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The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 The Role of Small Missions in Planetary and Lunar Exploration 2 The Small-Mission Concept EXPLORERS IN ASTROPHYSICS, SPACE PHYSICS, AND EARTH SCIENCES In all disciplines of the space sciences except planetary exploration, scientific problems currently are addressed by missions that range in cost and complexity from small to large. Thus in astrophysics, projects with narrow objectives and limited lifetimes (e.g., the Submillimeter Wave Astronomy Satellite) exist side by side with other projects that are of higher cost and extended lifetimes (e.g., the Hubble Space Telescope). Similarly, NASA's major thrust in terrestrial studies-the comprehensive Earth Observing System-is complemented by missions with focused goals that fall under the Earth Probes budget line. Examples of the latter include the planned Total Ozone Mapping REPORT MENU NOTICE Spectrometer and the Tropical Rainfall Measuring Mission. MEMBERSHIP PREFACE Generally speaking, the Explorer line is judged to be a success. EXECUTIVE SUMMARY According to a 1986 report by the Committee on Space Astronomy and CHAPTER 1 Astrophysics, "the Explorer program has established an outstanding record of CHAPTER 2 CHAPTER 3 scientific accomplishments in a variety of space science fields including CHAPTER 4 astronomy and astrophysics, space plasma physics, and solar physics" and CHAPTER 5 "there is no doubt that the Explorer program has resulted in outstanding scientific APPENDIX discoveries and continues to contribute in a vital way to the progress of space research."1 These conclusions reflect those of an earlier assessment by the Committee on Solar and Space Physics, which commented that "science ideas of high priority can be addressed with Explorers" and that "a high frequency of flight opportunities is warranted."2 Moreover, the record of the past and plans for the near future testify to the high quality of innovative science that is achieved by peer-selected Explorer missions. The primary complaint from these communities is that flights are much too scarce compared with the numerous scientific problems that can be addressed by low-cost missions. This problem has been exacerbated in these disciplines because many Explorers have exceeded the original guidelines of the program in terms of development schedule and funding file:///C|/SSB_old_web/smlch2.html (1 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 limits.3-5 SMALL MISSIONS IN PLANETARY EXPLORATION In view of the success that other space science disciplines have enjoyed with low-cost missions having specific scientific, technical, and programmatic guidelines, COMPLEX addressed whether such missions are also appropriate for planetary exploration. In particular, COMPLEX considered how NASA's proposed Discovery program can be designed to bring the benefits of small missions to the planetary science program. At the outset COMPLEX notes that NASA has not yet established a program for low-cost planetary missions. Thus, any assessment of the scientific viability of such a program becomes problematic, because the full scope and scientific potential of missions satisfying these cost and time constraints remain uncertain. Nevertheless, the completed Clementine mission and the development of the NEAR and Mars Pathfinder missions provide some calibration as to the level of the returned science that might be expected from small planetary missions. Examples of typical programs that may be possible under the general heading of small planetary science missions include Earth-orbital telescopes, flyby or orbital missions, and in situ sampling probes. How these small missions can be utilized to address the primary objectives outlined in COMPLEX's Integrated Strategy6 is also discussed below. The list of possible mission types is not comprehensive, nor is it intended to imply any special priority for the topics discussed. Earth-orbital missions with planetary science objectives could carry out a variety of spectroscopic and imaging observations of solar system bodies, as well as contribute to the search for extrasolar planets. For example, the Earth-orbiting Infrared Astronomical Satellite (IRAS) and the International Ultraviolet Explorer (IUE), while not specifically designed for planetary studies, have made significant contributions to our knowledge of the solar system. Telescopes in Earth orbit are able to observe in wavelength regions (e.g., ultraviolet and infrared) unavailable to ground-based observatories owing to atmospheric absorption. Moreover, moderate-size Earth-orbital telescopes can achieve spatial resolutions better than those possible with ground-based telescopes, whose images are degraded by atmospheric turbulence; adaptive optics could, when fully mature, lessen this advantage. Requirements for observing solar system objects are often inconsistent, to some degree, with the operation of large, general-purpose astrophysical observatories such as the Hubble Space Telescope. Frequently, these large observatories operate under constraints that prevent observations of some solar system targets and make continuous surveillance of time-variable phenomena difficult. For these reasons, Earth-orbital telescopes dedicated to file:///C|/SSB_old_web/smlch2.html (2 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 solar system targets hold promise for achieving significant advances, while still meeting the goals of a low-cost, rapid-development, and focused program. The scientific benefits and the engineering feasibility of Earth-orbital missions have been amply proven by the Explorer line in astrophysics and space physics. Various flyby and orbital missions to solar system targets-planets, satellites, asteroids, and comets-will also be possible in a small-missions program. Indeed, many proponents of the Discovery approach hold that this type of mission actually promises the greatest science return for the funds invested. This class of mission can address significant issues such as the surface composition of rocky bodies, interior structure, geologic evolution (including impact history), studies of particles and fields, and the composition, dynamics, and thermal structure of atmospheres. The Clementine and NEAR missions fall into this class. Since these latter missions have returned or will likely return valuable data, they suggest the level of science that can be accomplished within the confines of a small-missions program. Many outstanding questions about the origin, evolution, and structure of atmospheres recognized in COMPLEX's Integrated Strategy can be addressed by small missions that make in situ measurements. These include determination of rare-gas and isotopic abundance ratios, measurement of atmospheric winds, specification of horizontal and vertical temperature profiles, and examination of atmospheric chemistry. In the past, such observations were made as part of larger multipurpose missions (e.g., Viking and Pioneer Venus). It may be a challenge to the innovation of the proposes of low-cost missions to provide comparable types of in situ instruments within the confines of a small-missions program. For example, will atmospheric sampling missions be restricted to inner solar system bodies, and are in situ measurements of solid surfaces possible to accomplish at' all within the Discovery constraints? Although it is clearly harder to fit sampling missions and flights to the outer solar system into Discovery's $150 million cost envelope, innovative ways to achieve these objectives may be attainable. Accordingly, since an important purpose of the Discovery program is to challenge technology, it is essential that such projects be considered in any open competition for funds in a small- missions program. RELATIONSHIP TO INTEGRATED STRATEGY Many of the ideas that have been proposed for small missions address widely recognized, fundamental objectives in solar system science and can answer key questions outlined in COMPLEX's Integrated Strategy. In this section COMPLEX considers whether small missions can accomplish priority objectives at those targets highlighted by the Integrated Strategy: comets, Mars, Jupiter, and the search for extrasolar planets. As an example, the Integrated Strategy file:///C|/SSB_old_web/smlch2.html (3 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 strongly recommends the detailed study of primitive bodies, especially comets. Small, lightweight spacecraft, built and operated under tight budgets, are well suited to rendezvous missions with such bodies, especially those on orbits that approach the Earth's. A mission that would fly alongside a comet, much as NEAR will do for an asteroid, can be envisaged. The highest-priority science for primitive bodies is detailed determination of bulk composition, which can then be used to delimit possible origin scenarios. Composition can be well constrained by remote sensing, but sample analysis- whether in situ or by returned materials-may be required for definitive results. Cometary missions ranging in complexity from coma analysis, through penetrators and coma sample return, and ultimately to surface sample return may be proposed or attempted as small missions; some of these may turn out to be too expensive to be executed as small missions. COMPLEX recalls that the return of a sample of a solar system body for analysis in terrestrial laboratories has been achieved-and then at tremendous expense-only by the U.S. Apollo and Soviet Luna programs. The continued study of Mars is another major priority of the Integrated Strategy. Small spacecraft with focused payloads could be employed to observe Mars from orbit. Remote sensing and compositional mapping of the surface, determination of atmospheric circulation and water vapor and dust transport, gravity and topography measurements, and aeronomy are important scientific goals that can be addressed in this way. Many of these issues were to be investigated by Mars Observer and may now be studied by the orbital component of the Mars Surveyor program. In addition, by deploying small landers, Mars Surveyor will focus on other high-priority science questions. Small, inexpensive landers-particularly if launched in clusters in cooperation with international partners-can address fundamental questions of Mars science. But small missions alone will not fulfill all the major science objectives for Mars. Some important objectives, as stated by COMPLEX, are studying Mars's climate history, chronology, and the evolution of possible organic compounds. These studies appear to require the use of instrumentation and technology (e.g., sample return and long-range rovers capable of complicated in situ analyses) incompatible with a small-missions approach. Nevertheless, most detailed exploration will require preliminary surveys, which are well suited to small missions. The Jupiter system (the planet, its magnetosphere, rings, and satellites) constitute the third high-priority target within our solar system, according to the Integrated Strategy. The distance and flight time to Jupiter may hinder implementation of a small-mission approach. However, some flyby missions to the jovian system may yield sufficient science, post-Galileo, to justify their cost.7 Moreover, Earth-orbiting telescopes may be effective in providing answers to some important questions. Nevertheless, the most significant advances in understanding the jovian satellites, rings, magnetosphere, or atmosphere (excluding probe studies) will likely require orbiters or landers, either of which file:///C|/SSB_old_web/smlch2.html (4 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 would add substantially to mission duration, complexity, and price. To some degree, the choice of priorities in the Integrated Strategy reflects an integration of important scientific questions posed by complex planetary targets, notably Mars and the Jupiter system. Accordingly, an individual small mission, with its strongly focused science objectives, can address only part of what the Integrated Strategy calls for at these two planets. The Integrated Strategy set as its fourth priority the detection and study of circumstellar disks and extrasolar planets. Small Earth-orbiting observatories may make valuable contributions to this effort by, for example, being able to infer circumstellar material (as IRAS did) or to detect extrasolar planets. However, it is likely, although much less studied, that larger, more expensive projects (such as long-baseline interferometers in space or on the Moon) will be necessary to acquire the level of detail on planetary orbits and atmospheric compositions ultimately called for in COMPLEX's recent recommendation. Before settling on the four priorities listed above, the Integrated Strategy surveys the major science issues across all solar system objects. This tabulation shows clearly that numerous important science questions exist outside the four main priority targets. In many cases, small missions are the most effective way to address these topics. From this perspective, a jovian-magnetosphere probe and a Mercury remote-sensing orbiter could, for example, be of comparable priority in a Discovery-type program. It thus appears that small missions can yield a valuable science return, whether addressing the primary targets listed in the Integrated Strategy or some of the more specific objectives described in the same document. THE NEED FOR A BALANCED PROGRAM IN PLANETARY AND LUNAR EXPLORATION The discussion above demonstrates that many high-priority scientific goals may be achievable with small missions. Nevertheless, as already described, not all high-priority scientific investigations fit within the restrictions of a small mission. For example, 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 mission. COMPLEX's Integrated Strategy also identifies the outer solar system (particularly, Neptune and Pluto/Charon) as the key to several questions about solar system origin and evolution. It is, in addition, fertile territory for studies of comparative planetology. 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 file:///C|/SSB_old_web/smlch2.html (5 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 technological innovation (e.g., development of new propulsion systems), small missions are not likely to contribute to this area of planetary science. Thus, it is to be expected that a small-missions program will emphasize the inner solar system. Even if it does prove feasible to investigate the outer solar system through a small-missions program, it may not be cost-effective to do so-that is, the use of small 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. Almost by definition, large missions such as Galileo and Cassini carry a comprehensive set of instruments. For studying complex phenomena, simultaneous measurements with a variety of instruments are therefore possible on big missions, but frequently not with more focused missions.8 Cassini, for example, carries instruments that will measure the solar and magnetospheric energy input into Titan's atmosphere, the chemical composition of the satellite's atmosphere, the distribution of aerosols in the atmosphere, and the physical state of the surface. These attributes of Titan's atmosphere and surface are all interlinked; accordingly, great scientific value can accrue from well-coordinated, contemporaneous measurements. Nevertheless, future comprehensive missions might benefit from application of some aspects of the philosophy of small missions, that is, use of streamlined management, innovative technology, and lightweight spacecraft. COMPLEX recommends that this approach be studied. ATTRIBUTES OF AN EFFECTIVE SMALL-MISSIONS PROGRAM FOR PLANETARY EXPLORATION Given that a program of small missions could play a valuable role in planetary exploration, what features should characterize such a program? Before listing desirable attributes, it is essential to stress that a reduction in mission scale must not be taken to imply any lessening in the quality of the science that must be produced: any space program should aim for nothing less than addressing the most important scientific objectives and use of the most capable instrumentation available, with missions being selected by fully open competition. The reason that care must be taken to ensure that only the highest-quality science is accepted is that, despite being less costly than most previous planetary spacecraft, Discovery missions still have significant costs so that, in a constrained budget, only very few will be flown. A small-missions program is able to focus on specific, well-defined scientific objectives with the expectation that definitive results will be produced, using the minimum investment of scarce resources. This is an important virtue. file:///C|/SSB_old_web/smlch2.html (6 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 An example might be analysis of the elemental composition of an asteroid surface, or precise measurement of trace constituents in a planetary atmosphere. Such experiments have traditionally been included as part of an instrument suite aboard a major mission but may prove more cost-effective if made the focus of a payload. For a population of solar system objects with significant interobject variability, such as asteroids or comets, any attempt to generate a comprehensive picture of the population would require a number of essentially identical, or very similar, missions; this requirement might be prohibitively expensive and/or complicated if fulfilled with typical planetary spacecraft but might become more practicable with small missions. A necessary corollary of utilizing small, focused missions to address planetary science goals is the requirement of a high flight rate. A high rate of flights is desirable from a scientific perspective because it allows the program as a whole to study a diverse set of targets. It is also desirable from a programmatic perspective as described in Chapter 3. A rate of one mission per year has been widely discussed in connection with the Discovery program and is strongly endorsed by COMPLEX. Small planetary missions create an opportunity to introduce management structures that differ from those used in the traditional large missions. One approach that has proven efficient and successful for instruments in larger missions, as well as for whole missions in other fields, is to have the work controlled by an individual principal investigator (PI) who proposes the science objectives and the instrumental approach to achieve them. The PI is best able to decide how to make the inevitable trade-offs throughout the project that would be in the best interest of achieving the science objectives within budget constraints. Making such trade-offs will require rigorous cost and schedule control in order to fit within the cost cap and a minimum of one launch per year, essential elements of any small-missions program. Small planetary missions can also bridge the programmatic gap between ground-based astronomy and more traditional deep-space missions. Small missions allow exploratory visits to close-by targets (such as the Moon) and smaller solar system objects, as well as the monitoring of temporal and spatial variability of the planets from Earth orbit. Furthermore, small missions can lay the groundwork for more comprehensive missions. In particular, ground-based observations, and indeed remote sensing from flyby missions (such as Giotto and Vega), have failed to provide the information about the physical properties of a comet nucleus that will be necessary input for the design of any comet-nucleus sample-return mission. A Discovery-class spacecraft could in principle supply such data, thus facilitating a more ambitious sample-return mission. By providing specific answers that may greatly elucidate previous information, small missions may also "fill in the gaps" left after larger programs have returned their data lode. The relatively brief development time of small planetary spacecraft, as in the proposed Discovery program, makes it feasible to address targets of opportunity, whereas a traditional mission lacks the requisite flexibility. For file:///C|/SSB_old_web/smlch2.html (7 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 example, a small mission might allow taking advantage of a particularly favorable arrangement of objects in space, or the opportunity to characterize more fully an unusual target, of which last year's impact of comet Shoemaker-Levy 9 with Jupiter is a dramatic example. Any small-missions program will provide a learning experience for the planetary community, industry, and NASA in what it takes to conduct successful low-cost missions. Although the choice of components (e.g., spacecraft bus) for such a mission must be left to the PI, a growing body of experience will be available to later proposers. Thus the scientific capabilities of small missions can be expected to grow with time, if, of course, rapid development, frequent launch rate, and steady funding are ensured. Planetary science contributes to NASA's overall mission of providing knowledge and demonstrating technical achievements in space. However, lengthening intervals between traditional solar system missions have made it increasingly difficult to maintain a vital work force in planetary science. A high flight rate will do much to maintain the skilled, experienced cadre of engineers and managers that is essential for a successful program. By according primary responsibility to Pls at universities, as proposed for at least some missions in the Discovery program, NASA will enhance the educational and training programs at those universities. The first-hand experience that many students will gain within such a program will strengthen the nation's technical expertise. Such a decentralized mission organization, located within an educational institution, will lend itself to even more wide-spread outreach, extending beyond the university to K-12 education. Finally, teaming among universities, industrial organizations, and NASA centers, as emphasized within the proposed Discovery program, will be useful in stimulating constructive interactions among those organizations. Clearly, cross- fertilization of this kind will benefit all partners and will increase the competitiveness of the U.S. space program. Furthermore, by fostering a management approach that is both interactive and more streamlined than has been customary in space research, a program of small missions can epitomize the new way of doing business within NASA. In summary, the attributes of an effective small-missions program include the following: The performance of the highest-quality science; The ability to address a broad spectrum of studies having tightly focused objectives; Reduced cost and fast turnaround; A high launch rate, preferably one per year; and file:///C|/SSB_old_web/smlch2.html (8 of 10) [6/18/2004 1:48:06 PM]
The Role of Small Missions in Planetary and Lunar Exploration: Chapter 2 Streamlined management with a principal investigator structure, minimized bureaucracy, and heightened cooperation among programmatic elements. REFERENCES 1. Space Science Board, National Research Council, The Explorer Program for Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1986, pp. I and 2. 2. Space Science Board, National Research Council, A Strategy for the Explorer Program for Solar and Space Physics, National Academy Press, Washington, D.C., 1984, page 5. 3. Board on Atmospheric Sciences and Climate and Space Studies Board, National Research Council, A Space Physics Paradox, National Academy Press, Washington, D.C., 1994. page 43. 4. Space Science Board, National Research Council, A Strategy for the Explorer Program for Solar and Space Physics, National Academy Press, Washington, D.C., 1984, page 9. 5. Space Science Board, National Research Council, The Explorer Program for Astronomy and Astrophysics, National Academy Press, Washington, D.C., 1986, page 7. 6. Space Studies Board, National Research Council, An Integrated Strategy for the Planetary Sciences: 1995-2010, National Academy Press, Washington, D.C., 1994. 7. Science Applications International Corp., Measure-Jupiter Mission Design Book, Nasa-Jet Propulsion Laboratory, Pasadena, Calif., 1994. 8. Space Studies Board, National Research Council, "Scientific Assessment of the CRAF and Cassini Missions," letter report from the Committee on Planetary and Lunar Exploration to Lennard Fisk (NASA), March 30, 1992. file:///C|/SSB_old_web/smlch2.html (9 of 10) [6/18/2004 1:48:06 PM]
