator to meet rapid schedules and cost constraints. In some cases, available technologies may satisfy science requirements and provide the best option for achieving new science. However, science tends to progress most rapidly when it can take advantage of the greatly increased capabilities enabled by new technologies.15 Moreover, smaller, shorter-duration missions are a natural arena for opening new observing and measurement windows that require instrumentation having no flight heritage. Absent a new source of technology and new instrument developments, the FBC approach could wither. The choice of spacecraft buses or instrument technologies from a stock list should not be imposed in ways that would discourage the development of new instruments, new subsystems, or new mission concepts. Long-Term Science Objectives Require Long-Term Planning Smaller, low-cost missions and their implied constraints on the duration of scientific measurements or observations make long-term planning essential for responding to the needs of long-term and interdisciplinary science. For example, the earlier and larger EOS missions, once part of a 15-year planning horizon, now appear as 5-year, single-satellite versions. Although the measurement requirements for decade-long observations have not changed, the planning outlook is shorter and includes repeated opportunities for missions such as ESSP for 3 to 5 year periods. Similarly, many Sun-Earth Connection missions must study phenomena over a substantial part of the 11-year solar cycle. From a science point of view, it is essential to provide a long-term science plan and to align new mission opportunities to be consistent with that plan. From a technology point of view, it is sensible and feasible to stress the longer duration and greater reliability of a mission as well as smaller size and lower costs. A 13   For more information on the Explorer selection process and potential pitfalls for proposers, see American Geophysical Union, SPA Section Newsletter VII(12), February 2, 2000. 14   SSB, Scientific Assessment of NASA SMEX-MIDEX Space Physics Mission Selections, 1997. 15   Contrast the above to the more traditional approach to executing large, complex missions. First, the scientific objectives of traditional missions had to be clearly defined, analyzed, and agreed to by a sizeable portion of the scientific community. Second, the payload instruments had to be developed to meet those scientific objectives. Third, the platform had to be designed to suit the payload instruments being developed. Finally, the spacecraft had to be integrated and tested to ensure full compliance with all the scientific objectives. If a mission like this is designed to meet a complex series of scientific objectives, it is highly unlikely that a specific payload instrument can be found among existing commercial or government sources that meets those objectives. The first order of business in traditional missions is development of the scientific instruments required to meet the ambitious scientific objectives. These specialized instruments often take a long time to develop and may be subject to schedule delays and overruns that impact the entire mission. Nevertheless, complex missions typically yield extraordinary scientific results, satisfy a broad scientific constituency, and also leave a heritage of technologies and instrumentation developments that benefits future missions.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions management strategy that incorporates scientific vision and emphasizes long-term planning is important for facilitating the balance between long-duration science missions and more frequent, shorter-duration missions. International Partnerships Help to Balance Mission Sizes in a Portfolio In the section on international cooperation in Chapter 1, the committee discussed how the FBC approach can affect international collaboration. This section discusses how such cooperation can contribute to the mix of mission sizes in NASA’s portfolio of Earth and space science missions. Cooperative international programs are a clear example of how flexibility in mission planning has produced excellent science at low costs. Many of the large missions (e.g., Cassini and UARS) would not be possible in anything resembling their present form had they not had international contributions. The ability to forge low-cost (to the U.S. taxpayer) international collaborations has sometimes meant that the United States has been able to avoid duplicative missions. For example, because ROSAT, a German-U.S.-U.K. astrophysics mission, and ASCA, a Japanese-U.S. mission, were the natural successor missions to Einstein (with an increase in sensitivity, resolution, and bandpass by a factor of between 3 and 5), the United States was able to leapfrog directly to Chandra, a Great Observatory. In space science, ESA has a well-defined, long-term plan called Horizon 2000 and Horizon 2000 Plus. Although its long-term plan is less well defined, Japan has a plan for the next 5 years. In some areas of endeavor (such as cosmic microwave background studies), the ESA mission (Planck) is larger and more sophisticated than the analogous U.S. mission (Microwave Anisotropy Probe, (MAP)) but will fly about 6 years later. In other areas of astrophysics (such as the Far Infrared and Submillimeter Telescope (FIRST) and the International Gamma-Ray Astrophysics Laboratory (INTEGRAL)), ESA’s missions have diminished the pressure for a U.S.-only mission because ESA’s goals address some of the same goals as the NASA strategic plan.16 ESA and the United States are cooperating closely in the CLUSTER-II space physics mission, with ESA having the lead role. Similarly, NASA and the Japanese have a very successful ongoing collaboration in space physics with the Geotail mission and in solar physics with the Yohkoh and the Solar-B missions; Japan’s Institute of Space and Astronautical Science is the lead agency for all three collaborations. In the Earth sciences, international contributions have enhanced many missions but have introduced discontinuities in data for others. The Tropical Rainfall Mapping Mission (TRMM) and most of the successful ESSP projects would not have been approved by NASA if there had not been substantial foreign involvement. Many U.S. researchers recognize the importance of radar altimetry for oceanic science, global weather forecasting, and the observation of long-term climate signals. France has been particularly important for precision radar altimetry, an area on which NASA is not currently focusing. The U.S.-French radar altimetry mission, TOPEX/ Poseidon, continues to collect important data on sea-surface measurements (see Box 1.1 in Chapter 1). France is taking the lead role in the follow-up to TOPEX/Poseidon, Jason-1, which is slated for launch in 2001. Similarly, NASA has chosen not to focus on synthetic aperture radar measurements, an area in which Europe, Japan, and Canada have been leading. An apparent characteristic of foreign missions is that many fit within (or approach) the medium-cost window of U.S. missions, as defined in Appendix B. As a result, the gap created by current NASA plans, at least with 16   The flight of the first very long baseline interferometry mission (HALCA) by the Japanese seems to have affected U.S. plans for a similar experiment and has stimulated future U.S. proposals. A space-based radio interferometry mission, Advanced Radio Interferometry between Space and Earth (ARISE), to be overseen by an international advisory group, is described as one of the possible new missions in the NASA strategic plan. The impact of the longer-range ESA Horizon 2000 Plus plan will not be known until early 2000, after the revision of the NASA strategic plan. The ESA plan includes GAIA, an advanced astrometric mission; the infrared space interferometry mission DARWIN, whose goals include the first direct detection of terrestrial planets in orbit around stars other than our Sun and the first high-spatial-resolution imaging in the 6-µ to 30-µ wavelength region; LISA, a gravitational wave experiment with close U.S. collaboration; and XEUS, a very-large-area X-ray imaging and spectroscopic mission. Since DARWIN overlaps with TPF, LISA has strong U.S. science community support, and since the science goals for XEUS overlap (but are much more ambitious than) those of Constellation-X, the committee anticipates that ESA plans will have a substantial impact on the NASA program. An approved ESA mission has never been canceled.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions respect to space-based astronomy, is somewhat filled. Similarly, in the planetary sciences, foreign-led missions are providing the larger platforms required for key scientific objectives that involve sample return. One example is Rosetta, a cometary mission that includes 12 instruments from several nations, among them the United States. It will rendezvous with comet Wirtanen in 2012. The mission also includes a lander to conduct in situ measurements of the comet’s nucleus. With NASA’s cancellation of the Comet Rendezvous Asteroid Flyby (CRAF) mission and the Champollion lander, Rosetta becomes a critical link in addressing the SSB’s most important near-term scientific priority, which is to explore comets and other primitive solar system bodies.17 International partnerships—both those led by NASA and those led by foreign partners—have been very successful in solar and space physics. NASA is the lead agency for the Global Geospace Science (GGS) program, which includes the Wind and Polar spacecraft, both of which have important international components. Conversely, ESA is the lead agency for Ulysses, the Solar and Heliospheric Observatory (SOHO), and Cluster II, and Japan is the lead for Yohkoh, Geotail, and Solar B—all examples of foreign missions with high scientific priorities and significant U.S. participation. The current and planned programs in solar and space physics continue to include foreign participation, which contributes to the NASA strategic plan in these disciplines. Despite the fact that international missions clearly contribute to a balance in the mission mix needed to address high-priority science goals, there is no way, at present, of including these missions in NASA’s long-term planning effort. Indeed, the current U.S. strategic planning process is limited in its ability to coordinate with other space agencies on existing and planned international missions and to include in the planning information about the follow-up to its own missions. Trade-off: Does More Frequent Science Mean Greater Risk? In addition to the management and technical risks that the FBC approach may pose for missions (Chapter 1, section on risk), the FBC paradigm can also threaten NASA’s ability to carry out its strategic plans and to address the priorities established by the scientific community. In astronomy and astrophysics, notwithstanding the success of such astronomy missions as SWAS and the Far Ultraviolet Spectroscopic Explorer (FUSE), the failure of the High Energy Transient Explorer (HETE)—owing to launch vehicle problems—and WIRE and the partial failure of ALEXIS indicate that the risks associated with small missions can be quite high. Several smaller, shorter-duration missions in the astronomy and astrophysics program await completion and launch (e.g., MAP, the Full Sky Astrometric Mapping Explorer (FAME), HETE-2, the Galaxy Evolution Explorer (GALEX), Swift, and the Cosmic Hot Interstellar Plasma Spectrometer (CHIPS)). If they succeed, they will justify flying more frequent missions at higher risk, and the scientific return will have been great indeed. However, if a substantial fraction of the smaller, shorter-duration missions are either partial or total failures, then the FBC concept will not have succeeded. In solar and space physics the principal experience with FBC includes the notable successes with recent Explorers (e.g., the three SMEX missions: SAMPEX, FAST, and TRACE) and the Student Explorer Demonstration Initiative (STEDI) mission SNOE. In contrast, another STEDI mission, TERRIERS, failed (as discussed in Chapter 1), and the Thermosphere-Ionosphere-Mesosphere Energetics and Dynamics (TIMED) mission faced budget cuts that led to a descoping of the science objectives, downsizing of the mission, and subsequent delays. Thus far—as long as the problems with STEDI and TIMED do not recur—missions aimed at providing more frequent science cannot be judged to have introduced significant risk into the solar and space physics program. In the planetary science program, however, the recent loss of the Mars Climate Observer and the Mars Polar Lander exemplified the potential risks that the FBC approach poses for the strategic plan. Earlier, in January 1999, the Near Earth Asteroid Rendezvous (NEAR) satellite failed to complete the first objective of its mission when the 17   As conceived in the mid-1990s, Rosetta was to carry two landers, the NASA-supplied Champollion and the Franco-German RoLand. However, a mismatch of schedules between NASA and ESA forced the deletion of Champollion from the Rosetta mission. NASA later restructured its comet-exploration efforts and resurrected Champollion as a stand-alone spacecraft under the aegis of the New Millennium technology demonstration program. Called Space Technology 4 (previously Deep Space-4), the new program was finally canceled for budgetary reasons in 1999.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions spacecraft did not execute a crucial engine burn required for rendezvous with the Eros asteroid. Unlike the Mars missions, NEAR had another opportunity to complete its original objectives and successfully executed a rendezvous with Eros on February 14, 2000. The loss of science from the Mars failures will probably affect the feasibility of the originally planned scientific investigations as well as the mission designs for the next Mars missions. Another dimension of risk lies in the mission selection process and the potential for Explorer AOs, which are segregated by size and cost (e.g., MIDEX only or SMEX only), to encourage overambitious science plans. The science objectives may be squeezed (or expanded) to fit the AOs since researchers may want to take as much advantage as possible of each flight opportunity. The committee wondered if AOs could be designed to enable mission size and cost to be better matched to the proposed science. NASA might wish to examine this approach. DISCIPLINE-SPECIFIC ISSUES AND CONCERNS The issues described in the first part of this chapter are important because they affect all space-based science programs. However, each discipline also has a unique set of questions or observing requirements that must be understood on its own. This section outlines discipline-specific issues for the Earth science, planetary science, solar and space physics, and astronomy and astrophysics fields. Earth Sciences Earth science includes diverse scientific disciplines such as oceanography, land processes, atmospheric sciences, meteorology, climate, and geodesy, all of which utilize observations and measurements from space. For more than 30 years these disciplines have relied heavily on observations from meteorological satellites that now include NOAA’s polar-orbiting operational environmental satellites (POES) and its geostationary operational environmental satellites (GOES), the Defense Meteorological Satellite Program (DMSP) polar-orbiting satellites, and their foreign counterparts. The U.S. operational polar-orbiting series is being converged into the National Polar-Orbiting Operational Environmental Satellite System (NPOESS), which is preparing to begin on-orbit operations about 2009. The fact that there are both research and operational Earth-observing satellites is significant for the present discussion because the future uses of these satellites will have to address both research and operational objectives.18 Mission Mix and NASA Plans For many years, mission scope and spacecraft size for space-based Earth science research grew to accommodate the increased size and capability of instruments. However, the budgets that accompanied large spacecraft such as those in EOS were cut and the program was redesigned significantly. In an effort to maintain program efficiencies, as well as to sustain Earth science observations over a longer time horizon, the revised EOS plan, as it appeared in 1995, depended on successive reflights of substantially similar systems. That plan suffered under budget pressure, leading to the cancellation of the second and third reflights of large satellites in the EOS plan, but the accompanying insertion of smaller satellites solved some of the budgetary and technical problems.19 (Box 2.1 shows examples of large and small Earth science missions.) One consequence, however, is that the Earth Science Enterprise (ESE) Science Implementation Plan does not respond to some of the criteria for evaluating the mix of 18   The integration of critical climate research measurements into operational missions such as NPOESS 1 entails risks for collecting these measurements. This is the subject of the SSB report Issues in the Integration of Research and Operational Satellite Systems for Climate Research: I. Science and Design, National Academy Press, Washington, D.C., 2000. Both NASA and NOAA are making diligent efforts to accommodate such risks because missions like these integrate research and operational capabilities for collecting long-term climate data. 19   See Space Studies Board, National Research Council, Earth Observations from Space: History, Promise, and Reality, National Academy Press, Washington, D.C., 1995.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions BOX 2.1 Earth Science Accomplishments with Large and Small Missions UARS One of NASA’s flagship Earth science missions, the Upper Atmosphere Research Satellite (UARS), requested by Congress in 1976 and launched in 1991, continues to provide scientists with a better understanding of the linked physical and chemical dynamics of the stratosphere.1 Weighing 6 metric tons and carrying 10 scientific payloads, the UARS spacecraft was large in both scale and cost (roughly $750 million in life-cycle costs). UARS is an international endeavor with contributions from France, the United Kingdom, and Canada. It has contributed directly to international research on changes in the chemistry of the atmosphere, including ozone depletion due to NOx and chlorofluorocarbons.2 QuikSCAT Launched 8 years after UARS, in June 1999, the NASA Quick Scatterometer (QuikSCAT) gathers all-weather, high-resolution measurements of near-surface winds over Earth’s oceans. Named for its quick replacement of the NASA Scatterometer carried on the lost Japanese Advanced Earth Observation Satellite (ADEOS) mission, QuikSCAT uses a spare spacecraft bus and instrument. Small in comparison to UARS, it weighs 970 kg and was developed in a faster-better-cheaper mode at a cost of approximately $95 million (life-cycle costs). The satellite carries the SeaWinds instrument, a sophisticated microwave radar that measures wind speed and direction.3,4 GRACE Planned for launch on June 23, 2001, from Plesetsk, Russia, the Gravity Recovery and Climate Experiment (GRACE), which consists of 380-kg twin satellites, will begin its mission to create a new model of the variations in Earth’s gravity field. The first mission in an FBC program known as the Earth System Science Pathfinder (ESSP), GRACE will map Earth’s gravity fields with unprecedented accuracy by measuring the distance between the two satellites using the Global Positioning System and a microwave ranging system. GRACE is an $85.9 million (for the U.S. contribution) project that also has extensive participation from Germany.5 1   Space Studies Board, National Research Council, and European Science Foundation, U.S.-European Collaboration in Space Science, National Academy Press, Washington, D.C., 1998, pp. 83-84. 2   Image source: <http://uarsfot08.gsfc.nasa.gov>. 3   See <http://winds.jpl.nasa.gov/missions/quikscat/quikindex.html>. 4   Image source: <http://winds.jpl.nasa.gov/missions/quikscat/quikindex.html>. 5   Image source: <http://essp.gsfc.nasa.gov/grace/index.html>.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions mission sizes listed in Appendix E.20 In addition, the time line for planning follow-up missions in the EOS program has been shortened. Sustaining critical measurements over many years is more complicated technically and programatically. The mix of missions sizes in NASA’s current plan provides less assurance that the Earth science program may be relied upon to answer fundamental and long-term science questions.21 Furthermore, important questions now surround long-term shifts in the coupled Earth system (the boundaries or interactions between ocean and land, land and atmosphere, ocean and atmosphere, and people and the natural world), and the answers to such questions may be useful for policy makers. Smaller, shorter-duration missions make such interdisciplinary investigations more difficult programmatically, because it is difficult to ensure that the data sets have sufficient overlap and duration to answer these questions.22,23 The need to blend different sources of Earth science data is another critical factor affecting decisions on the portfolio of mission size in NASA’s Earth science program. Certain science goals endorsed by the science community, such as measuring the sea-level rise, verifying the stabilization of atmospheric ozone depletion, and detecting an increase or decrease in the mass of polar ice, require long time horizons, good data continuity, and well-characterized and well-calibrated instruments. To separate short-term variability in these phenomena from long-term trends often requires a blend of satellite, in situ, and model data. In each case, the mission design from concept to implementation must be in tune with the scientific objectives. Data from smaller or shorter missions as well as larger or longer ones can contribute to valuable long-term measurement programs if well planned and executed. Thus, instrumentation sufficient to support a long-term science objective does not necessarily imply many years of large annual expenditures. In addition, the ESE Technology Development Plan24 should be encouraged, but not to the exclusion of the science themes. NASA is to be commended for the recently introduced Instrument Incubator Program (IIP), aimed at bringing newly proposed measurement techniques from the stage of concept to the stage where they can compete for a space flight opportunity. However, it is difficult to plan and implement the transitioning of research instrumentation developed under very tight schedules to operational satellite systems, which are designed to incorporate new technology more slowly. Planetary Sciences The four principal scientific goals identified in the SSB’s planetary strategy are intensive studies of (1) comets, (2) Mars, and (3) the Jupiter system, and (4) the search for extrasolar planets.25 Each goal is addressed within NASA’s strategic plan. The medium and small missions of the Discovery line have enabled the community to address a wide variety of other science goals (e.g., studies of the Moon by Lunar Prospector and of asteroids by NEAR and Deep Space-1 (a New Millennium mission)). Nevertheless, the vast distances from Earth, extreme 20   High on the list of problems that were meant to be solved by the U.S. Global Change Research Program (USGRP) were the need for well-calibrated observations, the need to maintain critical observations, and the need for a focused scientific strategy. ESE’s use of smaller, shorter missions and the pressure to off-load continuing observations onto systems of the operational agencies runs counter to those needs. 21   Space Studies Board, “Report of the Task Group on Assessment of NASA’s Plans for Post-2002 Earth Observing Missions,” letter from Claude R. Canizares, Chair, Space Studies Board; Marvin A. Geller, Chair, Task Group on Assessment of NASA’s Plans for Post-2002 Earth Observing Missions; Eric J. Barron and James R. Mahoney, Co-chairs, Board on Atmospheric Sciences and Climate, and Edward A. Frieman, Chair, Board on Sustainable Development, to Ghassem Asrar, associate administrator, Earth Science Enterprise, NASA, April 8, 1999. 22   NASA, Earth Science Enterprise, Earth Science Implementation Plan, Version 1.0, April 1999, p. 61. 23   For example, a large part of the Earth science data that address several of the science goals identified in the GCRP priorities highlighted in the Pathways report is provided by SAR satellites. Save for NASA’s Seasat (1978), NASA has participated in SAR satellite programs only peripherally through international agreements with Canada, the European Space Agency, and Japan, whose successful satellite programs have led the field for the past 20 years. NASA has flown several SARs on one-week demonstration shuttle missions, but the impact of brief in-space operations on many Earth science questions is minimal. Although the cost of any realistic SAR satellite would place it in the medium to large category, the data would serve several high-priority science objectives. 24   NASA, Earth Science Enterprise Technology Development Plan, 1999. 25   Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions differences in (and poor knowledge of) operating environments, and long trip times make planetary missions inherently more complex and, hence, more expensive than missions that are flown closer to Earth. The growth in spacecraft size, number of instruments, mission complexity, and duration all contributed to the escalating costs of planetary missions in the late 1980s (typified by Mars Observer and Cassini).26 The concomitant risk associated with large, infrequent planetary missions was a major impetus for the FBC approach. Specifically, after the failure of Mars Observer in 1993, the risks in Mars exploration were managed by launching two spacecraft at every opportunity (which happened to be every 26 months) within a mission line of $145 million in FY98, $228 million in FY99, and $250 million in FY00 (although this plan was not sufficient to eliminate risk, as evidenced by the MCO and MPL failures). The creation of the Discovery class of missions, whose mission objectives are selected via an open competition, allowed a rapid response to new ideas for scientific exploration. However, care must be taken when comparing a small mission such as Lunar Prospector (~$70 million) with a large mission such as Cassini (~$2.5 billion). If Cassini were to be repeated with current technology and an FBC approach, the costs would be much less (how much less would depend on the degree of risk the developers would be willing to bear), but it would still be a large, complex mission. Cassini’s comprehensive set of 12 instruments and the Huygen’s probe allows a systematic exploration, including simultaneous measurements, of the complexities of Saturn’s atmosphere, rings, and moons over several years. While the five instruments on Lunar Prospector have provided useful information about the composition of the lunar surface and gravity field, the potential scientific accomplishments of Cassini could not be achieved with a series of multiple missions of the Lunar Prospector class. Thus, solar system exploration is optimized by a mixed portfolio of mission sizes, which includes occasional large missions for major objectives (particularly necessary in the outer solar system), many medium-size missions, and a few small missions for focused targets (in, say, a 1:10:5 ratio).27 (Box 2.2 shows examples of large and small planetary science missions.) Mission Mix and NASA Plans The Discovery line of missions (initiated in NASA’s FY94 budget) is intended to collect data to answer questions about the solar system within a total life-cycle cost of $300 million per mission (including launch). The initial skepticism over the scientific value of such smaller, shorter-duration missions as Mars Pathfinder, NEAR, Lunar Prospector, and Clementine has been moderated by the valuable and sometimes unexpected science return from these missions. They have, for example, contributed to a better understanding of the presence of hydrogen at the Moon’s poles (Lunar Prospector); the small-scale geology and chemical composition of rocks on Mars (Mars Pathfinder); the magnetic stripes on Mars (Mars Global Surveyor); and the low density of the asteroid Mathilde (NEAR). The FBC approach clearly has many benefits, such as flexibility, frequency, and timeliness of missions, as well as controlling excessive growth of mission costs and development time.28 However, tight constraints on mission costs can result in underestimating the funds necessary to return and analyze the data (e.g., Lunar Prospector (see Box 2.2)); reliance on spare instruments from previous missions (e.g., Stardust);29 reliance on nonmission funds (e.g., PIDDP)30 to support instrument development (most missions);31 or dependence on supplemental funds to pay for the launch. 26   SSB, The Role of Small Missions, 1995, p. 4. 27   SSB, Small Missions, 1995, p. 14; SSB, Integrated Strategy, 1994, pp. 182-183. 28   SSB, Integrated Strategy, 1994, p. 30; SSB, Small Missions, 1995, p. 15. 29   Stardust was launched in 1999 to collect material from a comet and interstellar dust and return it to Earth for analysis. 30   “The Planetary Instrument Definition and Development Program supports the advancement of spacecraft-based instrument technology that shows promise for use in scientific investigations on future planetary missions. The goal of the program is to define and develop instruments or instrument components to the point where the instruments may be proposed in response to future announcements of flight opportunity without additional extensive technology development.” See “Research Opportunities in Space Science 1998,” NRA 98-OSS-03, issued February 5, 1998, Appendix A 3.5 Planetary Instrument and Development Program, available at: <http://spacescience.nasa.gov/nra/98-oss-03>. 31   SSB, Small Missions, 1995, p. 19.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions BOX 2.2 Planetary Science Accomplishments with Large and Small Missions Cassini Cassini, launched October 15, 1997, is a mission to study Saturn’s atmosphere, magnetic field, rings, and moons. The 2,160-kg spacecraft is a joint NASA, European Space Agency (ESA), and Italian Space Agency endeavor. The Cassini spacecraft, including the orbiter’s 12 instruments and the 6 instruments on the Huygens probe, is one of the largest, heaviest, and most complex interplanetary spacecraft ever built, having cost $2.55 billion (including operations and science analysis). Its scientific objectives are to conduct the following observations: Orbital remote sensing of Saturn’s atmosphere, icy satellites, and rings; In situ orbital measurements of charged particles, dust particles, and magnetic fields; and Detailed measurements with six instruments on the Huygens probe during descent through Titan’s dense nitrogen atmosphere to the surface.1 These science objectives respond directly to the NRC scientific strategy, which calls for the exploration of the outer planets, including an intensive study of Saturn—the planet, satellites, rings, and magnetosphere—as one its highest priorities. Cassini is expected to reach Saturn in 2004 and begin its 4-year primary orbiter mission.2,3 Lunar Prospector Lunar Prospector is the first in a class of planetary probes, the Discovery line of missions. Developed at a total cost of $68 million, it was launched on January 6, 1998, beginning its 5-day trip to the Moon, where it remained in orbit for 18 months. The 300-kg spacecraft was equipped with a gamma-ray spectrometer, a neutron spectrometer, a magnetometer-electron reflectometer, an alpha-particle spectrometer, and equipment for a Doppler gravity experiment. The critical scientific objectives of the Lunar Prospector were as follows: To prospect the lunar crust and atmosphere for potential resources, including minerals, water ice, and certain gases; To map the Moon’s gravitational and magnetic fields; and To learn more about the size and content of the Moon’s core.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Lunar Prospector data were used to develop the first precise gravity map of the entire lunar surface and the first global maps of the Moon’s elemental composition, two primary scientific objectives recognized in an earlier NRC report, Strategy for Exploration of the Inner Planets, 1977-1987.4 In addition, despite the fact that the Moon’s magnetic field is relatively weak, the Lunar Prospector was able to confirm the presence of local magnetic fields. In a final attempt to detect water on the Moon, on July 31, 1999, it was crashed into a crater near the south pole of the Moon. However, no signature of water was detected.5,6 NEAR On February 17, 1996, the Near Earth Asteroid Rendezvous (NEAR) mission was launched to make the first quantitative and comprehensive measurements of an asteroid’s dimensions. The primary scientific goals are to assess the following: Bulk properties: size, shape, volume mass, gravity field, and spin state; Surface properties: elemental and mineral composition, geology, morphology, and texture; and Internal properties: mass distribution and magnetic field.7 The 805-kg spacecraft is managed by the Johns Hopkins University Applied Physics Laboratory and is the first launch of NASA’s Discovery program, an initiative for small planetary missions with a maximum 3-year development cycle and a cost cap of $224 million (life-cycle costs). Despite earlier complications, on February 14, 2000, NEAR rendezvoused with Eros, a large near-Earth asteroid, inserted itself into orbit around Eros, and began the year-long mission.8 1   SSB and ESF, U.S.-European Collaboration, 1998. 2   SSB, Integrated Strategy, 1994; Space Studies Board, “On the Scientific Viability of a Restructured CRAF Science Payload,” letter from Space Studies Board Chair Louis J. Lanzerotti and Committee on Planetary and Lunar Exploration Chair Larry W. Esposito to Lennard A. Fisk, associate administrator for NASA’s Office of Space Science and Applications, August 10, 1990; Space Studies Board, “On the CRAF/Cassini Mission,” letter from Space Studies Board Chair Louis J. Lanzerotti, transmitting a report of the Committee on Planetary and Lunar Exploration to Lennard A. Fisk, associate administrator for NASA’s Office of Space Science and Applications, March 30, 1992. 3   Image source: Painting by Michael Carroll, available electronically at <http://www.jpl.nasa.gov/cgi-bin/gs?/cassini/moreinfo/pix/dropoff.jpg>. 4   SSB, Strategy for Exploration of the Inner Planets: 1977-1987, National Academy of Sciences, Washington, D.C., 1978. 5   A.B. Binder, 1998. “Lunar Prospector: Overview,” Science 281:1480-1484; Space Science Board, National Research Council, Strategy for Exploration of the Inner Planets: 1977-1987, National Academy of Sciences, Washington, D.C., 1978; Space Science Board, National Research Council, A Strategy for Exploration of the Outer Planets: 1986-1996, National Academy Press, Washington, D.C., 1986; SSB and ESF, U.S.-European Collaboration, 1998. 6   Image source: <http://george.arc.nasa.gov/dx/basket/storiesetc/lpcrapix.html>. 7   Space Studies Board, National Research Council, The Exploration of Near-Earth Objects, National Academy Press, Washington, D.C., 1998. 8   Image source: <http://near.jhuapl.edu/NEAR/images/near2.gif>.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Achieving Major Science Objectives with a Series of Missions When major science objectives can be implemented by using a series of small and medium-size missions, there are recognizable benefits. As an example, replacing previously large-scale missions (e.g., Mars Observer and Mars sample return) with a series of small and medium missions has improved mission resiliency and provided an opportunity to address new scientific questions. Splitting up the original payload from the lost Mars Observer mission into three missions (Mars Global Surveyor, the now-failed Mars Climate Observer, and the Mars Surveyor 2001 orbiter) not only addressed the scientific objectives of Mars Observer but also provided an opportunity to fly three new instruments. Similarly, the redundancy inherent in the current Mars sample-return architecture enables multiple samples to be collected by a variety of means at widely separated sites. In addition, the use of two Mars ascent vehicles improves the likelihood that at least one set of samples will be returned to Earth. Nevertheless, the series of Mars missions will not accomplish the scientific goals set by NASA if the mission architecture relies heavily on developing new technology under tight schedules, constrained costs, and without means to recover from failures. Squeezing Large Science Objectives onto Medium-Size Missions Important scientific objectives such as sample returns, surface landers, and flights to the outer solar system cannot be achieved without larger missions. Nevertheless, as noted previously, a number of important scientific goals best addressed by large missions are being implemented as medium-sized (or nearly so) missions. This has led to programmatic problems (e.g., cost overruns and schedule delays) that may have contributed to the cancellation of ST-4/Champollion and an overreliance on alternative sources of funding to develop instruments and spacecraft technologies. These risks may be most evident in the Outer Planets program, which includes the Europa Orbiter and the Pluto/Kuiper Express missions as the first in a series of probes to explore organic-rich environments. Pluto/Kuiper Express (full life-cycle costs estimated at $354 million) and Europa Orbiter (full life-cycle costs estimated at $460 million) are characterized as large, according to the size categories set for this study. Both missions are technically demanding in their need to reach the outer parts of the solar system. Many researchers question, however, whether the allocated budgets will be sufficient to meet both the technical challenges and the science objectives, or whether the science objectives will be cut severely in order to stay within budget. As a consequence of promising to do more with less, there has been a serious reduction not only in the number of scientific instruments per mission but also in the fractional mass of the scientific payload on planetary missions (e.g., Europa Orbiter and Pluto/Kuiper Express).32 While there is clearly value in limiting the tendency to always want to attach just a few more instruments onto a spacecraft, there comes a point when there are too few instruments and too little scientific return to justify the cost of getting there. The key is to optimize the scientific return with a mission sized just large enough to adequately address the priority scientific objectives. Independent of mission size, squeezed budgets tend to affect mainly the later parts of a mission (the analysis, synthesis, and presentation of data), which then severely limits the mission’s scientific value33 (as happened, for instance, with Magellan, Galileo, Clementine, Lunar Prospector, Mars Global Surveyor, and all aspects of the International Solar-Terrestrial Physics program). 32   The mass of scientific instruments as a fraction of the total spacecraft mass (including propellant) is typically 11 percent for Discovery-size missions (Clementine, NEAR, Mars Global Surveyor). Missions to the outer solar system require large propellant masses (particularly for orbiters) and so tend to have lower payload mass fractions. Moreover, the science payload mass fraction for outer solar system missions has been decreasing with time: 13 percent for Voyager, 6.8 percent for Cassini, 2.5 percent for Pluto/Kuiper Express, and 2 percent for Europa Orbiter. See Sarsfield, Cosmos; D. Matson and J.-P. Lebreton, “The Cassini/Huygens Mission to the Saturian System,” Space Science Reviews, in press. 33   SSB, Small Missions, 1995, pp. 22-23.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Solar and Space Physics Solar and space physics are the primary disciplines involved in NASA’s Sun-Earth Connection (SEC) theme in the Office of Space Science (OSS). The studies of the Sun are examining its interior, surface, atmosphere, magnetic field, and solar variability in all of its manifestations. The solar atmospheric studies extend outwards to include the solar wind and the heliosphere and are also studying the physical interactions between the Sun and planetary environments, particularly Earth’s. The study of the space environment of Earth encompasses the magnetosphere, the ionosphere, and the upper atmosphere; coupling and energy transfer processes into, out of, and within the magnetosphere; and the various effects of solar-induced activity throughout geospace. The study of the magnetosphere and ionosphere is not restricted to Earth but extends to other planets, comets, and bodies in the solar system. There are complementary, and at times partially overlapping, interests in space physics and planetary science, and this has been appropriately reflected in the scientific objectives and accomplishments of several major missions through the years, such as Pioneer, Voyager, Galileo, and Cassini. Mission Mix and NASA Plans The slate of recommended missions in the new SEC Roadmap34 builds on, and is generally consistent with, the OSS Strategic Plan of 1997 and with the Committee on Solar and Space Physics’ assessment of that plan.35 Thus, each STP and Frontier Probe contributes significantly to one or more of the scientific objectives of the OSS Strategic Plan. The OSS Strategic Plan of 1997 does not specifically address mission cost or cost caps, but it does support the FBC approach, as discussed in the plan’s appendix on metrics.36 The new SEC Roadmap incorporates a strategy to implement the plan’s science objectives primarily using STP missions (less than $250 million), which are medium-size, and occasionally by using Frontier Probes (greater than $250 million), which are medium-size to large. The Roadmap also adopted the FBC approach by adhering to cost constraints in the mission planning process itself, which meant that the mission plans accepted for the Roadmap were ones that met cost-cap requirements after evaluation by the Roadmap technology teams. The Roadmap does not explicitly address whether a recommended mission fell short of optimal scientific objectives because of the set cost categories. However, in all likelihood some of the STPs will find it very difficult to meet optimal science objectives without exceeding the $250 million cost cap. The mission definition team for the Solar Terrestrial Relations Observatory (STEREO) has evidently already exceeded the funding ceiling, and the team for the Magnetospheric Multi Scale (MMS) is struggling to finalize mission designs and objectives that will keep the mission within the budget ceiling. An exciting and truly novel mission such as Magnetospheric Constellation, which would consist of 100 or so spacecraft networked in orbit, would undoubtedly have to make scientific compromises to meet the cost constraints of an STP mission. Scientific Priorities and the Mission Portfolio In the new SEC Roadmap, scientific priorities are couched in the form of quests and campaigns. Quests are major questions for which answers are needed to understand solar variability and its effects on the solar system and life on Earth. Implementation is achieved through organized campaigns. Quests address the following: Why does the Sun vary? How do Earth and the planets respond to solar variability? 34   NASA, Sun-Earth Connection Roadmap: Strategic Planning for 2000-2025, 1999. 35   See NASA, The Space Science Enterprise Strategic Plan: Origin, Evolution, and Destiny of the Cosmos and Life, November 1997; Space Studies Board, National Research Council, An Assessment of the Solar and Space Physics Aspects of NASA’s Space Science Enterprise Strategic Plan, National Academy Press, Washington, D.C., 1997; NASA, Sun-Earth Connection Roadmap: Strategic Planning for 2000-2025, 1999. 36   See NASA, The Space Science Enterprise Strategic Plan, 1997, Appendix B.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions How do the Sun and the galaxy interact? How does solar variability affect life and society? Campaigns address the following: The origins of solar variability; The effects of solar variability on the corona and the solar wind; The geospace environment; Comparative planetary space environments; The heliospheric boundary and nearby galactic environment; and Space weather. Each recommended mission in the solar and space physics program must make significant contributions to one or more of the SEC campaigns. The order of the mission queue is based on the following: Relative importance of the scientific objectives; Likelihood of achieving the scientific objectives by the mission plan; Potential for discovery and understanding; Breadth of the science (does it contribute to more than one campaign?); Urgency and relevance to society; Programmatic issues: timeliness (do it now or later?); Budgetary impact; Technology readiness; and Development costs and capability. All recommended missions are required to have an approved plan for education and outreach in accord with SEC and NASA expectations. SEC missions are designed to have direct relevance to other NASA science themes and the interests of other government agencies. In space and solar physics there is a clear need for a portfolio of mission sizes. Certain focused scientific objectives can often be accomplished with small or medium-size missions, which could be conducted through the Explorer or STP programs. Other scientifically more challenging and complex objectives would require far greater financial resources and large missions to be successful. Some of the Frontier Probes would be large missions. (Box 2.3 shows examples of one large mission, SOHO, and two small space physics missions, SAMPEX and TRACE.) An implementation plan might have a number of reasons for including a large mission, depending on the science to be accomplished. Some of the conditions that could lead to large missions are a long observation time line (solar variations or sunspot cycle effects); multiple instruments of high resolution (microphysics of particles and fields); highly stable platforms (for remote observations); vast physical parameter ranges in the operating environment (heliospheric observations outwards to interstellar space); interdisciplinary missions (planetary missions to investigate both the planet and its environment); and use of multiple spacecraft or constellations (to separate spatial and temporal effects or to make complementary observations simultaneously). Astronomy and Astrophysics Astrophysical sources radiate at all wavelengths of the electromagnetic spectrum, yet Earth’s atmosphere is transparent in only a few windows. Even at visible wavelengths accessible to ground-based telescopes, there are gains in angular resolution, dynamic range, and astrometric precision that are achievable only from space. Space astronomy is thus essential for progress across the whole field of astrophysical research.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions BOX 2.3 Space and Solar Physics Accomplishments with Large and Small Missions SOHO “The most comprehensive space mission ever devoted to the study of the Sun and the heliosphere,”1 the Solar and Heliospheric Observatory (SOHO) is a joint international project between the European Space Agency and NASA. Measurements and images taken from SOHO are helping scientists better understand the structure and dynamics of the solar interior using helioseismology techniques. In addition, researchers analyzing SOHO data are gaining insight into the physical processes that form and heat the Sun’s corona and into the solar wind and its acceleration processes. The 1,850-kg spacecraft is equipped with 12 instruments, including helioseismology instruments to study the structure and dynamics of the solar interior from the deep core to the outermost layers and remote-sensing instruments, including extreme ultraviolet and ultraviolet imagers, spectrographs, and coronagraphs to view the outer solar atmosphere and corona. Since SOHO began its 3-year mission in 1995, the observatory has provided the first image of the convection zone of a star; the first tracing of the slow-speed solar wind near the equatorial current sheet; the first detection of elements and isotopes in the solar wind; and the first observations of coronal mass ejections (CMEs) that generated subsequent disturbances. These disturbances were observed by other spacecraft to establish a cause-and-effect relationship for a solar system event that extended from the Sun to the solar wind to Earth’s magnetosphere and ionosphere. Having completed its original mission, SOHO was extended another 5 years, to 2003.2,3 SAMPEX Space physicists have a more complete understanding of the highly energetic, charged particles emanating from the magnetosphere and cosmic rays around the Sun and Earth as a result of the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX). This research includes particles trapped in Earth’s magnetosphere and those that enter the magnetosphere from interplanetary space. In particular, SAMPEX studied the composition and charge state of anomalous cosmic rays, which are not of solar, galactic, or extragalactic origin. Launched in 1992, weighing 170 kg, and costing $80 million (life-cycle costs), this FBC-style satellite has contributed to the fundamental understanding of anomalous cosmic rays in interplanetary space, a high-priority goal identified in previous NRC strategy reports.4

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions The SAMPEX satellite carries a set of four detectors designed with a high resolution and sensitivity to sense anomalous, galactic, and magnetospheric energetic particles. Data collected from the SAMPEX instruments have shown definitively that anomalous cosmic rays are mostly singly ionized and that upper atmosphere NOx changes with the level of flux of the precipitating energetic electrons. SAMPEX is currently in an extended mission phase.5,6 TRACE At 224 kg, the Transition Region and Coronal Explorer (TRACE), launched in 1998 and costing approximately $72 million (life-cycle costs), is contributing to scientific understanding of the processes that lead to solar variability. It is providing continuous observations of the Sun at the ultraviolet and extreme ultraviolet wavelengths. These observations are taken from a single, high-resolution telescope.7 The images and observations made from TRACE provide insight into the three-dimensional magnetic structure emanating from the Sun and help define the geometry and dynamics of the upper solar atmosphere, known as the transition region and corona. In addition, the telescope is acquiring solar images taken through filters that select different spectral features. By comparing the temporal evolution of events as seen through different filters, investigators are gaining critical information about the origin and evolution of local energy-release processes and the rearrangement of coronal structures such as coronal holes.8 1   SSB and ESF, U.S.-European Collaboration, 1998, p. 51. 2   SSB, SMEX-MIDEX, 1997, pp. 5-6. 3   Image source: <http://sohowww.nascom.nasa.gov/gallery/SC>. 4   SSB, A Science Strategy for Space Physics, 1995. 5   SSB, SMEX-MIDEX, 1997. 6   Image source: <http://lepsam.gsfc.nasa.gov/www/public/sampex.html>. 7   SSB, SMEX-MIDEX, 1997, p. 9. 8   Image source: <http://vestige.lmsl.com/TRACE/Public/Gallery/Images/>.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Mission Mix and NASA Plans Scientific merit is the first criterion, as noted in Appendix E, for evaluating the balance in a portfolio of missions. The goals of the space astronomy and astrophysics program outlined in the NRC report A New Strategy for Space Astronomy and Astrophysics37 involve obtaining answers to a set of fundamental questions about planets, star formation, and the interstellar medium; stars and stellar evolution; galaxies and stellar systems; and cosmology and fundamental physics. (Examples of large and small astronomy and astrophysics missions and their contributions to the field are shown in Box 2.4.) The highest-priority goals include the following, ranked according to their priority: Determination of the geometry and content of the universe by measuring the fine-scale anisotropy of the cosmic microwave background radiation; Investigation of galaxies near the time of their formation at very high redshift; Detection and study of planets around nearby stars; and Measurement of the properties of black holes of all sizes. Other important, unranked goals include the following: Study of star formation by, for example, high-resolution far-infrared and submillimeter observations of protostars, protoplanetary disks, and outflows; Study of the origin and evolution of the elements; Resolution of the mystery of the cosmic gamma ray bursts; and Determination of the amount, distribution, and nature of the dark matter in the universe. Scientific Priorities and the Mission Portfolio In mapping the scientific goals to the portfolio of missions, the first-ranked goal, which involves studies of the fine-scale anisotropy of the cosmic background radiation, can be accomplished with a small mission such as MAP. The second-ranked goal, the study of galaxies near the time of their formation, requires a large-aperture infrared telescope that must be housed on a large platform designed to accommodate telescope size and weight (NGST). One of the unranked scientific goals, resolution of the mystery of the cosmic gamma-ray bursts, can be pursued with a small mission such as HETE-2 and a medium-size mission such as Swift. Other goals, such as studying the origin of the elements and measuring the properties of black holes, can be done well with missions in the medium cost range. Since 1994, when NASA adopted the FBC paradigm for conducting missions, the agency has selected an excellent set of small and medium missions. However, the fact that very few of these had actually flown as of early 2000 means that it is difficult to decide if the change in emphasis to smaller missions has been successful.38 In the 10-year period from 1998 to 2007, NASA is planning for 15 UNEX missions, 10 SMEX missions, and 8 MIDEX missions to be shared among three themes: the Sun-Earth Connection, the Astronomical Search for Origins, and the Structure and Evolution of the Universe. NASA has also pursued a more vigorous program with its somewhat 37   Space Studies Board, National Research Council, A New Science Strategy for Space Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1997. 38   The Bahcall committee (formally known as the Astronomy and Astrophysics Survey committee, which authored the NRC report The Decade of Discovery in Astronomy and Astrophyics, 1991) recommended that the number of Explorers in the medium-cost category be increased to six. Ongoing medium-cost missions such as ACE and FUSE were completed. GLAST, which might be considered medium, is in the planning stages. In general, however, the astronomy and astrophysics portfolio has contained few medium-cost missions. Swift and FAME, which are also in the planning stage, are in the lower range of the medium category. See also Space Studies Board, “On ESA’s FIRST and Planck Missions,” letter to Wesley T. Huntress, Jr., NASA Associate Administrator for Space Science, from Claude R. Canizares, Chair, Space Studies Board, and Robert Dynes, Chair, Board on Physics and Astronomy, February 18, 1998.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions BOX 2.4 Astronomy and Astrophysics Accomplishments with Large and Small Missions Hubble Space Telescope Ranked as the highest priority in the 1970s astronomy decadal survey, the “Greenstein report,”1 the Hubble Space Telescope was launched in April 1990. It was the first of the Great Observatories designed for sensitive, high-angular-resolution observations in the ultraviolet through near-infrared spectral range. It features a suite of instruments that are upgraded by periodic shuttle missions. It is the largest orbiting observatory ever built; the total mission cost since its inception has been over $8 billion (to NASA, life-cycle costs including use of the shuttle) and about $550 million (to ESA). Since the refurbishment mission that corrected the spherical aberration induced by the 2.4-m primary mirror, the telescope has delivered images with a sharpness close to the limit imposed by diffraction. These images are significantly sharper than those delivered by ground-based telescopes, revealing entirely new phenomena at smaller physical scales. The mission has made crucial contributions across the whole of astrophysics, from planets (impact of comet Shoemaker-Levy 9 with Jupiter) to candidate supermassive black holes (nuclear regions of galaxies) to cosmic evolution (morphological structures in the most distant known galaxies). The general outcome is that the Hubble Space Telescope has had the greatest impact of any observatory-type facility available in space.2,3 Submillimeter Wave Astronomy Satellite The Submillimeter Wave Astronomy Satellite (SWAS) is a small Explorer, launched in December of 1998 and designed to study the chemical composition of interstellar gas clouds. Its primary objective is to survey water, molecular oxygen, carbon, and isotopic carbon monoxide emissions in a variety of star-forming regions in the Milky Way. The spacecraft is making detailed 1 degree x 1 degree maps of these species in giant molecular and dark cloud cores with an angular resolution of 4 arcminutes. The overall goal of the mission is to gain a greater understanding of star formation by determining the composition of interstellar clouds and by establishing the means by which these clouds cool as they collapse to form stars and planets. Other SWAS targets include external galaxies, circumstellar envelopes, planetary nebulae, and solar system objects (e.g., water features in Jupiter and in comets). The spacecraft spent its first viewing year exploring the Milky Way through a number of targets. It will return to its original orbit, from which it operates in an observation mode, and make more detailed studies of selected targets.4,5 1   National Research Council, Astronomy Survey Committee, Astronomy and Astrophysics for the 1970s, National Academy Press, Washington, D.C., 1972. 2   SSB and ESF, U.S.-European Collaboration, 1998, p. 44. 3   Image source: <http://www.stsci.edu/hst>. 4   See <http://cfa-www.harvard.edu/cfa/oir/research/swas.html>. 5   Image source: <http://cfa-www.harvard.edu/cfa/oir/research/swas.html>.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions larger and more capable Delta-class Explorers. A recent example of a Delta-class Explorer is the FUSE mission, which was launched in June 1999. At a total cost of $204 million, FUSE would be classified as a medium-size mission. NASA’s current flight mission program in space astronomy comprises mostly large new starts (such as SIM, NGST, Constellation X, and Terrestrial Planet Finder (TPF), each with total costs exceeding $550 million) or missions with total costs of less than $140 million. There are few current or planned missions in the $200 to $550 million range. The small number of true medium-sized missions has split science goals into those that are accommodated on small and MIDEX platforms and those that engage the broad community and result in programs such as NGST. Portfolio and Planning In addition to increasing the use of smaller missions, the move toward an FBC paradigm has led to changes in the process for choosing missions. In the past, NASA, through its advisory committees, working groups, and external and internal experts, chose the area of science to which a new mission would be devoted (e.g., the AO that resulted in the Rossi X-ray Timing Explorer (RXTE) mission called specifically for an X-ray timing mission). Under present policy, the AO calls for any mission that fits within the cost caps and is consistent with the general guidelines of the AO. The thematic approach to AOs allowed the science community to respond to certain scientific goals in NASA’s strategic plans, while the new approach, although it has yielded excellent scientific proposals, does not define scientific areas and therefore cannot be incorporated into long-term plans.