1
Introduction

The Sun is the source of energy for life on Earth and is the strongest modulator of the human physical environment. In fact, the Sun’s influence extends throughout the solar system, both through photons, which provide heat, light, and ionization, and through the continuous outflow of a magnetized, supersonic ionized gas known as the solar wind. The realm of the solar wind, which includes the entire solar system, is called the heliosphere. In the broadest sense, the heliosphere is a vast interconnected system of fast-moving structures, streams, and shock waves that encounter a great variety of planetary and small-body surfaces, atmospheres, and magnetic fields. Somewhere far beyond the orbit of Pluto, the solar wind is finally stopped by its interaction with the interstellar medium…. (From The Sun to the Earth—and Beyond, p.11)

Space is far from empty—an often gusty solar wind flows from the Sun through interplanetary space, forming the heliosphere (see Figure 1.1 and Box 1.1). Bursts of energetic particles (also known as cosmic rays) arise from acceleration processes at or near the Sun and race through this wind, traveling through interplanetary space, impacting planetary magnetospheres, and finally penetrating beyond our solar system. 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.

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). 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 an expected resource profile during 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 our ability to predict the space weather experienced by exploring spacecraft. The purpose of this report is to consider research priorities in the light of the space exploration vision.

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., 2004.



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement



Below are the first 10 and last 10 pages of uncorrected machine-read text (when available) of this chapter, followed by the top 30 algorithmically extracted key phrases from the chapter as a whole.
Intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text on the opening pages of each chapter. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

Do not use for reproduction, copying, pasting, or reading; exclusively for search engines.

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration 1 Introduction The Sun is the source of energy for life on Earth and is the strongest modulator of the human physical environment. In fact, the Sun’s influence extends throughout the solar system, both through photons, which provide heat, light, and ionization, and through the continuous outflow of a magnetized, supersonic ionized gas known as the solar wind. The realm of the solar wind, which includes the entire solar system, is called the heliosphere. In the broadest sense, the heliosphere is a vast interconnected system of fast-moving structures, streams, and shock waves that encounter a great variety of planetary and small-body surfaces, atmospheres, and magnetic fields. Somewhere far beyond the orbit of Pluto, the solar wind is finally stopped by its interaction with the interstellar medium…. (From The Sun to the Earth—and Beyond, p.11) Space is far from empty—an often gusty solar wind flows from the Sun through interplanetary space, forming the heliosphere (see Figure 1.1 and Box 1.1). Bursts of energetic particles (also known as cosmic rays) arise from acceleration processes at or near the Sun and race through this wind, traveling through interplanetary space, impacting planetary magnetospheres, and finally penetrating beyond our solar system. 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. 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). 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 an expected resource profile during 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 our ability to predict the space weather experienced by exploring spacecraft. The purpose of this report is to consider research priorities in the light of the space exploration vision. 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., 2004.

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration FIGURE 1.1 The heliospheric system—the Sun, the solar wind and space environment of Earth (lower right), the Moon (bottom), and Mars (upper right). This sketch is not to scale; for example, in reality the Sun is 100 Earth-diameters across and the Sun-Earth distance is 108 solar-diameters; Mars is half the size of Earth and 1.5 times farther from the Sun. The report of the President’s Commission on Implementation of United States Space Exploration Policy—A Journey to Inspire, Innovate, and Discover (the Aldridge Commission report)3—set forth 15 recommendations to address factors critical to achieving NASA’s vision for space exploration. The commission report considered science in two contexts: enabling science, which is research that provides new knowledge or capability that facilitates exploration, and enabled science, which is research to create new knowledge by means of exploration.4 The report also organized basic science around three themes—origins, evolution, and fate—that are defined broadly and that include exploration to understand the origin and evolution of the universe, the formation of planets and planetary systems, the origin and extent of life, and the environment and habitability of our own Earth (see Appendix C). That concept for a research agenda in the context of exploration explicitly includes (under “fate”) studies of temporal 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. 4   Finding 7 from the commission report (p. 36) states, “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.”

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration BOX 1.1 Energetic Particles in Space The gas in space is a composite of several distinct classes of particles. In the interplanetary environment the dominant class is the solar wind (mostly ionized hydrogen, i.e., protons and electrons) that blows outward from the expanding corona of the Sun at supersonic velocities of 400 to 1000 km/s to fill the solar system with a hot, dilute plasma. This high-speed plasma not only fills interplanetary space but also controls the energy that drives aspects of space weather. These aspects include the very energetic and intense radiation belt particles that populate planetary environments, such as that of Earth and Jupiter, and the electrical currents and auroral particle acceleration that also characterize planetary environments. A second important class comprises galactic cosmic rays, moving at close to the speed of light (c) and infiltrating in through the magnetic fields in the solar wind from the surrounding interstellar space. They are primarily protons plus a smaller number of heavier nuclei and a few electrons. Galactic cosmic rays are always present, although their intensity in the inner solar system is reduced somewhat as the solar wind drags the Sun’s magnetic field out through interplanetary space. Outside the protecting magnetic field and atmosphere of Earth each square centimeter (about the area of a fingernail) is penetrated once or twice per second by a cosmic-ray proton. The lowest-energy cosmic rays (0.1 to 1.0 GeV, velocities of 0.4 to 0.9 c) are strongly suppressed during the years of maximum activity in the sunspot cycle. Above 1 GeV the number of cosmic-ray particles, and their reduction by the solar wind, decline rapidly with increasing energy. At 20 GeV (0.999 c) the reduction is at most only a few percent. The particles above 1 GeV pose a particularly difficult problem for human interplanetary travel, because their enormous energy makes them difficult to shield against. Upon collision with the nucleus of an atom, for example, in Earth’s atmosphere or a spacecraft wall, a proton of 1 GeV or more produces many secondary fast particles (pions, gamma rays, electron-positron pairs, protons, and neutrons), which in turn create more fast particles as they collide with other nuclei. Therefore, the first 50 to 100 gm/cm2of shielding serves only to increase the number of fast particles. The higher the initial proton energy, the worse this becomes. Fortunately, the 1000 gm/cm2 represented by the full terrestrial atmosphere is enough to stop most of the secondary particles, except for the neutron component and the muons. This provides adequate protection here at the surface of Earth. Out in space, however, devising a practical means for protecting astronauts remains a major technical challenge. Finally, there are the energetic particles emitted by flares on the Sun, or accelerated in shock fronts near the Sun and in interplanetary space, that are typically referred to as solar energetic particles or solar cosmic rays. These particles (mostly protons, a few heavier nuclei, and some electrons) are usually at much lower energies (10 MeV to 10 GeV) than the galactic cosmic rays. However, their enormous numbers can do fatal damage to exposed electronics and astronauts. The problem is that these solar cosmic rays are highly variable and appear intermittently in unanticipated intense events—solar proton events (SPEs)—associated with individual flares and coronal mass ejections at the Sun. It is essential, therefore, to understand the physics of solar activity to know when such an event is likely to occur. Astronauts can then be warned not to stray far from shelter in case a potentially lethal burst occurs. Unfortunately, about once in 20 to 30 years there is an exceptional flare that produces a spectacular burst of particles with energies up to 20 GeV or more, supplying a potentially lethal dose of radiation that cannot be readily shielded against. The physics of these remarkable events (such events occurred in 1956, 1972, and 2003) has yet to be properly understood. Research to date indicates that the acceleration of solar energetic particles in SPEs is related primarily to fast coronal mass ejections (CMEs), possibly via the shock wave driven by them, at distances of ~2 to 40 solar radii (~0.01 to 0.2 AU) from the Sun (inner heliosphere), and to a lesser extent solar flares. However, some very fast CMEs are observed that do not appear to produce SPEs, and similarly fast shocks at 1 AU generally accelerate particles only up to MeV/nucleon energies, not the >10 to 100 MeV/nucleon energies of particles in SPEs. Thus, current understanding of the production of SPEs is very poor, although gaining the ability to recognize the magnetic configurations on the Sun that creates them would be an important next step.

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration BOX 1.2 Exploring the Universe Through Space Plasmas “Our solar system, and stellar systems in general, are rich in the dynamical behaviors of plasma, gas, and dust organized and affected by magnetic fields. These dynamical processes are ubiquitous to highly evolved stellar systems, such as our own, but also play important roles in their formation and evolution. Stellar systems are born out of clumpy, rotating, primordial nebulas of gas and dust. Gravitational contraction, sometimes aided by shock waves (possibly from supernovas), passage through dense material, and other disruptions, forms condensation centers that eventually become stars, planets, and small bodies. Magnetic fields moderate early-phase contractions and may also play vital roles in generating jets and shedding angular momentum, allowing further contraction. The densest of the condensation centers become protostars surrounded by accretion disks. Dynamo action occurs within the protostars as the heat of contraction ionizes their outer gaseous layers, resulting in stellar winds. In similar fashion, rotating solid and gaseous planets form, and many of these also support dynamo action, producing magnetic fields. Ultraviolet and x-ray photons from the central stars partially ionize the upper atmospheres of the planets as well as any interstellar neutral atoms that traverse the systems. Viewed as a whole, the resulting plasma environments are called asterospheres, or in the Sun’s case, the heliosphere. In its present manifestation, the heliosphere—the local cosmos—is a fascinating corner of the universe, challenging our best scientific efforts to understand its diverse machinations. It must be appreciated at the same time that our 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. 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.” SOURCE: Reprinted from National Research Council, Plasma Physics of the Local Cosmos, p. 77, The National Academies Press, Washington, D.C., 2004. variations in solar output so as to understand their consequences and to have a basis for making predictions.5 NASA’s solar and space physics program is conducted by the Sun-Earth Connection (SEC) Division of the Office of Space Science.6 NASA operates a range of SEC missions—from major multi-spacecraft programs to small, focused missions—with the goal of understanding the heliospheric system. The basic research thrust of SEC reflects the growing realization that the processes that control Earth’s space environment are important throughout the universe,7 and hence the SEC research constitutes an intrinsic form of exploration in its own right (see Box 1.2). Moreover, SEC exploration contributes to the broader goals of understanding the origin and evolution of planetary and astrophysical systems, as illustrated by the example of exploration of the heliosphere discussed in Box 1.3. Some of the most exciting basic space research involves the underlying physical processes that are common to plasmas (i.e., the electrically ionized gases that permeate space). For example, the process of magnetic reconnection in a plasma (Box 1.4)—the dynamic change in the topology of a magnetic field—likely plays an important role in the ejection of energetic particle beams from the Sun as well as in triggering magnetic storms at Earth, and is likely to be a basic physical property of astrophysical plasmas ranging from stellar systems to supermassive black hole accretion disks. Similarly, the physical processes associated with particle acceleration, shocks, and turbulence occur in or near Earth’s magnetosphere, and in all probability, around other planets and throughout the wider cosmos. These 5   A Journey to Inspire, Innovate, and Discover: Report of the President’s Commission on Implementation of United States Space Exploration Policy, p. 38, ISBN 0-16-073075-9, U.S. Government Printing Office, Washington, D.C., 2004. 6   Subsequent to the completion of the committee’s report NASA implemented a reorganization that placed the Sun-Earth Connection program in a new headquarters program office—the Science Mission Directorate. 7   National Research Council, Plasma Physics of the Local Cosmos, The National Academies Press, Washington, D.C., 2004.

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration BOX 1.3 Heliosphere and the Local Interstellar Medium: Example of SEC Study of Origins and Evolution The central contribution of the SEC program to scientific exploration is illustrated by the exploration of the heliosphere.1 After the Voyager mission encounters with Jupiter, Saturn, Uranus, and Neptune over the period from 1979 to 1989, the two spacecraft continued their flights into the outer reaches of the solar system, where the science that they were accomplishing became as much the science of the interstellar medium as of the solar wind. Indeed, the interplanetary medium beyond about 10 AU is dominated, by mass, by neutral atoms of interstellar origin rather than by solar wind. Thus, exploration of the outer heliosphere offers the opportunity to learn about both the interplanetary and the interstellar medium, and the manner in which they interact. The detailed interaction between the local interstellar medium (LISM; i.e., that region of space in the local galactic arm where the Sun is located) and the solar wind is not understood. This lack of understanding demonstrates the need for direct observations and for knowledge of the LISM’s basic physical parameters. From physical reasoning, researchers know that boundary regions must separate the solar wind from the LISM. However, these regions are completely unexplored since they are so far out, well beyond the planets of our solar system. The boundary regions are likely separated by several enormous shocks. The innermost shock may be a site where cosmic rays are accelerated, thereby providing a link to supernova shocks thought to accelerate galactic cosmic rays. In the past year scientists working with data from Voyager-1 raised the exciting possibility that Voyager may be in the vicinity of the heliospheric boundary. There is indirect evidence for a "hydrogen wall" where the flow of neutral hydrogen from the LISM is slowed down, compressed, and heated before it penetrates the solar wind. Obtaining direct observations of the interstellar interaction remains a high priority for scientific discovery at the outer frontier of solar and space physics. Sending future spacecraft to the boundaries of our heliosphere to begin the exploration of our galactic neighborhood will be one of the great scientific enterprises of the new century—one that will capture the imagination of people everywhere. Interstellar space is a largely unknown frontier that, along with the Sun as the source of the solar wind, determines the size, shape, and variability of the heliosphere, the first and outermost shield against the influence of high-energy cosmic rays. The interstellar medium is the cradle of the stars and planets, and its physical state and composition hold clues to understanding the evolution of matter in our galaxy and the universe. With plentiful bodies of all sizes and dust in the Edgewood-Kuiper Belt and in the Oort Cloud, the outer heliosphere is a repository of frozen and pristine material from the formation of the solar system. After the contents of our solar system, which is 4.5 billion years old, the LISM provides a second, more recent, sample of matter in our galaxy and in fact the only sample of the interstellar medium that can be studied close-up and in situ. Last but not least, the heliosphere is the only example of an asterosphere that is accessible to detailed study. These perspectives provide a natural bridge and synergism between in situ space physics, the astronomical search for the origins of life, and astrophysics. 1   For a more complete discussion of the exploration of the heliosphere see National Research Council, Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report, The National Academies Press, Washington, D.C., 2004. fundamental processes play key roles in the origin and evolution of planetary and astrophysical systems and tie the results of SEC programs to the scientific goals of exploration. By studying the physical processes that are the ultimate causes of space weather, we stand the best chance of making scientific breakthroughs of ultimately the highest practical importance to space weather prediction and addressing the goal (under “fate”) of “temporal variations in solar

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration BOX 1.4 Reconnection Explosive events in the Sun’s corona, including solar flares and coronal mass ejections, and in planetary magnetospheres, including auroral and magnetic storms, are driven by the conversion of magnetic energy into high-speed plasma flows and high-energy particles. These explosions are the driver of space weather, and the penetrating radiation from these events poses significant hazards to unprotected spacecraft and their human and technological assets. One way for this energy to be released is for oppositely directed magnetic fields to annihilate in a process called magnetic reconnection, so named because magnetic fields must change their structure by “breaking” and “reconnecting” with their neighbors (see Figure 1.4.1). Significant progress in understanding how magnetic field lines “break” has been made though direct satellite measurements in Earth’s magnetosphere and comparisons with theoretical predictions based on computer models. The mechanisms for particle energization and what determines the onset of the explosive energy release—critical for space weather forecasting—remain less fully understood. The broad importance of this topic is reflected in the high priority given in the decadal survey report1 to the Magnetospheric Multiscale (MMS) mission, a four-satellite mission designed to explore the fundamentals of reconnection. FIGURE 1.4.1 Regions of reconnection (boxed areas) occur in many locations in astrophysical systems, including (a) Earth’s magnetosphere and (b) the solar corona. Panel (c) shows a computer simulation of reconnection showing magnetic field lines (white) and strong electrical currents. Oppositely directed magnetic field lines, together with the plasma, flow toward the center of the picture from the sides (light arrows). The field lines reconnect at the center and accelerate strongly outward (up and down) in a slingshot action. The resulting release of magnetic energy produces high-speed plasma flows (dark arrows) and large numbers of energetic particles. Electrons that are accelerated by electric fields to velocities close to the speed of light power the aurora and drive radio bursts from the Sun. SOURCES: (a) Committee on the Assessment of the Role of Solar and Space Physics in NASA’s Space Exploration Initiative, (b) Yohkoh SXT science team, and (c) M. Shay, University of Maryland. 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.

OCR for page 5
Solar and Space Physics and Its Role in Space Exploration output—monitoring and interpretation of space weather as relevant to consequence and predictability.”8 Continued aggressive pursuit of the basic research goals of SEC is crucial both to our eventual understanding of space plasma phenomena and to the effectiveness of the more applied work of the space weather and Living With a Star (LWS) programs. 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. Research activities in space physics have provided critical information on space weather and on the conditions under which it can have disruptive and even hazardous effects on humans and their technological systems both in space and on Earth. The tremendous synergy among SEC space missions is enhanced by the theoretical and ground-based research programs of the NSF and by space-based measurements performed by NOAA and DOD spacecraft. The significant impact that space weather phenomena can have on technological systems on Earth and in Earth orbit has led to the establishment of the multi-agency National Space Weather Program. A significant space-based addition to this program is being developed by NASA through its LWS mission line (for specific mission descriptions see Appendix B). As NASA moves forward on its vision for space exploration the concept of space weather quite properly, and quite feasibly, will take on an expanded meaning in which the Sun’s influence on the environment in interplanetary space and at other planets becomes as important as the need to understand effects in the terrestrial environment. SEC science relates to the space exploration vision in two key ways. First, as noted above and in Boxes 1.2, 1.3, and 1.4, the scientific research in solar and space physics is a form of exploration that is closely aligned with those goals of exploration that focus not only on establishing presences in the solar system but also on understanding the histories and characteristics of various environments and their suitability for life, past and present. Second, from the perspective of providing science that enables exploration, new knowledge gained in understanding our Sun-Earth system will improve our knowledge of and our ability to explore new worlds safely. The new vision for space exploration for a long-term human and robotic program to explore the solar system and beyond will require that humans and our technology survive and operate successfully in a diversity of environments, including interplanetary space and planetary magnetospheres, ionospheres, and atmospheres. SEC missions will tackle the fundamental questions that must be answered to ensure the survival and performance of humans and robots. What is the long-term variability of the environments where our explorations will lead? How can we predict the occurrence of extreme hazardous conditions to safeguard our missions? How can we effectively combine our need to develop new technologies with our desire for scientific exploration and discovery? To address these questions we need to understand the workings of the pieces of the puzzle as well as how the pieces are interconnected into a whole system. 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. 8   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.