E

Material Provided by Space Studies Board Discipline Committees

Two of the assignments for the Space Studies Board discipline committees (see Appendix C) were to identify examples of ongoing and planned missions in different sizes that address near-term (10 years) and far-term (10 to 20 years) science objectives in the discipline, and to identify criteria for evaluating the mix of missions chosen for the portfolio. This appendix presents the material submitted by the board’s discipline committees in response to the assignment. In addition, each discipline summary includes a table showing a nonexhaustive set of ongoing and planned missions (including international missions) and their scientific objectives or parameters, life-cycle costs, size (as defined by NASA for the study), status, and estimated lifetimes.

EARTH SCIENCE

Arguments for a Portfolio of Mission Sizes

There are powerful arguments for having a broad portfolio of mission sizes to achieve near-term (10 years) and far-term (10 to 20 years) goals in Earth science. In each application, the scientific objectives and the specific mission goals will generally dictate the size range of the spacecraft. The need continues for the capabilities offered by several larger platforms such as NASA’s Earth Orbiting System Chemistry Satellite (EOS-CHEM) mission: multiple instruments, extensive redundancy, and long lifetimes. Smaller missions aimed at well-focused and relatively short-term objectives allow for separate new starts to maintain pace with emerging scientific interests and needs. NASA’s Earth System Science Pathfinder (ESSP) program is an excellent example of this small mission paradigm.1 Smaller spacecraft can also reduce the time to “first science,” if the instruments are already available (i.e., no development is required).

1  

The ESSP program was initiated by NASA in 1996. The program is intended to apply the spirit of FBC to the Earth Science Enterprise. Proposals submitted by the community in response to announcements of opportunity must offer principal-investigator-led, end-to-end flight missions within tight constraints. The proposed missions must also have (1) a significant and well-focused science objective, (2) a cap on the cost to NASA, typically $120 million, (3) minimal reliance on unproven or high-risk technology, (4) time to launch of less than 3 years, and (5) flight mission duration of nominally 2 years. NASA’s plan is to conduct ESSP opportunities every 2 years or so, selecting 2 missions each round.



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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions E Material Provided by Space Studies Board Discipline Committees Two of the assignments for the Space Studies Board discipline committees (see Appendix C) were to identify examples of ongoing and planned missions in different sizes that address near-term (10 years) and far-term (10 to 20 years) science objectives in the discipline, and to identify criteria for evaluating the mix of missions chosen for the portfolio. This appendix presents the material submitted by the board’s discipline committees in response to the assignment. In addition, each discipline summary includes a table showing a nonexhaustive set of ongoing and planned missions (including international missions) and their scientific objectives or parameters, life-cycle costs, size (as defined by NASA for the study), status, and estimated lifetimes. EARTH SCIENCE Arguments for a Portfolio of Mission Sizes There are powerful arguments for having a broad portfolio of mission sizes to achieve near-term (10 years) and far-term (10 to 20 years) goals in Earth science. In each application, the scientific objectives and the specific mission goals will generally dictate the size range of the spacecraft. The need continues for the capabilities offered by several larger platforms such as NASA’s Earth Orbiting System Chemistry Satellite (EOS-CHEM) mission: multiple instruments, extensive redundancy, and long lifetimes. Smaller missions aimed at well-focused and relatively short-term objectives allow for separate new starts to maintain pace with emerging scientific interests and needs. NASA’s Earth System Science Pathfinder (ESSP) program is an excellent example of this small mission paradigm.1 Smaller spacecraft can also reduce the time to “first science,” if the instruments are already available (i.e., no development is required). 1   The ESSP program was initiated by NASA in 1996. The program is intended to apply the spirit of FBC to the Earth Science Enterprise. Proposals submitted by the community in response to announcements of opportunity must offer principal-investigator-led, end-to-end flight missions within tight constraints. The proposed missions must also have (1) a significant and well-focused science objective, (2) a cap on the cost to NASA, typically $120 million, (3) minimal reliance on unproven or high-risk technology, (4) time to launch of less than 3 years, and (5) flight mission duration of nominally 2 years. NASA’s plan is to conduct ESSP opportunities every 2 years or so, selecting 2 missions each round.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Larger Spacecraft The principal arguments in support of larger spacecraft are (1) beneficial aggregation of instruments and (2) accommodation adequate to support large instruments. Increased science requirements or the need for more comprehensive data sets lead to significant growth in several instruments central to Earth science applications.2 This growth is driven by increases—sometimes by large factors—in the required spatial resolution, number of spectral bands, and signal amplitude resolution (bits per sample).3 These and similar enhancements call for larger apertures, higher data rates, greater power load, more severe thermal control requirements, and more stringent pointing requirements, among other things. The demand from the scientific community for increased performance has outstripped the ability of technology to keep pace.4 Smaller Spacecraft The principal arguments in support of smaller spacecraft are that (1) they suffice to meet modest, well-focused objectives, (2) they minimize the time to science, and (3) they reduce the cost of achieving certain science objectives. A significant corollary is that with more new programs, there is a greater potential to increase the number of investigators and students involved from start to finish in space-based Earth observation. Small satellites, and by implication less complex missions, tend to be most effective when directed toward well-focused, short-term objectives. They can provide a rapid response for some missions if the required instruments have already been developed, presumably through a separate, adequately funded preparatory program. The concept of small satellites may also, however, connote higher risk and development shortcuts.5 “Cheaper” should be interpreted as “more cost-effective” rather than “short-changed” if the faster-better-cheaper (FBC) philosophy is to have value in the long run. Small satellites also have the reputation of being short-lived, although there is no intrinsic reason for this to be so.6 New technology coupled with cost-effective design and implementation practices would make multiple small satellite options appealing alternatives to large, multi-instrument systems for certain science applications. However, these faster-better-cheaper and (sometimes) smaller missions must not be invented ad hoc but should fall within a comprehensive long-term science plan. 2   Space Studies Board, National Research Council, Earth Observations from Space: History, Promise, Reality, National Academy Press, Washington, D.C., 1995. 3   As noted in Chapter 3, research advances in long-term weather forecasting and the development of climate computer models have increased the requirements for horizontal and vertical resolution for temperature and moisture atmospheric profiles. The need for 1-km vertical resolution will necessitate larger instruments. 4   SSB, Earth Observations from Space, 1995. 5   See Space Studies Board, National Research Council, The Role of Small Satellites in NASA and NOAA Earth Observation Programs, National Academy Press, Washington, D.C., 2000, p. 54, which discusses the risks involved in employing small satellites: The small satellite approach carries with it several risks concerning scientific return: 1. Rapid development missions are often focused on “small” problems. Missions are not designed for long life and are sometimes viewed as “one shot” opportunities. 2. Missions employing small satellites are more likely to be developed as part of a program of technology demonstrations as opposed to a program in which the science return is paramount. 3. Small missions require a well-defined focus to keep them simple and the cost low. This approach may not work well for scientific studies that require measurements of many processes. 4. Data processing and distribution may be related to relatively lower priority, thus making it difficult for nonproject scientists to gain access to the data. This problem could be exacerbated in the case of missions led by a principal investigator (PI) should research investigations become centered on an individual’s personal scientific interests. 5. With more single-sensor missions, the proportion of funds spent on satellite hardware and launch vehicles will increase. Such funds might otherwise be spent on scientific research. 6   For example, one of the early and very small Transit navigation satellites maintained full operational status for 22 years.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions A satellite should be as large as necessary, but no larger than needed to carry out its mission; the overall program should be funded sufficiently, but no more than necessary to complete its tasks. Objective cost trade-offs can be used to determine the right size for a spacecraft.7 If budget is the dominant consideration, low-cost missions generally bear higher risk, both in development and in operation, as was made all too clear by Lewis and Clark and the more recent Mars missions.8 If reliability is compromised by a lack of redundancy or for other cost-reduction reasons, the spacecraft life may be curtailed. Some of the smaller, cheaper programs have been successful (e.g., QuikSCAT), but in general, short schedules and mission success depend on the availability of previously developed instruments and a baseline spacecraft as well as efficient and effective mission management and operations.9 Need for Long-Term Measurements Programs such as the Earth Observing System (EOS) that have long-term science objectives (the measurement of climate variables, for example) cannot be replaced by short-term limited missions. Long-term and broad-based systematic measurements require a long-term commitment. Such a commitment could be met by a mix of satellite sizes if well-planned and coordinated.10 The SSB’s Committee on Earth Studies recently completed an analysis of issues related to the transitioning of NASA research satellite instrumentation for NOAA’s operational use and conducted a workshop on joining the Integrated Program Office (IPO)/NPOESS and NASA/Earth Science Enterprise capabilities for climate research. Among the conclusions, the Committee on Earth Studies notes that climate studies require long-term measurements, revision and independent validation of the algorithms, ability to reprocess older data, and good characterization and calibration of instruments.11 7   SSB, Earth Observations from Space, 1995, p 134. 8   The Small Spacecraft Technology Initiative (SSTI) was developed by NASA’s Office of Space Access and Technology to advance the state of technology and reduce the costs associated with the design, integration, launch, and operation of small satellites. In July 1994, TRW and CTA Space Systems were each awarded a contract by NASA to design and launch small Earth-observing satellites, one named “Lewis,” the other named “Clark.” Both contracts called for substantial infusion of new technology into both payload and spacecraft bus and for delivery of the satellites to launch in only 24 months following contract start. Both missions were unsuccessful. In the case of Lewis, the satellite development was completed within the allotted 24-month period and, after a 1-year delay before its Athena-I (LMLV-I) launch vehicle was deemed flight-ready, the satellite was successfully placed into its initial orbit in August 1997 and was subsequently lost. The Clark mission suffered excessive schedule delays and projected cost growth, ultimately leading to termination of the contract. 9   “The often complex evaluation of whether the use of a small satellite is appropriate is driven by mission-specific requirements, including those related to the policy and execution of the program, fiscal constraints, and the scientific needs of the end users. Considering the many issues involved, the design of an overall mission architecture, whether for operational or research needs, requires a complete risk-benefit assessment for each particular mission. For some missions, a mixed fleet of small and large satellites may provide the most flexibility and robustness, but the exact nature of this mix will depend on mission requirements” (SSB, The Role of Small Satellites, 2000, p. 5). 10   An excellent example is provided by ongoing measurements of atmospheric ozone, particularly over Antarctica. TOMS, the Total Ozone Mapping Spectrometer, has flown on four different spacecraft since 1978. TOMS was one of several instruments on three previous large satellites, but the current mission is a small (295 kg) Earth probe. Since TOMS is only a 35-kg instrument, that option works well. From a climate point of view, however, ozone is only part of the story. Climate, or the trend in long-term weather, depends on atmospheric gases and particulates, clouds, moisture, temperature, and their circulation and interaction with the surface, especially the sea’s surface. Climate observations must track all of these key variables for many years. Several of the instruments required are considerably larger than can be accommodated on a small spacecraft. 11   Issues raised by the integration of research and diverse climate measurements into the operational NPOESS have been studied by the SSB Committee on Earth Studies. Its phase I report, Issues in the Integration of Research and Operational Satellite Systems for Climate Research: I. Science and Design, emphasizes the potential science value of operational weather observations and examines the fit between NPOESS and the climate requirements with respect to a set of eight environmental variables. Issues in the Integration of Research and Operational Satellite Systems for Climate Research: II. Implementation, which is forthcoming, focuses on technology and related spacecraft issues.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Example of a Portfolio of Mission Sizes Examples abound in the Earth sciences of a wide variety of Earth-observing missions and their satellites. Selected Earth observation missions drawn from NASA’s current plan12 are provided in Table E.1. Missions shown in the table are meant to be representative of the current and projected mix of satellite sizes. Terra, the first satellite in the EOS program to be launched, and its companions, EOS-PM (now Aqua) and EOS-CHEM, are survivors after a decade of recurrent revisions of NASA’s EOS plans. All three are large13 satellites, with a design life of 5 or 6 years. The original EOS plans for these missions spanned 15 years, to be implemented by a series of three nearly identical satellites in each sequence. All were to share a large common bus, which promised certain economies of scale to reduce their aggregate cost; however, the common-bus approach has been abandoned. Landsat-7 has a long history, the Landsat series having been returned to NASA after a complicated path originating at NASA in the 1970s, passing first to the commercial world, next to NOAA, and then back to NASA. It is part of a sequence of Earth-imaging systems that provide an important source of data for environmental and land process studies. Whether Landsat-like observations will continue beyond Landsat-7 is under discussion. The weather satellites GOES, POES, and NPOESS are first and foremost operational weather satellites. Their data, which now span more than 40 years, provide the most important space-based record of climate and environmental variables. These data are and will continue to be in demand for scientific studies. The NPOESS Preparatory Project satellite appears to be unique, providing a one-time opportunity to validate new technologies and instruments of interest for NASA research and NOAA’s operational activities. UARS, launched on the space shuttle in 1991, continues to provide an important series of upper atmospheric observations. Its large size is as much a reflection of the relatively unconstrained mass limitations of the shuttle as it is a consequence of its mission. The Ocean Topography Experiment (TOPEX)/Poseidon and Jason-1 are the two currently funded precision ocean altimeter missions in which the United States has a major role. SeaWiFS, an instrument flown in a precedent-setting commercial partnership with the private sector, provides an important source of ocean color data.14 EO-1, the first of the New Millennium satellites,15 is designed primarily as a technology demonstration platform; science (if any) is secondary. The Gravity Recovery and Climate Experiment (GRACE) (ESSP-2) is a good example of the faster-better-cheaper system encouraged by the ESSP program. Total mission costs are about twice those billed to NASA, thanks to major foreign contributions. GRACE would not be possible without extensive prior developments, including a large science base and an existing spacecraft design, Challenging Minisatellite Payload (CHAMP), for a European mission. Note also that the science objective of GRACE cannot be met by one satellite, as it takes two satellites to capture the level of detail in Earth’s gravity field to be mapped by GRACE. In turn, a twin-satellite mission would not be feasible, technically or financially, without the motivation afforded by the spirit of FBC. 12   Available at <http://www.earth.nasa.gov/missions/2002/index.html>. 13   Small, medium, and large are meant to convey total system cost, as defined by NASA. Because such numbers are often controversial and the cost may be complicated by extensive international participation, spacecraft mass is also listed, where known, to provide an objective indication of the size of each satellite. 14   NASA’s coastal zone color scanner (CZCS), hosted on a Nimbus-7 satellite in the 1970s, first proved the importance of such data. Neither CZCS nor its equivalent was adopted by NOAA as an operational instrument. 15   The New Millennium program is a technology demonstration program to validate technology in spaceflight that will lower the risks to future science missions using the technologies. The program draws on existing government-funded research and development efforts.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions TABLE E.1 Selected Earth Science Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation ONGOING QuikSCAT Ocean winds, sea ice storm patterns Small, 970 kg, $95 million Ongoing 1999-(2-yr design) SeaWiFS Ocean color (commercial system) N/Aa Ongoing 1997 TRMM Tropical rainfall Medium, $296 million 3,600 kg, Ongoing 1996- POES Meteorology/weather (original TIROS–NASA) Large, 2,250 kg Several in orbit 1960-2010 (2-yr design) GOES Meteorology/weather (original SMS–NASA) Large, 2,100 kg Several in orbit 1974- UARS Upper atmosphere (10 instruments) Large, 6,800 kg, $750 million Ongoing 1991- TOPEX/ Poseidon Ocean altimetry Large, 2,500 kg, $450 million Ongoing 1992-(5-yr design) Landsat-7 Medium-resolution, multiband Earth imager Large, 2,200 kg, $447 million Ongoing 1999-(5-yr design) Terra (EOS-AM) Earth and atmospheric study Large, 5,190 kg, $1.2 billion Ongoing 1999-(5-yr design) PLANNED ESSP-2 (GRACE) Gravity field mapping (two spacecraft constellation) and cooperation with Germany Small, NASA, $86 million; Germany, $45 million Planned 2001 (2-yr design) EO-1 Technology: land mapping instruments Medium, 530 kg, $162 million Planned 2000 ICESat Laser altimetry measuring ice-sheet topography and temporal changes, cloud and atmospheric Medium, $227 million Planned July 2001 Jason-1 Ocean altimetry Medium, Planned 2001 500 kg, (5-yr design) $160 million (French-led mission; NASA contribution, $94 million) Aqua (EOS-PM) Earth and atmospheric study Large, 3,120 kg, $880 million Planned 2001 (6-yr design)

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation EOS-CHEM Tropospheric chemistry Large, 2,970 kg, $670 million Planned 2002 (5-yr design) NPP Flight of opportunity, plus new instruments Large, Estimated $800 million: (5-yr design) $500 million from NASA; remainder from DOD and NOAA Planned 2005 NPOESS Meteorology/weather Large, $ N/Aa Planning 2008-2020 (10-yr design) aNA, not available. Criteria for Evaluating the Mission Mix Previous Space Studies Board reports identified several criteria that can be used for evaluating the balance of missions in the Earth science portfolio.16 In addition to being assessed on their individual scientific merit, missions are assessed based on the extent to which they do the following: Address the high-priority scientific goals;17 Serve the needs of the U.S. Global Change Research Program; Reflect the need for sustained, long-term observations as well as for short-term, focused research; Respond to the need to facilitate the transition from research to operations; Incorporate and justify appropriate new technology without being technology-driven; Involve the science community in overall planning; Allow for increased risk in individual, specialized missions; Balance spacecraft design against instrument reliability and mission time horizon; and Recognize the need for stable funding for a diligent, sustainable, and productive program. Several of the criteria underscore the need for long-term planning in the space-based Earth science program, as stated in two Space Studies Board reports: The planning process should be an orderly one that is aimed at minimizing the continual changes. The process also must include more attention to intricate issues associated with … ensuring the continuity of key long time-series measurements.18 and 16   Space Studies Board, National Research Council, “Report of the Task Group on Assessment of NASA’s Plans for Post-2002 Earth Observing Missions,” letter to Ghassem Asrar, associate administrator, Office of Earth Science, NASA, April 8, 1999, pp. 5-9. 17   Board on Sustainable Development, National Research Council, Global Environmental Change: Research Pathways for the Next Decade, National Academy Press, Washington, D.C., 1999. 18   Space Studies Board, National Research Council, “Report of the Task Group on Assessment of NASA’s Plans for Post-2002 Earth Observing Missions,” letter to Ghassem Asrar, associate administrator, Office of Earth Science, NASA, April 8, 1999, p. 15.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions … the CES [Committee on Earth Studies] believes that the needs of research in the Earth sciences and applications should not be continually deferred until the development of new, unproven technologies. The prospect of lower cost is always attractive, but the practice of placing new technology developments ahead of the conduct of basic and applied research has been disruptive to the Earth sciences for more than two decades.19 PLANETARY SCIENCES Arguments for a Portfolio of Mission Sizes The Space Studies Board’s strategy report for the planetary sciences, An Integrated Strategy for the Planetary Sciences: 1995-2010, covers a diversity of topics and objectives, including studies of protoplanetary disks, planetary systems about other stars, primitive bodies, the origin and evolution of life, the surfaces and interiors of solid bodies, and planetary atmospheres, rings, and magnetospheres.20 The scientific, technical, and operational aspects of planetary exploration require a range of mission sizes.21,22 Large missions are necessary to approach future high-priority scientific goals such as a sample return from a comet nucleus or from the surface of a planet or satellite, a comprehensive survey of a giant planet (with atmospheric and satellite probes), and extensive exploration of Mars in preparation for human missions.23 Small missions are critical for continuing the introduction and infusion of new technology and for addressing very tightly focused scientific goals. The current mix of planetary-science missions seems to reflect this balance. However, there is a growing emphasis on medium-size missions that is affecting the balance. Example of a Portfolio of Mission Sizes Table E.2 shows examples of planetary science missions that span the range of sizes. It includes planned missions as well as missions that are currently operating. Many of the ongoing and planned missions are medium-size missions (or nearly so) in the Discovery line. The few low-cost missions are either at the low end of Discovery (e.g., Contour) or are non-NASA or international missions for which the total costs are uncertain (e.g., Nozomi). Some missions (e.g., Europa Orbiter and Pluto-Kuiper Express) have officially been classified as large missions, but they are near the medium-size cap of $350 million. Many larger missions are either conglomerate missions (Mars Sample Return) or missions involving international collaboration (e.g., Rosetta or Mars Express), whose costs are uncertain. 19   SSB, Earth Observations from Space, 1995, p. 134. 20   Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994, pp. 3-6. 21   “… Priority scientific investigations [identified by the Integrated Strategy] can be addressed by the full gamut of techniques, including small (inexpensive) and large (expensive) robotic probes, ground- and space-based observatories, and laboratory studies and theoretical modeling” (SSB, Integrated Strategy, p. 186). 22   “Many diverse objects across the solar system must be studied to achieve the broad goals of planetary and lunar exploration [outlined by COMPLEX]. An effective program for lunar and planetary exploration also dictates a mix of mission sizes, ranging from comprehensive missions with multiple objectives, such as Galileo and Cassini, down to relatively low cost missions, such as those in the Discovery program” (SSB, The Role of Small Missions, 1995, p. 27). 23   “The long travel times between Earth and the outer solar system require long-lived components, specialized power systems, and complex, high-powered communications. This implies that, with current technology, any mission sent beyond the asteroid belt must be very capable. In addition, many [priority studies] require concurrent coordinated observations between the different components of a particular planet or comet (e.g., simultaneous in situ and remote-sensing observations of Titan’s atmosphere by Huygens and Cassini, respectively). Thus, COMPLEX believes that many solar system missions, especially those to the outer solar system, cannot be adequately accomplished by reconfiguration of large spacecraft into one or more small spacecraft” (SSB, Integrated Strategy, pp. 182-183).

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions TABLE E.2 Selected Planetary Exploration Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation ONGOING NEAR Asteroid rendezvous Medium, 503 kg, $224 million Ongoing 1996 4-yr cruise 1-yr operations Mars Global Surveyor Mars mapping mission Medium, 1,030 kg, $273 million Ongoing 1996 5 years Nozomi Atmosphere and ionosphere of Mars; U.S. contributed NMS instrument Medium, Japanese-led mission; U.S. contribution, $6 million Ongoing Launched 1998 Stardust Collect comet material Medium, 380 kg, $205 million Ongoing February 1999 7 years Galileo Jupiter orbiter and probe Large, $1,425 million Ongoing Launched 1988, in orbit 1995- Cassini Saturn system including Titan probe Large, 5,650 kg, $2,550 million Ongoing October 1997, over various timescales up to 11 years PLANNED Contour Imaging and spectral maps of three comets Small, 489 kg, $144 million Planned 2002 6 years, 3 flybys Genesis Solar wind sample return Medium, 648 kg, $216 million Planned January 2001 2-yr operations Messenger Mercury Orbiter Medium (Discovery), $339 million Planned Launch 2004 1-yr orbital operations Deep Impact Image subsurface of a comet Medium, 600 kg, $240 million Planned Launch January 2004 1.5 years Mars 2001 (orbiter and lander) Mars geochemical mapper Large, 1,460 kg, $415 million Planned 2001, 3-yr orbiter Europa Orbiter Europa search for oceans Large, 1,600 kg, $460 million   Planned Launch in 2003, 6 years Mars Sample Return (1 French orbiter, 2 U.S. landers) Mars sample return Large, 1,800 kg each, $1,100 million Planned 2005 and 2007 launches, sample return in 2010 Rosetta (ESA mission, NASA providing 4 instruments) Comet lander and surface investigation Large total (U.S. contribution, $39 million) Planned January 2003 10-yr cruise plus landed operations

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation Mars Express (ESA mission; NASA providing components of ASPERA-3 (Energetic Neutral Atoms Analyzer) plus other: radio frequency section, transmitter, antenna subsystems for the radar instruments) Interaction of solar wind with Mars atmosphere Large total (U.S. contribution, $6.6 million for ASPERA, $27 million for other contributions) Planned June 2003 3 years Pluto-Kuiper Express Surface and atmosphere Large, 225 kg, $354 million Planned 2004 More than 9 years Criteria for Evaluating the Mission Mix The following criteria are proposed for evaluating the current mix of missions for solar system exploration: Addresses high-priority scientific goals; Optimizes science return for the money spent; Exhibits compatibility between mission goals and scale; Demonstrates a balanced-risk strategy; Considers future application of new technologies; Shows balance between technology and science; Involves community in mission/instrument/technology selection; Promotes stable funding and continuous planetary exploration; Is consonant with Deep Space Network (DSN) and mission operations and data analysis (MO&DA) support; Uses diverse modes of mission implementation (principal-investigator-led, university-industry-NASA team, NASA-led); and Incorporates education and public outreach. SPACE AND SOLAR PHYSICS Arguments for a Portfolio of Mission Sizes Solar and space physics are mature sciences in which exploration, discovery, and observations have been carried out in space for more than four decades. The disciplines are now essentially beyond the exploration and discovery phases, although—remarkably—discoveries are still made. The disciplinary maturity requires that future missions must address sophisticated questions that require a coordinated and novel approach—typically involving multiple instruments and even multiple spacecraft—to measure the many physical variables involved and to separate spatial from temporal physical effects. In the past, the disciplines were driven by science questions and priorities and not by mission size per se. That approach—science-driven missions—led naturally to a portfolio of mission sizes that included small, medium, and large missions. As explained below, the diversity and maturity of Sun-Earth Connection (SEC) science and current scientific priorities, even though they are being implemented using FBC principles, continue to necessitate medium-size and occasionally large missions. Thus,

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions the new SEC Roadmap sets forth a community science plan that requires Explorers (small missions), Solar-Terrestrial Probes (medium-size missions), and Frontier Probes (medium and large missions) for implementation. Interdisciplinary missions to the planets (space physics and planetary science) would most likely require large missions (more than $350 million) unless conducted through the Discovery program. Example of a Portfolio of Mission Sizes Table E.3 includes an array of currently operating and planned missions in space and solar physics. Some of the missions are important both for addressing science questions and for understanding current space weather conditions. TABLE E.3 Selected Space Physics Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation ONGOING IMP-8 Near-Earth solar wind monitor Small (Explorer), $ n/a Ongoing Launched 10/25/73; far into extended-phase operations SAMPEX Observations of solar energetic particles, cosmic rays, precipitating relativistic electrons Small (SMEX), $80 million Ongoing Launched 7/3/92; continues to 2003 FAST Electron and ion acceleration, plasma dynamics above auroral zone Small (SMEX), $74 million Ongoing Launched 8/21/96, 1-year primary plus extended mission as permitted SNOE Nitric oxide density and variation in Earth’s upper atmosphere Small (UNEX), $12 million Ongoing Launched 2/26/98 TRACE Solar magnetic structures, heating of solar atmosphere, flare onsets Small (SMEX), $72 million Ongoing Launched 4/1/98, 1-year primary phase, in extended phase Geotail Magnetotail dynamics, the near-Earth neutral line, and the magnetopause Medium, $150 million (Japanese-led mission) Ongoing Launched 7/24/92, 2-year primary and extended phase through 2002 ACE Interplanetary particles, composition, energy spectrum, solar wind plasma Medium (Explorer), $215 million Ongoing Launched 8/25/97, 3-year primary plus 2-year extended mission Wind Comprehensive measurements of solar wind plasmas, fields, and radio waves upstream of Earth Large, $360 million Ongoing Launched 11/1/94, extended to 2003 SOHO Measurements of solar electromagnetic emissions, the solar interior, the inner heliosphere, and the solar wind (ESA-led mission) Large, $430 million, cost is U.S. portion only for instruments, launch, and tracking and missions support Ongoing Launched 12/2/95, now in extended phase through 2005

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation Polar Energy flow into magnetosphere and the ionosphere in polar regions Large, $420 million Ongoing Launched 2/25/96, extended to 2003 PLANNED HESSI Solar flare high-resolution spectroscopy and imaging (3 keV to 2 MeV) Small (SMEX), 281 kg, $76 million In development Launch 7/4/00, planned 3-year lifetime IMEX Dynamics of inner magnetosphere and storms Small (UNEX), 350 lb, $13 million Under study Launch 8/00 IMAGE Imaging magnetospheric plasma, boundary layers, and auroras Medium (MIDEX), 536 kg, $154 million Waiting for launch Launch 3/15/00, 2-yr primary plus extended phase TIMED Energy flow and dynamics in the 60- to 180-km region of Earth’s atmosphere, by remote sensing Medium (STP), $208 million In development Launch 7/01, 2 years of operation Solar B Solar magnetic field evolution at photosphere, lower corona Medium, 875 kg, $154 million (Japanese-led mission) Phase A/B Launch 2003 STEREO Observe stereoscopically CMEs and solar energetic particles from the photosphere to Earth Medium (STP), $318 million (2 spacecraft) Phase A/B Launch 2004 on a Taurus Magnetospheric Multiscale Turbulence, reconnecting plasma entry at plasma boundaries Medium (STP), $ N/Aa STD in process Launch 2005 on a Delta 7325 Global Electrodynamic Constellation Plasma and electrodynamic coupling in Earth’s upper atmosphere/ ionosphere Medium (STP), $ N/A STD in process Launch 2007 on a Delta 7325 Magnetotail Constellation 3-D dynamic imaging of the outer magnetosphere Medium (STP), $ N/A STD in process Launch 2008 on a Delta 7325 Solar Polar Imager Solar polar fields, origin of solar wind and activity cycle Medium (Frontier Probe), $ N/A Under study Launch 2010 Solar Probe Coronal heating, acceleration mechanism of solar wind Medium (Outer Planets), $156 million Planning, under study Launch 2010 (?) Interstellar Probe Explore outermost heliosphere and interaction with local instellar medium Large (Frontier Probe), $ N/A Under study Launch ~2010 CLUSTER II 3-D study of plasma using 4 identical spacecraft at magnetospheric boundaries and magnetotail, separating space and time variations Large (ESA/NASA mission, NASA contribution $14.3 million to replace the U.S. instruments) In development Launch June and July 2000 (by 2 Soyuz rockets) aN/A, not available.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Criteria for Evaluating the Mission Mix The recent Roadmap exercises conducted by the Sun-Earth Connection theme reflect the portfolio of mission sizes recommended by the solar and space physics science communities to address current scientific priorities.24 Determined and pragmatic efforts were made in these exercises to adhere to NASA’s expectations of FBC and NASA’s budgetary trends, with the result that most of the missions recommended were made to fit into the Solar-Terrestrial Probe (STP) program. An STP is capped at $250 million and has an anticipated launch rate of about one every 18 months. Quoting from the 1997 Roadmap document: The majority of the candidate missions described in the Roadmap would be implemented under NASA’s STP program, which offers a continuous sequence of flexible, cost-capped missions designed for the systematic study of the Sun-Earth system. The strategy embodied in the STP mission line is to use a creative blend of in-situ and remote sensing techniques and observations, often from multiple platforms, (i) to provide understanding of solar variability on time scales from a fraction of a second to many centuries, with an underlying activity cycle of approximately 11 years; and (ii) to determine cause (solar variability) and effect (planetary and heliospheric response) relationships over vast spatial scales. The latter objective generally requires innovative multi-spacecraft and/or missions operating concurrently.25 A portfolio of missions to carry out the scientific objectives in space and solar physics is reflected in Table E.3, which shows that both ongoing and planned missions span the full spectrum of small, medium, and large sizes. Small and medium missions in the plan have focused scientific objectives. Certain other scientific requirements can be met only by large missions: 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 outward 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 Arguments for a Portfolio of Mission Sizes The importance of having a mix of mission sizes and costs to pursue space astronomy and astrophysics has long been recognized by the astronomy and astrophysics community. For example, the previous astronomy and astrophysics survey committee (the Bahcall committee) noted in its 1991 report, The Decade of Discovery in Astronomy and Astrophysics, that a vigorous program in space astronomy and astrophysics requires a proper mix between small, moderate, and large missions.26 The goals identified by the Bahcall committee and NRC strategies27 are broad and diverse and can be answered in a cost-effective way only through a coordinated program. In addition to suiting the scientific and physical requirements, a mix of mission sizes provides for continuity and follow-up in the various subfields of space astronomy and astrophysics. To achieve that diversity, the Bahcall committee recommended that the Explorer program be substantially expanded to allow flying six Delta-class astronomy and astrophysics Explorer missions and five SMEX-class missions for astrophysics during the 1990s.28 Most of the Delta-class missions would fall into the medium cost category. 24   NASA, Office of Space Science, Sun-Earth Connection Roadmap: Strategic Planning for 2000-2025, 1999. 25   NASA, Sun-Earth Connection Roadmap: Strategic Planning for the Years 2000-2020, 1997. 26   National Research Council, Astronomy and Astrophysics Survey Committee, The Decade of Discovery in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1991, pp. 15-16. 27   See, for example, Space Studies Board, National Research Council, A New Science Strategy for Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1997. 28   National Research Council, The Decade of Discovery, 1991, p. 23.

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Example of a Portfolio of Mission Sizes Table E.4 lists current and planned missions in astronomy and astrophysics. As shown, there are several small and medium missions under development; few are currently operating. TABLE E.4 Selected Astronomy and Astrophysics Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation ONGOING SWAS Submillimeter spectrum molecules in star-forming regions Small, $97 million Ongoing December 1998-present ACE Particles, isotopic, elemental composition of planetary and interstellar space Medium, $203 million Ongoing August 1997 2-5 years FUSE Far-UV spectrum, deuterium, H2, hot gas Medium, $204 million Ongoing June 1999 3 years HST Optical, UV, and near-IR observations Large, $9.1 billion (including operations, data analysis, and use of shuttle) Ongoing April 1990-present Compton Gamma Ray Observatory Gamma-ray astrophysics Large, $ N/Aa Ongoing April 1991-present Chandra X-ray Observatory X rays, supernovae, compact stars, AGNs Large, $2,800 million Ongoing July 1999 5+ year lifetime XMM European-led mission with U.S. guest observer program X rays to faint flux limits Large, $ N/A Ongoing December 1999 PLANNED, Short-Term HETE-2 Gamma-ray bursts/fast response Small, 125 kg, $23 million Planned 2000 5-2 years MAP CMBR anisotropy <1 deg Small, 800 kg, $149 million Planned Fall 2000 2 years GALEX UV surveys/galaxy evolution Small, 280 kg, $76 million Planned 2 years CATSCAT Origin and nature of gamma-ray bursts Small (UNEX), $ N/A Planned July 2001 CHIPS EUV spectrum, hot local ISM Small, $12 million Planned April 2002 1 year

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Spacecraft Parameters/Goals Mission Size/Mass/ Life-Cycle Costs (real $) Status Time Scale of Observation Swift Gamma-ray bursts Medium, 1,270 kg, $163 million Planned 2003 3 years FAME All-sky stellar astrometry Medium, 1,030 kg, $162 million Planned 2004 5 years GLAST Gamma rays/AGNs, bursts, pulsars, supernovae remnants Medium, 4,500 kg, $330 million Planned September 2005 5 years SOFIA (aircraft) Suborbital infrared/star, planet formation Large, $1,351 million (including 20 years of operations) Planned November 2002 20 years GP-B Gyroscopes/test general relativity Large, 3,300 kg, $556 million Planned Fall 2001 SIRTF Infrared/brown dwarfs, protoplanetary disks, AGNS, distant galaxies Large, 905 kg, $880 million Planned December 2001 2.5-5 years ACCESS Cosmic ray experiment Large, $ N/A Planned 2005 PLANNED, Long-Term SIM Optical interferometer/parallax, proper motion, planet detection Large, 5,000 kg, $900 million est. Planned June 2006 5 years NGST Near-IR/high-redshift galaxies Large, 3,300 kg, $1,700 million Planned 2008 5 years LISA Interferometer/gravitational radiation with contributions from several countries Large, $ N/A Under study 2009 6 years Constellation-X X rays (imaging and spectroscopy) Large, $ N/A Planned 2010 3-5 years TPF IR interferometer/planet detection, processes related to star and planet formation, AGNs Large, $ N/A Planned 2011 >5 years FIRST/Planck Image the anisotropies of the cosmic background radiation field over the whole sky Large, $ N/A (ESA missions with NASA contributions) Planned 2003 for FIRST, 2007 for Planck aN/A, not available

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Assessment of Mission Size Trade-offs for NASA’s Earth and Space Science Missions Criteria for Evaluating the Mission Mix Mission selection criteria were well described in a 1986 report by the NASA Advisory Council29 and are excerpted below: Scientific merit Significance of the scientific objectives Potential for new discoveries and understanding Generality of interest; Programmatic considerations Feasibility and readiness Infrastructure requirements Cost effectiveness Institutional implications; and Societal and other implications Potential for stimulating technological development Contributions to scientific awareness of the public Contributions to international understanding Contributions to national pride and prestige. To evaluate the mix of mission sizes, the criteria need to be weighted according to the mission costs. For example, a large mission would need to have a very broad impact while a small mission might be selected to pursue a narrow science problem and to stimulate technological development. More risk could be accepted with medium-size and small missions. 29   NASA Advisory Council, Space and Earth Science Advisory Committee, The Crisis in Space and Earth Science: A Time for New Commitment, 1986, pp. 55-58.