4
An Interstellar Probe to the Boundaries of the Heliosphere and Nearby Interstellar Space

During the next few years the Voyagers are expected to cross the termination shock, if Voyager 1 has not yet already done so (Krimigis et al., 2003), and make fundamental discoveries about the size of the heliosphere and the nature of its boundaries. Precisely when this occurs may depend on the motion of the shock itself, which may be moving outward owing to solar cycle changes in ram pressure, on both 11-year and shorter time scales. It should be realized, however, that the Voyagers carry in situ instrumentation that was designed 30 years ago with the primary objective of exploring planetary magnetospheres. Beyond this initial reconnaissance work, a new mission—an interstellar probe—carrying modern instrumentation is needed if we are to make detailed measurements in the outer solar system and at the heliospheric boundaries and to then exit the heliosphere and begin the in situ exploration of the space between the stars. To relate the particles and fields measured along the spacecraft trajectory to models of the global heliosphere, it is important that Interstellar Probe carry a complementary package of both in situ and remote-sensing instruments.

A voyage by an interstellar probe from Earth to beyond 200 AU could enable the comprehensive measurements of plasma, neutral atoms, magnetic fields, dust, energetic particles, cosmic rays, and infrared emission from the outer solar system, through the boundaries of the heliosphere, and on into the interstellar medium. Such an exploratory journey could address key questions about the distribution of matter in the outer solar system, the processes by which the Sun interacts with our galaxy, and the nature and properties of the nearby galactic medium. An interstellar probe could also directly gather the data that are necessary to address the four principal science objectives (see Chapter 1) of a mission to the border of our galaxy:

  1. To explore the nature of the interstellar medium and its implications for the origin and evolution of matter in our galaxy and the universe. Interstellar space is a largely unknown frontier that holds many keys to understanding our place in the Galaxy. The nearby interstellar medium includes species that are predominantly ionized (e.g., C, S, and Si), those that are mostly neutral (H, He, N, O, Ne, and Ar), and others that are mainly locked up in grains (e.g., Al, Ca, and Fe) (Slavin and Frisch, 2002). Although pickup



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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report 4 An Interstellar Probe to the Boundaries of the Heliosphere and Nearby Interstellar Space During the next few years the Voyagers are expected to cross the termination shock, if Voyager 1 has not yet already done so (Krimigis et al., 2003), and make fundamental discoveries about the size of the heliosphere and the nature of its boundaries. Precisely when this occurs may depend on the motion of the shock itself, which may be moving outward owing to solar cycle changes in ram pressure, on both 11-year and shorter time scales. It should be realized, however, that the Voyagers carry in situ instrumentation that was designed 30 years ago with the primary objective of exploring planetary magnetospheres. Beyond this initial reconnaissance work, a new mission—an interstellar probe—carrying modern instrumentation is needed if we are to make detailed measurements in the outer solar system and at the heliospheric boundaries and to then exit the heliosphere and begin the in situ exploration of the space between the stars. To relate the particles and fields measured along the spacecraft trajectory to models of the global heliosphere, it is important that Interstellar Probe carry a complementary package of both in situ and remote-sensing instruments. A voyage by an interstellar probe from Earth to beyond 200 AU could enable the comprehensive measurements of plasma, neutral atoms, magnetic fields, dust, energetic particles, cosmic rays, and infrared emission from the outer solar system, through the boundaries of the heliosphere, and on into the interstellar medium. Such an exploratory journey could address key questions about the distribution of matter in the outer solar system, the processes by which the Sun interacts with our galaxy, and the nature and properties of the nearby galactic medium. An interstellar probe could also directly gather the data that are necessary to address the four principal science objectives (see Chapter 1) of a mission to the border of our galaxy: To explore the nature of the interstellar medium and its implications for the origin and evolution of matter in our galaxy and the universe. Interstellar space is a largely unknown frontier that holds many keys to understanding our place in the Galaxy. The nearby interstellar medium includes species that are predominantly ionized (e.g., C, S, and Si), those that are mostly neutral (H, He, N, O, Ne, and Ar), and others that are mainly locked up in grains (e.g., Al, Ca, and Fe) (Slavin and Frisch, 2002). Although pickup

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report ions and anomalous cosmic rays provide information on neutral species, we currently have no information on the elemental and isotopic composition of ionized species, nor do we know to what extent refractory elements have condensed into grains. In addition, we have almost no knowledge of the direction and strength of the interstellar magnetic field, or of the intensity and composition of low-energy cosmic rays outside the heliosphere. Direct sampling of the composition of interstellar matter would provide a benchmark for comparison with solar-system abundances and provide constraints on galactic chemical evolution. Abundance measurements of isotopes such as 2H, 3He, 13C, 18O, 22Ne, 26Mg, and 30Si would constrain cosmological and nucleosynthesis models and provide a more accurate picture of the evolution of the solar system, the Galaxy, and the universe. To explore the influence of the interstellar medium on the solar system, its dynamics, and its evolution. The heliosphere and its boundaries provide a unique laboratory for studying plasma processes and the interaction of a star with its environment. The termination shock is thought to be a powerful accelerator, with particle energies reaching more than 1 GeV. In situ studies of shock structure, plasma heating, and acceleration processes will serve as a model for other astrophysical shocks. Beyond the termination shock, in the heliosheath, the solar wind flow turns to match the flow of the diverted interstellar plasma. At the boundary between the solar wind and interstellar plasmas, there could be magnetic reconnection between interplanetary and interstellar magnetic field. A bow shock might be created in the interstellar medium ahead of the nose of the heliosphere, depending on the unknown interstellar magnetic field strength. Energetic neutrals created by charge exchange in the heliosheath can be used to image the three-dimensional structure of the heliosphere. Charge-exchange collisions cause a pileup of neutral hydrogen at the heliosphere nose, referred to as the “hydrogen wall.” Interstellar Probe will pass through these boundary regions and make in situ measurements to answer questions about the size, structure, and dynamics of the heliosphere and processes occurring at the boundaries. As the solar wind pressure varies over the solar cycle, the termination shock and heliopause are expected to move by ~10 to 20 AU (Zank and Mueller, 2003). Over the course of the Sun’s journey through the Galaxy, these boundaries were undoubtedly much closer in during times when the heliosphere encountered more dense interstellar regions. To explore the impact of the solar system on the interstellar medium as an example of the interaction of stellar winds with their environments. Models show that the interstellar medium experiences considerable heating by charge exchange of fast, hot neutrals created in the inner heliosheath. These neutrals transport heat anomalously across the heliopause, and via a secondary charge exchange they heat the very local interstellar medium. It has also been suggested that the newly created pickup ions in the local interstellar medium (LISM) act to heat electrons (Cairns and Zank, 2002), making this region more likely to produce radio emissions when global merged interaction region shocks of solar origin sweep through. The plasma physics of the LISM is therefore likely to be affected by solar wind neutrals, as well as by anomalous cosmic rays diffusing out. By observing the plasma state of the LISM, Interstellar Probe will directly address this question. To explore the outer solar system for clues to its origin and the nature of other planetary systems. By carrying a small charge-coupled device (CCD) camera, Interstellar Probe could survey the population of Kuiper Belt objects >1 km in size and determine the radial extent of the Kuiper Belt and the primordial solar nebula. Interstellar Probe could also survey the composition and distribution of dust due to collisions of Kuiper Belt objects and could possibly investigate the nature and evolution of organic material in the outer solar system. NASA’s last four Sun-Earth Connection roadmaps (2000-2003) have included a mission to explore the boundaries of the heliosphere and nearby interstellar space, and interstellar probe became part of the

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report NASA Strategic Plan in 2000.1 The recently published NRC decadal research strategy in solar and space physics recognized the scientific importance of an interstellar probe mission but listed it as a “deferred high-priority flight mission” because of its requirement for advanced propulsion (NRC, 2003). Specifically, the report included the following recommendation: “NASA should assign high priority to the development of advanced propulsion and power technologies required for the exploration of the outer planets, the inner and outer heliosphere, and the local interstellar medium” (pp. 10-11). To be sure of crossing the heliopause and making a significant penetration into interstellar space, Interstellar Probe should reach at least 200 AU within a reasonable time frame (e.g., 10 to 20 years), requiring an escape velocity of 10 to 20 AU/year. This requires propulsion technology well beyond that of Voyager 1, which is on an escape trajectory with an average velocity of ~3.6 AU/year.2 Over the past decade, three approaches have emerged that, following some development, could achieve the required spacecraft velocities carrying an advanced scientific payload. The first of these, which has been studied at various times in the past, makes use of a Jupiter flyby, followed by a powered solar flyby passing within 4 solar radii of the center of the Sun. By performing a rocket burn deep in the Sun’s gravitational well, the resulting change in velocity can be translated to escape velocities well in excess of those normally achieved with chemical propulsion and planetary gravity assists (Mewaldt et al., 1995; Ehricke, 1972). This approach and related technology issues have recently undergone additional study through funding by NASA’s Institute for Advanced Concepts program, resulting in a mission concept that might achieve escape velocities of 12 to 15 AU/year (McNutt et al., 2001, 2003a,b). Of course, before this trajectory is attempted with an interstellar probe it will be necessary to successfully demonstrate the heat shield and other technologies required for a close solar flyby by carrying out a successful solar probe mission. The other two propulsion technologies that hold promise for an interstellar probe are nuclear-electric propulsion (NEP) and solar sail propulsion (see Gavit et al., 2001). Solar sail propulsion is more efficient closer to the Sun, where the radiation pressure is greater. In the mission concept adopted in a 1999 study (JPL, 1999b),3 a 200-m-radius sail with an areal density of 1 g/m2 (sail material plus support structure) would be used to maneuver the spacecraft for a swing-by at 0.25 AU, where the increased radiation pressure could accelerate the spacecraft at escape velocities of ~14 AU/year. Because almost all of the propulsion takes place within the first few AU, it is possible to jettison the sail at ~5 AU, thereby avoiding possible effects that a large, conducting sail might have on in situ particles and fields measurements as well as on other instruments. Although solar sail propulsion holds great promise for a number of new missions (see, e.g., The Sun-Earth Connection Roadmap 2003-2028, NASA, 2003; and The Sun to Earth—and Beyond: A Decadal Research Strategy in Solar and Space Physics, NRC, 2003), a solar-sail-propelled spacecraft has never been tested in space. However, funding for solar sail development has increased significantly during the past few years, largely owing to NASA’s OSS-funded In-Space Propulsion Program and technology development within the New Millennium Program.4 Even before a NASA-funded test flight could occur, solar-sail 1   On page 21 of the 2000 NASA Strategic Plan, in the section “Long Term Plans 2012-2025,” is the statement, “Obtain x-ray images and composition of chemical elements created in supernovas, and directly measure the composition of the gas outside our solar system with an interstellar probe.” See the 2000 NASA Strategic Plan at http://www.hq.nasa.gov/office/codez/plans/pl2000.pdf. 2   According to the Voyager Web site, the escape speeds are 3.3 AU/yr for Voyager 2 and 3.6 AU/yr for Voyager 1. See http://voyager.jpl.nasa.gov/mission/interstellar.html. 3   See also Liewer et al. (2001) and NASA’s Interstellar Probe Web site at http://interstellar.jpl.nasa.gov/interstellar/probe/index.html. 4   Solar sail technology development at NASA is summarized at http://solarsails.jpl.nasa.gov/index.html. Information about the New Millennium Program is available at http://nmp.jpl.nasa.gov/program/program.html.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report demonstration flights are planned for 2004 by the Planetary Society5 and for 2005 by a private consortium named Team Encounter.6 A realistic roadmap for solar sail propulsion will most likely require at least one flight of a somewhat less ambitious mission like Solar Polar Imager or PASO (Particle Acceleration Solar Orbiter), for which the requirements on sail size and areal density are more easily achieved.7 However, it should be recognized that large technical differences may exist between near- and long-term solar sail propulsion systems, and it is possible that evolving the technology from a smaller to a larger sail may not be straightforward. With the announcement of NASA’s 2003 budget, a new era in spacecraft propulsion began when NASA and the Department of Energy (DOE) embarked on a bold, new initiative named Project Prometheus, which will develop nuclear-powered propulsion capabilities that promise to revolutionize our ability to explore the solar system. Over the next decade it is expected that Project Prometheus will invest several billion dollars in the development of nuclear-electric propulsion (in which a nuclear reactor is used to power ion-drive engines), and it will also develop advanced radioisotope power systems that are needed for a number of deep-space missions. The first mission slated to use this new propulsion capability is the Jupiter Icy Moons Orbiter (JIMO), which will search for evidence of global subsurface oceans on Jupiter’s three icy moons: Europa, Ganymede, and Callisto.8 In addition to providing abundant electrical power for in-space propulsion, these new nuclear systems would provide revolutionary capabilities for powering instruments and for returning data. The science working team that is studying JIMO is also charged with identifying other mission concepts that could make use of this new capability. Nuclear-electric propulsion is also potentially suited for accelerating Interstellar Probe to the required speeds, as illustrated in Figure 4.1, which compares the solar sail and NEP trajectories for two mission concepts studied in 1999 (JPL, 1999). NEP has been studied in the context of an interstellar probe mission at many times in the past due to its potential for this type of mission.9 Note that while the solar sail achieves essentially all of its acceleration within the first few AU of the Sun, the NEP approach involves slow, continuous acceleration that can, in principle, eventually reach even greater speeds. Because the environment surrounding a nuclear reactor will likely interfere with some in situ measurements (plasma, magnetic fields, low-energy charged particles, and so on), it may be desirable to shut down the reactor periodically, to deploy a small, tethered instrument package, or to jettison the reactor once the required terminal velocity has been achieved (e.g., at 80 to 100 AU). At that point an advanced radioisotope power system could provide power for the spacecraft.10 5   The Planetary Society is developing and conducting a privately funded solar sail project with the Cosmos Studios. The spacecraft is being built in Russia by the Babakin Space Center under a contract to the society. It will also be launched and operated from Russia. See description of the project at http://www.planetary.org/solarsail/missions/planetary_solar_sai.html. 6   See description at http://www.teamencounter.com/faq/about_us.asp#anchor1. 7   PASO and Solar Polar Imager are discussed in the 2003 SEC Roadmap at http://sec.gsfc.nasa.gov/sec_roadmap.htm. 8   JIMO is currently planned for a launch no earlier than 2012. See the NASA mission Web site at http://www.jpl.nasa.gov/jimo/. 9   See Jaffe and Ivie (1979), Jaffe and Norton (1980), Jaffe et al. (1980), Jones and Sauer (1984), and Pawlik and Phillips (1977). See also Nock (1987). 10   A fourth, low-thrust concept, not as well studied to date as the others, is radioisotope electric propulsion (Noble, 1999; Oleson et al., 2001). Ion thrusters are used as with NEP, but the power supply is based on radioisotope power systems, so that the mass is at a premium as with solar sailing and the ballistic approach.

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Exploration of the Outer Heliosphere and the Local Interstellar Medium: A Workshop Report FIGURE 4.1 Possible Interstellar Probe trajectories based on nuclear-electric propulsion (NEP) and solar sail propulsion are compared with that of Voyager 1. It was assumed that NEP was terminated after 10 years. The estimated location of various heliospheric boundaries is also indicated. See The Interstellar Probe Mission Architecture and Technology Report, JPL-D-18410, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif., October 1999, or http://interstellar.jpl.nasa.gov/interstellar/probe/index.html.