What causes the complex and often violent activity on the nearest star, the Sun? What are the effects of solar activity on Earth and other planetary bodies, the interplanetary environment, and the local interstellar medium? How do the phenomena observed on the Sun and in the complex of plasma, energetic particles, and magnetic fields that fill the solar system relate to those seen in more distant astrophysical environments? These are some of the fundamental questions motivating research in solar and space physics. And although this research reflects the deep-seated human impulse to know and understand the workings of nature, the lessons learned also promise to yield important practical benefits in areas such as understanding the effects of global climate change and the impact of the solar-terrestrial environment on human technology.
As explained in the solar and space physics (SSP) decadal survey released in 2003, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics,1 the scientific challenges to be addressed by the solar and space physics community in the coming decade are as follows:
Understand the structure and dynamics of the Sun’s interior, the generation of solar magnetic fields, the origin of the solar cycle, the causes of solar activity, and the structure and dynamics of the corona;
Understand heliospheric structure, the distribution of magnetic fields and matter throughout the solar system, and the interaction of the solar atmosphere with the local interstellar medium;
Understand the space environment of Earth and other solar system bodies and their dynamical response to external and internal influences;
Understand the basic physical principles manifest in processes observed in solar and space plasmas; and
Develop a near-real-time predictive capability for understanding and quantifying the impact on human activity of dynamical processes at the Sun, in the interplanetary medium, and in Earth’s magnetosphere and ionosphere.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion 3 Applications of Nuclear Power and Propulsion in Solar and Space Physics: Background SCIENTIFIC AND PROGRAMMATIC CONTEXT The Goals of Solar and Space Physics What causes the complex and often violent activity on the nearest star, the Sun? What are the effects of solar activity on Earth and other planetary bodies, the interplanetary environment, and the local interstellar medium? How do the phenomena observed on the Sun and in the complex of plasma, energetic particles, and magnetic fields that fill the solar system relate to those seen in more distant astrophysical environments? These are some of the fundamental questions motivating research in solar and space physics. And although this research reflects the deep-seated human impulse to know and understand the workings of nature, the lessons learned also promise to yield important practical benefits in areas such as understanding the effects of global climate change and the impact of the solar-terrestrial environment on human technology. As explained in the solar and space physics (SSP) decadal survey released in 2003, The Sun to the Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics,1 the scientific challenges to be addressed by the solar and space physics community in the coming decade are as follows: Understand the structure and dynamics of the Sun’s interior, the generation of solar magnetic fields, the origin of the solar cycle, the causes of solar activity, and the structure and dynamics of the corona; Understand heliospheric structure, the distribution of magnetic fields and matter throughout the solar system, and the interaction of the solar atmosphere with the local interstellar medium; Understand the space environment of Earth and other solar system bodies and their dynamical response to external and internal influences; Understand the basic physical principles manifest in processes observed in solar and space plasmas; and Develop a near-real-time predictive capability for understanding and quantifying the impact on human activity of dynamical processes at the Sun, in the interplanetary medium, and in Earth’s magnetosphere and ionosphere.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion High-Priority Missions in the SSP Decadal Survey Overcoming these challenges will, as described in the SSP decadal survey, require a systems approach to theoretical, ground-based, and space-based research that encompasses the flight programs and focused campaigns of NASA, the ground-based and basic research programs of the National Science Foundation (NSF), and the complementary operational programs of other agencies such as the Department of Defense, the Department of Energy, and the National Oceanic and Atmospheric Administration. Elements of this program consist of large (>$400 million), medium (between $250 million and $400 million), and small (<$250 million) space- and ground-based projects backed up by theoretical, computational, and modeling activities and related research and data-analysis programs. The SSP decadal survey identified only one large (>$400 million) program, the Solar Probe, a spacecraft intended to study the heating and acceleration of the solar wind. It will do this through in situ measurements and some remote-sensing observations during one or two passes through the innermost region of the heliosphere (i.e., the region from ~0.3 AU to as close as 3 solar radii above the Sun’s surface). In addition, the SSP decadal survey selected nine moderate-cost ($250 million to $400 million) projects as being especially important. These are as follows, in priority order: Magnetospheric Multiscale. A four-spacecraft cluster to investigate magnetic reconnection, particle acceleration, and turbulence in magnetospheric boundary regions. Geospace Network. Two radiation-belt mapping spacecraft and two ionospheric mapping spacecraft to determine the global response of the geospace environment to solar storms. Jupiter Polar Mission. Polar-orbiting spacecraft to image the aurora, determine the electrodynamic properties of the Io flux tube, and identify magnetosphere-ionosphere coupling processes. Multispacecraft Heliospheric Mission. Four or more spacecraft with large separations in the ecliptic plane to determine the spatial structure and temporal evolution of coronal mass ejections and other solar-wind disturbances in the inner heliosphere. Geospace Electrodynamic Connections. Three to four spacecraft with propulsion for low-altitude excursions to investigate the coupling among the magnetosphere, the ionosphere, and the upper atmosphere. Suborbital Program. Sounding rockets, balloons, and aircraft, equipped with advanced instrumentation, to perform targeted studies of solar and space physics phenomena. Magnetospheric Constellation. Fifty to 100 nanosatellites to create dynamic images of magnetic fields and charged particles in the near magnetic tail of Earth. Solar Wind Sentinels. Three spacecraft with solar sails positioned at 0.98 AU to provide earlier warning than L1 monitors and to measure the spatial and temporal structure of coronal mass ejections, shocks, and solar-wind streams. Stereo Magnetospheric Imager. Two spacecraft providing stereo imaging of the plasmasphere, ring current, and radiation belts, along with multispectral imaging of the aurora. The small, space-based projects identified by the SSP decadal survey were as follows, in priority order: L1 Monitor. Continuation of solar-wind and interplanetary magnetic field monitoring, in support of Earth-orbiting space physics missions; Solar Orbiter. Instrument contributions to a European Space Agency spacecraft that periodically corotates with the Sun at 45 solar radii to investigate the magnetic structure and evolution of the solar corona; and University-Class Explorer. Revitalization of the so-called UnEx line of PI-led missions designed to provide frequent access to space for focused research projects. Recent Scientific Developments Major discoveries and significant trends since the release of the SSP decadal survey in 2003 can be summarized under the following headings:
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Priorities in Space Science Enabled by Nuclear Power and Propulsion The Sun, the inner heliosphere, and space weather; The outer heliosphere; Planetary magnetospheres; and Basic physical processes. The Sun, the Inner Heliosphere, and Space Weather In October–November 2003, the most intense solar flares ever detected by spacecraft occurred, saturating the Geostationary Operational Environmental Satellite (GOES) soft x-ray detectors. The Reuven Ramaty High Energy Solar Spectroscopic Imager (RHESSI), a Small Explorer mission, imaged, for the first time, flare gamma-ray line emissions produced by energetic ions at the Sun. The fast coronal mass ejections (CMEs) and shock waves associated with these flares accelerated solar energetic particles (SEPs) to relativistic energies in the inner heliosphere. The numbers of SEPs observed at 1 AU by the Advanced Composition Explorer are surprisingly similar to the numbers inferred by RHESSI for energetic particles at the Sun. These CMEs and shock waves then propagated outward and were observed by all operating spacecraft in the heliosphere, including Cassini, Ulysses, and Voyager. In late July 2004, Earth’s radiation belts were pumped up to the highest levels ever observed, with relativistic (>2 MeV) electron fluxes, several times the maximum fluxes recorded during the last >18 years. These new observations provided a severe test of researchers’ understanding of the interconnections of space weather and how solar effects are manifested throughout the heliosphere and in Earth’s magnetosphere. Missions currently under development, including the Solar Dynamics Observatory and the Solar Terrestrial Relations Observatory, promise to provide more important vantage points for understanding this complex system. The ultimate proof that solar effects can be accurately modeled throughout the global heliosphere will come from consideration of conditions at the inner boundary (obtained from the data collected by Solar Probe, which will also provide ground truth for remote solar observations), and propagating these effects to an interstellar probe in the outer heliosphere and the local interstellar medium. In 2004, NASA’s Living with a Star program established a science and technology definition team for the Solar Sentinels mission that will explore how space weather is influenced by events on the Sun and conditions in the inner heliosphere. The long-term goal is to reach a level of understanding sufficient to accurately model solar effects throughout the global heliosphere. The Outer Heliosphere Observations by the Voyager spacecraft since the 2003 SSP decadal survey may be consistent with multiple observations of the termination shock of the solar wind. Although particle spectra and composition appear consistent with a crossing into the shocked solar wind, the lack of magnetic and plasma wave signatures has led to controversy about how the observations should be interpreted. These new observations have shown that: Scientists are likely on the verge of definitively determining the size of the unshocked solar wind cavity; and The dynamics of the outer heliosphere remain a mystery and reinforce the SSP decadal survey’s emphasis on characterizing this region far from the Sun and understanding how the heliosphere interacts with the very local interstellar medium (i.e., the region within some 2,000 AU of the Sun). These goals are further explored in the report of a recent NRC workshop.2 Planetary Magnetospheres The space environment of Jupiter has been probed with separate but simultaneous measurements by the Galileo and the Cassini spacecraft during the latter’s flyby of that planet in late 2000. In showing that interplanetary shocks drive magnetospheric dynamics, these measurements have reemphasized the need for multiple-point space physics measurements at the outer planets. In addition, combining auroral imaging with in situ measurements has yielded new information on how shocks affect auroral emissions. Finally, the new technique of neutral
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Priorities in Space Science Enabled by Nuclear Power and Propulsion atom imaging for obtaining global views of space physics environments—which was pioneered on Imager for Magnetopause-to-Aurora Global Exploration (the first Midsize Explorer mission) spacecraft at Earth—provided new insight on the jovian system structure (as seen from Cassini during its flyby) and promises even more insight into the complexities of the Saturn system as the Cassini orbital tour evolves. The continuing observations of Sun-grazing comets are changing perceptions of the importance of the origins and evolution of dust in the inner heliosphere, a topic that is being reviewed during the development of the Solar Probe. Basic Physical Processes Recently, the Polar and the Wind spacecraft made the first observations inside the magnetic reconnection regions at the dayside magnetopause and in the distant magnetotail, allowing for in situ study of the fundamental plasma process, which controls the dynamics of the magnetosphere (and, by extension, solar flares and other astrophysical phenomena). These spacecraft penetrated the ion diffusion region (and possibly the electron diffusion region) and detected the Hall fields that are predicted by theory and simulation. Multispacecraft observations with much higher temporal resolution and accuracy, such as those planned for the Magnetospheric Multiscale mission, are required to understand the microphysics controlling this process. Programmatic Context The scientific goals of the solar and space physics community, as articulated in the SSP decadal survey, remain essentially unchanged by the recent discoveries just outlined. There has, however, been a major change in the direction of U.S. space activities as a result of policy responses to President Bush’s Vision for Space Exploration, announced on January 14, 2004.3,4 This new exploration initiative comes at a time when the solar and space physics community is rapidly developing a connected view of deep-space radiation systems. Researchers are working to understand the network of space radiation environments, to develop capabilities to warn of radiation hazards, to understand how space radiation has affected Earth’s climate and biosphere in the past, and to project the effects of these radiation environments on future global sustainability. Solar and space physicists are striving to understand how steady and punctuated ejections of matter and energy from the Sun propagate through the solar system and in many cases form shocks that cause large transient changes in the radiation environments at Earth and other planets. Researchers are also learning how the conditions in the local interstellar medium control the radiation environment in the solar system and may change the solar system’s radiation environment in the future. This development will be critical for understanding not only the radiation environment of interplanetary space, but also the radiation system of Earth and its impact on global sustainability. Key to all of these developments are spacecraft observations of solar phenomena and their effects both at Earth and at many other locations throughout the solar system. Some researchers would even argue that an inability to monitor the solar system’s space environment significantly threatens the viability of the President’s exploration initiative. Indeed, without the ability to assess, monitor, and develop advanced warning capabilities for space radiation hazards, extended voyages by crews beyond low Earth orbit may not be realizable.5 A substantial improvement in understanding of the interconnectivity of the solar system radiation environment will enhance the ability to provide advanced warning of radiation hazards, and will provide more detailed knowledge of the radiation environments near the surfaces of the Moon, Mars, and other planets in the solar system. Because of the essential role that solar and space physics is likely to play in supporting the exploration initiative, the discipline’s scientific priorities are being reexamined and a strategy is being developed for the solar and space physics community to engage fully in this initiative.6 At issue in this report are the new space science territories that will be opened up by the availability of nuclear power and propulsion systems. Space physicists are familiar with radioisotope power systems (RPSs) from their use on spacecraft—e.g., Pioneer 10 and 11, Voyager 1 and 2, and Ulysses—and will likely make use of RPSs again on future missions such as the Solar Probe and the Interstellar Probe (see Chapter 4). In contrast, the use of fission reactors for power and propulsion is something new to the space science community. Although technologies such as nuclear-electric propulsion will likely open up new capabilities and enable types of missions not previously
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Priorities in Space Science Enabled by Nuclear Power and Propulsion envisaged, this is likely to entail a programmatic cost. Given the budgetary pressures facing the solar and space physics community, advanced nuclear systems are likely to enhance research capabilities only if they do not displace the small and medium-sized missions that are so important both to the health and the vitality of the community and to the implementation of the exploration initiative. IMPLEMENTATION AND TECHNIQUES A variety of implementation features common to many solar and space physics missions are rather different from those typical of astronomical or planetary missions. The characteristics and requirements detailed below influence the application of fission reactors and RPSs to space physics missions as well as to many other aspects of mission design. Mission concepts highlighting some of these characteristics are described in Chapter 4. Features important for the implementation of solar and space physics goals include the following: In situ observations are important. Space physics has traditionally been dominated by in situ observation of solar, heliospheric, and magnetospheric phenomena. As the discipline has matured, the ability to access new environments near the Sun and in the solar system and beyond will become increasingly important. Nuclear propulsion systems clearly have a role to play in enabling access to observing locations not easily accessible using existing chemical propulsion systems. Another trend seen in recent years has been the development and increasing importance of remote-sensing techniques—for example, energetic neutral atom (ENA) imaging—as opposed to the in situ techniques that have traditionally dominated space physics. Space physics instruments do not, in general, require fine pointing accuracy or knowledge; generally a pointing knowledge of ~0.1° to 1° will suffice. In situ and remote-sensing instruments are prominent features of most, if not all, of the space physics mission concepts described in Chapter 4. Mass, power, and data rates are modest. There is considerable flexibility in the design of in situ sensors, and in many circumstances considerable progress can be made using instruments with modest mass and power requirements—generally less than ~10 kg and 10W, depending on the instrument—delivered to suitable orbits in the heliosphere. Although it is true that some investigations of microphysics occurring on short timescales demand high instantaneous data rates, the use of onboard data storage means that only low-to-moderate average data rates—say 10 kilobits per second (kbps)—have traditionally been used. On the other hand, processes such as cosmic ray modulation can be studied using data averaged over periods of days or longer, and require modest telemetry rates. Abundant electrical power from nuclear systems potentially opens up the possibility of using new types of instruments, such as those that actively probe extraterrestrial particle and field environments by the use of such techniques as incoherent scatter radar. The use of these and related approaches has, to date, generally been limited to terrestrial applications because the relevant instruments have very large power requirements. The availability of abundant power can also enable larger datasets to be returned to Earth than have typically been used in space physics experiments. One possible benefit of such improved data transmission rates is the ability to routinely sample all three electric and magnetic components of plasma waves, together with three-dimensional plasma distribution functions, rather than limited data subsets as is now typical. This complete set of measurements allows for direct determination of the source and the nature of wave-particle interactions such as sources of free energy for the waves or pitch-angle scattering of the charged particles by the waves. The ability to return complete datasets of this type and/or the inclusion of active instruments would likely be features of concepts such as the Neptune-Triton System Explorer mission (see Box 6.5). Space physics experiments as secondary payloads. The modest power, mass, and data rate requirements of most space physics instruments make them ideally suited for inclusion as secondary payloads on solar system exploration missions. Because many space physics investigations address interplanetary phenomena, the ability to conduct cruise-phase investigations on planetary missions has been particularly important for the development of this discipline. The ability of nuclear propulsion systems to transport comprehensive scientific payloads to diverse planetary environments (see Chapter 6) will necessarily increase the available opportunities to include secondary space physics instruments. Moreover, the potential development of a new generation of small RPSs (see Table 1.3) creates additional opportunities since they could, for example, be used to power small, deployable subsatellites.7
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Priorities in Space Science Enabled by Nuclear Power and Propulsion The Titan Express/Interstellar Pioneer concept (see Box 6.4) is an example of a mission in which an elaborate, secondary payload addressing space physics goals is piggybacked onto what is primarily a solar system exploration mission. Diverse locations must be accessed. Space physics instruments have to be delivered to widely separated and diverse locations, including the corona within a few solar radii of the Sun’s surface, high heliographic latitudes, planetary magnetospheres, regions of interaction of the solar-wind plasma with planets and their satellites, regions of interplanetary space far removed from any planet, and the distant boundary of the heliosphere and beyond. Nuclear propulsion systems clearly have an important role to play in enabling access to observing locations that may be too difficult, if not impossible, to access using more conventional propulsion systems. The Interstellar Observatory (see Box 4.1), the Solar Coronal Cluster (see Box 4.2) and the Solar System Disk Explorer (see Box 4.3) are examples of missions using nuclear-electric propulsion systems to place scientific payloads in locations—e.g., the outermost and innermost regions of the heliosphere—that might be inaccessible otherwise. Simultaneous multipoint measurements are advantageous. It is commonly the case that simultaneous measurements at different locations are needed to resolve basic physical processes. Using similar instrumentation on multiple spacecraft to conduct simultaneous observations is enabling because it allows for the resolution of space-time ambiguities that are characteristic of single-point measurements, and it provides stereoscopic viewing for understanding inherently three-dimensional structure and dynamics. The results from the Cluster 2 mission indicate that the use of four probes can provide fundamentally new information through global views of large dynamical systems. For example, to resolve microphysical processes such as turbulence and reconnection requires measurements from multiple spacecraft separated by short distances in what typically are relatively compact three-dimensional regions. The Jupiter Magnetosphere Multiprobe Mission (see Box 4.4) is an example of a concept that combines the propulsive capacities of nuclear-electric propulsion with the capabilities of RPSs to enable multipoint observations of the global structure of the jovian magnetosphere. Observing timescales are extensive. Solar and space physics missions address phenomena that vary on a wide range of timescales from sub-millisecond plasma processes through decade-long variations in solar, heliospheric, and magnetospheric environments driven by the Sun’s 22-year magnetic cycle. In general, a statistically significant description of such phenomena requires long-term observations with relatively few data gaps. These observations have most often been achieved using dedicated space physics missions—e.g., Interplanetary Monitoring Platform 8—and by cruise-phase operation of space physics payloads on planetary spacecraft such as Voyager 1 and 2. The demonstrated multidecade lifetimes of RPSs and the potentially long life of reactor-based power systems are ideally suited to support extended-duration observations. Environmental factors must be controlled. In situ measurements generally depend on sensitive instruments requiring careful control of interference and background. This has important implications for spacecraft design in areas such as control of spacecraft charging and protection from stray magnetic fields, electromagnetic emissions, and ionizing radiation. A nuclear-powered spacecraft may have requirements for long booms, shielding, or other measures to mitigate the potentially detrimental effects of the spacecraft’s power source and other systems on its payload. In summary, space physics observations are typically made using small, focused payloads that require limited spacecraft resources. However, experience indicates that the science return could benefit significantly from spacecraft with more robust mass, power, volume, telemetry, and propulsion capabilities enabled by nuclear power and propulsion systems. Solar and space physics priorities will also advance through joint initiatives between solar and space physicists and astronomers, astrophysicists, and planetary scientists. REFERENCES 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 Research Council, Exploration of the Outer Heliosphere and the Local Interstellar Medium, The National Academies Press, Washington, D.C., 2004.
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Priorities in Space Science Enabled by Nuclear Power and Propulsion 3. National Aeronautics and Space Administration, The Vision for Space Exploration: February 2004, National Aeronautics and Space Administration, Washington, D.C., 2004. 4. President’s Commission on Implementation of United States Space Exploration Policy, A Journey to Inspire, Innovate, and Discover, U.S. Government Printing Office, Washington, D.C., 2004. 5. See, for example, National Research Council, Scientific Prerequisites for the Human Exploration of Space, National Academy Press, Washington, D.C., 1993, pp. 3 and 17–24. 6. See, for example, National Research Council, Solar and Space Physics and Its Role in Space Exploration, The National Academies Press, Washington, D.C., 2004. 7. See, for example, “Subsatellite Missions,” section 2.5 of Enabling Exploration with Small Radioisotope Power Systems, R.D. Ableson, ed., JPL Publication 04-10, Jet Propulsion Laboratory, Pasadena, Calif., 2004.