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Solar and Space Physics and Its Role in Space Exploration Executive Summary In 2003, the National Research Council published the first decadal survey for Solar and Space Physics, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics (referred to here as the decadal survey report).1 The survey report recommended a research program for NASA and the National Science Foundation (NSF) that would also address the operational needs of NOAA and DOD. The report included a recommended suite of NASA missions, which were ordered by priority, presented in an appropriate sequence, and selected to fit within the expected resource profile for the next decade. In early 2004, NASA adopted major new goals for human and robotic exploration of the solar system,2 exploration that will depend, in part, on an ability to predict the space environment experienced by robotic and piloted exploring spacecraft. The purpose of this report is to consider solar and space physics priorities in light of the space exploration vision. In June 2004 the President’s Commission on Implementation of United States Space Exploration Policy (also known as the Aldridge Commission) issued a report in which it described a broad role for science in the context of space exploration.3 The report treated science as being both an intrinsic element of exploration and an enabling element: Finding 7 – The Commission finds implementing the space exploration vision will be enabled by scientific knowledge, and will enable compelling scientific opportunities to study Earth and its environs, the solar system, other planetary systems and the universe. The commission also presented a notional science research agenda that comprises the three broad themes of origins, evolution, and fate (see Appendix C). Research in solar and space physics appears centrally under the topic “temporal variations in solar output—monitoring and interpretation of space weather as relevant to consequence and predictability” as an element of the fate theme, and it contributes in key ways to many aspects of several components of the origins and evolution themes. In light of the commission’s findings, the Committee on the Assessment of the Role of Solar and Space Physics in NASA’s Space Exploration Initiative chose to interpret its charge in the broadest sense and to examine 1 National Research Council, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, The National Academies Press, Washington, D.C., 2003. 2 National Aeronautics and Space Administration, The Vision for Space Exploration, NP-2004-01-334-HQ, NASA, Washington, D.C., February 2004. 3 A Journey to Inspire, Innovate, and Discover: Report of the President’s Commission on Implementation of United States Space Exploration Policy, ISBN 0-16-073075-9, U.S. Government Printing Office, Washington, D.C., 2004.
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Solar and Space Physics and Its Role in Space Exploration the fundamental role of solar and space physics research both in scientific exploration and in support of enabling future exploration of the solar system. From a purely scientific perspective, it is notable that the solar system, and stellar systems in general, are rich in the dynamical behaviors of plasma, gas, and dust that are organized and affected by magnetic fields. These dynamical processes are ubiquitous in highly evolved stellar systems, such as our own, and they play important roles in their formation and evolution. Magnetic fields produced in rotating solid and gaseous planets in combination with ultraviolet and x-ray photons from the planetary system’s central stars create plasma environments called asterospheres, or in the Sun’s case, the heliosphere. In its present manifestation, the heliosphere is a fascinating corner of the universe, challenging our best scientific efforts to understand its diverse workings. Consequently this “local cosmos” is a laboratory for investigating the complex dynamics of active plasmas and fields that occur throughout the universe, from the smallest ionospheric scales to galactic scales.4 Close inspection and direct samplings within the heliosphere are essential parts of the investigations that cannot be carried out by a priori theoretical efforts alone. Finding 1. The field of solar and space physics is a vibrant area of scientific research. Solar and space physics research has broad importance to solar system exploration, astrophysics, and fundamental plasma physics and comprises key components of the Aldridge Commission’s main research themes of origins, evolution, and fate. Interplanetary space is far from empty—a dynamic solar wind flows from the Sun through the solar system, forming the heliosphere, a region that encompasses all the solar system and extends more than three times the average distance to Pluto. Gusts of energetic particles race through this wind, arising from acceleration processes at the Sun, in interplanetary space, in planetary magnetospheres, and outside our solar system (galactic cosmic rays). It is these fast particles that pose a threat to exploring astronauts. The magnetic fields of planets provide some protection from these cosmic rays, but the protection is limited and variable, and outside the planetary magnetospheres there is no protection at all. Thus, all objects in space—spacecraft, instrumentation, and humans—are exposed to potentially hazardous penetrating radiation, both photons (e.g., x-rays) and particles (e.g., protons and electrons). Just as changing atmospheric conditions on Earth lead to weather that affects human activities on the ground, the changing conditions in the solar atmosphere lead to variations in the space environment—space weather—that affect activities in space. The successful exploration of the solar system on the scale and scope envisioned in the new exploration vision will require a prediction capability sufficient to activate mitigation procedures during hazardous radiation events. The development of such a capability will require understanding of the global system of the Sun, interplanetary medium, and the planets. This is best achieved by a mixed program of applied space weather science and basic research. A balanced, integrated approach with a robust infrastructure that includes flight mission data analysis and research, supporting ground and suborbital research, and advanced technology development must be maintained. The strategy outlined in the solar and space physics decadal survey report was designed to accomplish these goals; the committee believes that NASA should retain a commitment to the achievement of the goals of the decadal survey. Indeed curtailing program elements that address the scientific building blocks of space weather research jeopardizes the goal of space weather prediction. However, in light of likely constraints on resources in future years, the committee offers findings and recommendations that address a realistic revision of mission timelines that will still permit a viable program. Space weather conditions throughout the heliosphere are controlled primarily by the Sun and by the solar wind and its interaction with the magnetic fields and/or ionospheres of the planets. While simple statistical statements (analogous to “March tends to be colder than June”) can be made as a result of empirical, short-term studies, accurate predictions (analogous to “a cold front will bring wind and rain late tomorrow afternoon”) will require longer-term studies of the underlying processes as well as of how the whole heliospheric system responds. Both basic science and applied studies are necessary components of a viable program that facilitates space weather predictions. 4 See National Research Council, Plasma Physics of the Local Cosmos, The National Academies Press, Washington, D.C., 2004.
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Solar and Space Physics and Its Role in Space Exploration Finding 2. Accurate, effective predictions of space weather throughout the solar system demand an understanding of the underlying physical processes that control the system. To enable exploration by robots and humans, we need to understand this global system through a balanced program of applied and basic science. NASA’s Sun-Earth Connection program depends on a balanced portfolio of spaceflight missions and of supporting programs and infrastructure, which is very much like the proverbial three-legged stool. There are two strategic mission lines—Living With a Star (LWS) and Solar Terrestrial Probes (STP)—and a coordinated set of supporting programs. LWS missions focus on observing the solar activity, from short-term dynamics to long-term evolution, that can affect Earth, as well as astronauts working and living in the near-Earth space environment. Solar Terrestrial Probes are focused on exploring the fundamental physical processes of plasma interactions in the solar system. A key assumption in the design of the LWS program was that the STP program would be in place to provide the basic research foundation from which the LWS program could draw to meet its more operationally oriented objectives. Neither set of missions alone can properly support the objectives of the exploration vision. Furthermore, neither set of spaceflight missions can succeed without the third leg of the stool. That leg provides the means to (1) conduct regular small Explorer missions that can react quickly to new scientific issues, foster innovation, and accept higher technical risk; (2) operate active spacecraft and analyze the LWS and STP mission data; and (3) conduct ground-based and suborbital research and technology development in direct support of ongoing and future spaceflight missions.5 Finding 3. To achieve the necessary global understanding, NASA needs a complement of missions in both the Living With a Star and the Solar Terrestrial Probes programs supported by robust programs for mission operations and data analysis, Explorers, suborbital flights, and supporting research and technology. The decadal survey report from the Solar and Space Physics Survey Committee recommended a carefully reasoned and prioritized program for addressing high-priority science issues within the constraints of what was understood to be an attainable timeline and budget plan (see Figure 3.1 (a) in Chapter 3 below). The integrated research strategy presented in the decadal survey for the period 2003 to 2013 is based on several key principles. First, addressing the scientific challenges that were identified in the survey report requires an integrated set of ground- and space-based experimental programs along with complementary theory and modeling initiatives. Second, because of the complexity of the overall solar-heliospheric system, the greatest gains will be achieved by a coordinated approach that addresses the various components of the system, where possible, in combination. Third, a mix of basic, targeted basic,6 and applied research is important so that the advances in knowledge and the application of that knowledge to societal problems can progress together. Finally, containing cost is an important consideration because the recommended program must be affordable within the anticipated budgets of the various federal agencies. Finding 4. The committee concurs with the principles that were employed for setting priorities in the decadal survey report and believes that those principles remain appropriate and relevant today. With those principles in mind, the decadal survey report recommended a specific sequence of high-priority programs as a strategy for solar and space physics in the next decade. To accomplish this task, the survey report presented an assessment of candidate projects in terms of their potential scientific impact (both in their own subdisciplines and for the field as a whole) and potential societal benefit (i.e., 5 For a full discussion of the roles and relationships of spaceflight missions to supporting research and technology programs, see National Research Council, Supporting Research and Data Analysis in NASA’s Science Programs, National Academy Press, Washington, D.C., 1998. 6 By “targeted basic” research the committee means research that is conducted at a relatively fundamental level but that is intended to provide the scientific basis for specific future applications. The term “strategic research” has sometimes been used synonymously.
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Solar and Space Physics and Its Role in Space Exploration with respect to space weather). The survey report also took into consideration the optimum affordable sequence of programs, what programs would benefit from being operational simultaneously, the technical maturity of missions in a planning phase, and what programs should have the highest priority in the event of budgetary limitations or other unforeseen circumstances that might limit the scope of the overall effort. The recommended sequence of missions was supported by a strong base of Explorer missions, mission operations and data analysis (MO&DA), suborbital activities, and supporting research and technology (SR&T) programs, which together provide the core strength of the Sun-Earth Connection (SEC) program research base. Finding 5. The committee concludes that, for an SEC program that properly fulfills its dual role of scientific exploration and of enabling future exploration of the solar system, the prioritized sequence recommended in the decadal survey report remains important, timely, and appropriate. Although the recommendations and schedule presented in the decadal survey report were formulated in 2002—before the adoption by NASA of the new exploration vision—the essential reasoning behind the conclusions of the survey report remains valid: to explore and characterize the solar system and to understand and predict the solar-planetary environment within which future exploration missions will take place requires a scientific approach that treats the environment as a complex, coupled system. The extension of exploration beyond the environment close to Earth will require accurate prediction of conditions that will be encountered. Without programs such as the STP mission line, which study the physical basis of space weather, the development of accurate predictive tools would be placed at serious risk. Recommendation 1. To achieve the goals of the exploration vision there must be a robust SEC program, including both the LWS and the STP mission lines, that studies the heliospheric system as a whole and that incorporates a balance of applied and basic science. A robust program of SEC research depends on four foundation programs—Explorers, MO&DA, the Suborbital program of flights, and SR&T—for basic research and for development of technologies and theoretical models. The vitality of the Explorer mission line depends on the orderly selection of a complement of Small Explorer (SMEX) and Medium-Class Explorer (MIDEX) missions. Recommendation 2. The programs that underpin the LWS and STP mission lines—MO&DA, Explorers, the Suborbital program, and SR&T—should continue at a pace and a level that will ensure that they can fill their vital roles in SEC research. In the event of a more constrained funding climate, the timing of near-term missions may have to be stretched out. The committee recognizes that there may be a need to re-evaluate the order and timing of far-term missions in light of the way the exploration initiative evolves while keeping in mind the full scientific context of the issues being addressed. Recommendation 3. The near-term priority and sequence of solar, heliospheric, and geospace missions should be maintained as recommended in the decadal survey report both for scientific reasons and for the purposes of the exploration vision. Even with an SEC program that preserves the priorities and sequence of recommended missions, there will be important consequences from delaying the pace at which missions are executed as a means of dealing with resource constraints. First, there will be losses of scientific synergy due to the fact that opportunities for simultaneous operation of complementary missions will be more difficult to achieve. Furthermore, a number of missions that were recommended in the decadal survey report will be deferred beyond the 10-year planning horizon. This could be the case for the Jupiter Polar Mission, Stereo Magnetospheric Imager, Magnetospheric Constellation, Solar Wind Sentinels, and Mars Aeronomy Probe. These issues will demand careful attention as NASA develops its overall plan for science in the exploration vision.
Representative terms from entire chapter: