B
The Interstellar Observatory

MISSION CONCEPT

The Interstellar Observatory—so named to distinguish the mission developed here from past interstellar-probe concepts—is envisaged as being equipped with instruments never before flown on missions to the outer solar system (Table B.1). As such, it will address a highly cross-disciplinary scientific agenda (see below) and be capable of making important new observations in planetary science, heliospheric physics, and astrophysics throughout its journey to some 200 AU from the Sun. A more complete description of a possible implementation of the Interstellar Observatory concept can be found in Box 4.1.

The Interstellar Observatory will begin to make fundamental new scientific observations shortly after launch as it moves through the solar system with advanced, highly sensitive instrumentation acquiring data at a high time resolution with high accuracy. This mission embodies the concept of exploration and will redefine the frontier of modern space science as it is propelled beyond our solar system to explore the local interstellar medium.

SCIENCE OBJECTIVES

The Interstellar Observatory concept is designed to address scientific issues in the following general areas:

  • Space physics and the heliosphere;

  • Zodiacal dust and the Kuiper Belt;

  • The edge of the solar system;

  • Cosmic infrared background radiation;

  • Search for organic molecules;

  • Composition and ionization state of interstellar matter;

  • Cosmic rays and modulation by the solar magnetic field; and

  • Cosmic rays and the energy density of the galaxy.

Each of these topics is discussed in detail below.



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Priorities in Space Science Enabled by Nuclear Power and Propulsion B The Interstellar Observatory MISSION CONCEPT The Interstellar Observatory—so named to distinguish the mission developed here from past interstellar-probe concepts—is envisaged as being equipped with instruments never before flown on missions to the outer solar system (Table B.1). As such, it will address a highly cross-disciplinary scientific agenda (see below) and be capable of making important new observations in planetary science, heliospheric physics, and astrophysics throughout its journey to some 200 AU from the Sun. A more complete description of a possible implementation of the Interstellar Observatory concept can be found in Box 4.1. The Interstellar Observatory will begin to make fundamental new scientific observations shortly after launch as it moves through the solar system with advanced, highly sensitive instrumentation acquiring data at a high time resolution with high accuracy. This mission embodies the concept of exploration and will redefine the frontier of modern space science as it is propelled beyond our solar system to explore the local interstellar medium. SCIENCE OBJECTIVES The Interstellar Observatory concept is designed to address scientific issues in the following general areas: Space physics and the heliosphere; Zodiacal dust and the Kuiper Belt; The edge of the solar system; Cosmic infrared background radiation; Search for organic molecules; Composition and ionization state of interstellar matter; Cosmic rays and modulation by the solar magnetic field; and Cosmic rays and the energy density of the galaxy. Each of these topics is discussed in detail below.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion TABLE B.1 Instruments for the Interstellar Observatory Instrument Measurement or Objective In Situ Package   Magnetometer Magnetic fields of heliosphere and interstellar medium Plasma and radio wave detector Interaction of solar wind and interstellar medium Solar-wind plasma ion and electron detector Thermal ion composition and charge state; ion and electron distribution functions Interstellar medium plasma ion and electron detector Thermal ion composition and charge state; ion and electron distribution functions Pickup and interstellar ion mass spectrometer — Interstellar neutral atom mass spectrometer Density, composition of neutral species in the interstellar medium Suprathermal ion mass spectrometer — Anomalous and galactic cosmic ray element/isotope spectrometer — Molecular analyzer for organic material Organic material in outer heliosphere and interstellar medium Dust composition analyzer — Suprathermal ion charge states detector — Gamma-ray burst detector Complement long-baseline grid to locate gamma-ray bursters accurately Imaging Package Infrared spectrometer—scans via spin Structure of solar system dust disk; cosmic infrared background radiation Energetic neutral atom imager Structure and dynamics of heliosphere Ultraviolet spectrometer (Lyman alpha) Backscatter from neutrals in the interstellar medium; heliospheric structure Space Physics and the Heliosphere The heliosphere is a large and complicated structure whose dimensions are not definitively known. Recent measurements suggest that the Voyager 1 spacecraft may have crossed the “termination shock” of the solar wind and passed intermittently into the interstellar medium (Figure B.1). Although the interpretations of the observations are controversial, it is quite certain that this region is unlike anything ever sampled previously. Continued tracking of the two Voyager spacecraft should provide the size of our heliospheric cavity within the next several years.1 The solar wind continually rams into the local interstellar medium, and through complex interactions forms the large-scale outer boundaries of our solar system. The latter has three distinct components: The termination shock where the solar wind is abruptly slowed and heated prior to being deflected by the interstellar medium; The heliopause that separates the solar wind from the interstellar medium; and The bow shock or bow wave where the interstellar flow is slowed, heated, and deflected by the solar wind. Between these boundaries that separate layers of solar wind from the interstellar medium, this “interstellar interaction” generates an approximate factor-of-two enhancement of neutral hydrogen (the “hydrogen wall”) in the upstream direction—the direction from which interstellar material is moving toward the Sun at ~25 km/s. Understanding the structure and dynamics of the interstellar interaction and our solar system’s outer boundaries is a primary goal of heliospheric science, and it is also relevant to understanding how other stars interact with the interstellar medium. A variety of indirect measurements have provided only limited information on the nature of the interstellar interaction. These indirect techniques include measurements in the inner heliosphere of interstellar neutral atoms

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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE B.1 A sketch of the heliosphere and its major features indicating the location of the planets of the solar system and the trajectories of Voyagers 1 and 2. and pickup ions produced by charge exchange between interstellar neutral atoms and solar wind ions. In the outer heliosphere, indirect tracers of the interstellar interaction include measurements of anomalous cosmic rays and radio emissions at very low frequencies, apparently from the heliopause (Figure B.2). There is also now a promising technique to image globally the interstellar interaction through measurements of so-called energetic neutral atoms—i.e., direct products of the interstellar interaction formed through charge-exchange between the hot solar wind beyond the termination shock and the inflowing interstellar neutral atoms (Figure B.3). Zodiacal Dust and the Kuiper Belt The Kuiper Belt, a remnant of our solar system’s formation, is a region that extends beyond the orbit of Pluto with bodies ranging in size from minute dust grains to small boulders and small planetary-sized objects. It is believed that the Kuiper Belt is continually eroded through collisions and may feed much of the dust-grain population throughout the solar system. To advance understanding of the formation and evolution of the solar system, greater understanding is needed of the inner parts of the solar system’s zodiacal dust cloud, the Kuiper Belt, and the extended dust disk beyond the Kuiper Belt. However, the structure of our solar system’s dust cloud beyond 3 AU cannot be studied remotely from vantage points near Earth because of the much larger emissions from the dust inside 3 AU.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE B.2 A simulation of the heliosphere showing interactions between the solar wind and the interstellar medium. Simulation courtesy of Gary Zank, University of California, Riverside. The Interstellar Observatory will map the structure of the solar system’s dust cloud, the outer zodiacal cloud, using the infrared emissions from interplanetary dust. In situ instruments will study the composition of both interstellar and interplanetary dust grains to provide clues to their origin. The Interstellar Observatory’s measurements of dust in the Kuiper Belt will probe the collisional evolution of the solar system, leading to a greater understanding of the evolutionary progression from dense protostellar disks to debris disks like those around Beta

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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE B.3 Simulation of the relative intensity of energetic neutral atoms seen across the entire sky at 1 AU produced at the termination shock. The intensities and patterns differ depending on the shock strength. Pictoris, and Alpha Lyrae, and finally to pristine systems like our own. Greater knowledge of dust in the solar system from this mission will enhance understanding of dust disks and the formation and evolution of planetary systems around other stars. The Interstellar Observatory would make the first observations of predicted structures in the zodiacal dust cloud such as a heliocentric ring of dust at Mars orbit, dust associated with Jupiter’s Trojan asteroids, and structure in the Kuiper disk associated with dust trapped in mean-motion resonances with Neptune. All of these structures are expected to have direct analogs in debris disks around other stars. Figure B.4 shows submillimeter-wavelength observations of a dust debris disk around the nearby (~10 light-years) solar-type star Epsilon Eridani; the structure of the disk is thought to be evidence of a planet around this star. Thus, the mission will make fundamental contributions to the goals of NASA’s Origins program, since understanding our own zodiacal emission is key to understanding the structures of dust around other stars, and eventually to searching through the haze of both local and extrasolar zodiacal clouds for planets around other stars. The Edge of the Solar System Recent measurements of Kuiper Belt objects (KBOs) have shown a steep drop-off in the density of material in the outer solar system beyond the orbits of Neptune and Pluto. Is this the edge of the solar system, or only an “inner” edge? Various models attempt to explain the observed drop-off, such as a close encounter with another star in the early history of the solar system (unlikely in the local stellar environment) or an inefficient accretion process at large heliocentric distances, with smaller bodies spiraling in due to gas drag. It is also possible that the Kuiper Belt formed closer to the Sun and migrated outward ~10 AU with Neptune early in early solar system history. Depending on how the solar system evolved, there remains the possibility that the density of circumsolar material will increase again at distances greater than 100 AU and return to the level predicted by extrapolation of the r−1.5 curve fit to the material within 50 AU (see Figure B.5). A mission to the interstellar medium would measure such an increase, if present, and thereby enhance our understanding of the evolution of the solar system, a central goal of both planetary and origins-related science.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE B.4 Submillimeter-wavelength radio observations of a dust debris disk around Epsilon Eridani; the structure of the disk is thought to be evidence of a planet in orbit around this star. The dust, particles, waves, and magnetic fields in the little-explored region of the outer heliosphere provide interaction sites for energetic particles, the sources of energetic neutral atoms, and pickup ions. How Kuiper Belt objects, grains, and the solar-wind populations in the outer heliosphere have interacted over time remains a subject of current and active research. Resolution of many of the questions about these populations will require the type of in situ measurements that only the Interstellar Observatory can provide. Cosmic Infrared Background Radiation Once the Interstellar Observatory has traveled some 5 to 10 AU from the Sun, the cosmic infrared background (CIRB) radiation will become visible as background uncontaminated by emissions from zodiacal dust. With a suitably cooled and shielded infrared detector, the Interstellar Observatory would enable measurement, for the first time, of the spectrum of the CIRB between 3 and 100 μm. These wavelengths correspond to highly redshifted (z ~7 to 10) rest-frame ultraviolet/optical wavelengths emitted by very distant objects formed early in the history of the universe. The CIRB contains information about the formation and evolution of galaxies and can be used to address questions relating to the following topics:

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Priorities in Space Science Enabled by Nuclear Power and Propulsion When did stars and galaxies form? Did stars form before galaxies? Were early galaxies dusty? (If so, they could be unobservable in the optical but visible in the infrared.) Thus, observations of the CIRB from a mission to the interstellar medium can be used to test fundamental hypotheses of modern-day cosmology. Search for Organic Molecules Organic material is found in the solar system—e.g., in asteroids, comets, meteorites, and interplanetary dust grains—and in the interstellar medium. Important questions relating to these materials include the following: Do the nonterrestrial organic materials have similar origins? Amino acids have been found in meteorites and tentatively identified in the interstellar medium in Sagittarius B2, but do they exist in the local interstellar medium as well? Did extraterrestrial organic material that reached Earth play a role in the emergence of life on our planet? These questions can only be answered with in situ measurements in the outer heliosphere and the local interstellar medium. A suitable instrument on the Interstellar Observatory would search for and analyze organic material in the outer solar system and in the nearby interstellar medium to determine the nature and chemical evolution of this organic material. FIGURE B.5 Mass density in the solar system showing the observed edge of the solar system at about 50 AU. Beyond 100 AU, the mass density may increase dramatically up toward the level expected from extrapolation of the solid curve. Courtesy of Michael Brown, California Institute of Technology.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion FIGURE B.6 A schematic showing the reduction of galactic cosmic rays reaching the inner heliosphere due to filtering provided by the interaction of the heliosphere and the local interstellar medium. Understanding time variations of this population is important for the safety of astronauts on interplanetary voyages. Composition and Ionization State of Interstellar Matter The chemical composition of the interstellar medium changes continually as it becomes enriched with material processed and reprocessed from stars, and then is released through stellar winds, novas, and supernovas. Understanding how the interstellar medium changes with time is the key to understanding the chemical evolution of our galaxy and the universe. By in situ sampling of interstellar matter, the Interstellar Observatory will determine directly how elements are distributed between solid (dust), neutral (gas), and plasma (ionized) states, as well as the ionization state of the interstellar medium, and how the isotopic composition of the present-day interstellar medium compares with that in the solar system. The current chemical inventory of our solar system has been established through the study of comets, asteroids, meteorites, Earth, the Moon, other planets, the Sun, and the solar wind. Analysis of the material brought back by the Genesis spacecraft will greatly enhance knowledge of the solar chemical inventory. How does the composition of the interstellar medium compare with that of the Sun and solar system? While the interstellar medium has evolved chemically over the last 4.6 billion years, the chemical inventory of the Sun and solar system has remained largely unchanged, and so is a record of the composition of the interstellar medium 4.6 billion years ago when the solar system was formed. What will this tell us about the chemical evolution of the galaxy? The abundances in the local interstellar medium that will be found by the Interstellar Observatory will be compared to solar abundances and those from more distant galactic regions. In the context of cosmogenic and nucleosynthetic models, the local interstellar

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Priorities in Space Science Enabled by Nuclear Power and Propulsion abundances found by the Interstellar Observatory will improve our understanding of how stars process matter and how the galaxy evolves, and it will improve our knowledge of the age of the universe. Cosmic Rays and Modulation by the Solar Magnetic Field In the heliosheath—the region beyond the termination shock, where the solar wind is heated and slowed—models show the formation of a large magnetic barrier that filters out the majority of the low-energy galactic cosmic rays (<100 MeV/nucleon) from the interstellar medium. In addition, the solar wind’s magnetic field—i.e., the interplanetary magnetic field—and its embedded large-scale magnetic disturbances exclude more galactic cosmic rays from the inner solar system over the ~2- to 4-year period when the Sun is most active during the 11-year solar cycle. Highly penetrating galactic cosmic rays are one of the most serious hazards for astronauts on long-duration missions beyond the protection of Earth’s magnetic field (Figure B.6). By directly passing through and sampling the heliosheath, measuring both galactic cosmic rays and the heliosheath’s magnetic field, the Interstellar Observatory will study directly how the solar system is shielded from the majority of galactic cosmic rays. Cosmic Rays and the Energy Density of the Galaxy The spectrum of interstellar cosmic rays is not known because particles with energies <100 MeV/nucleon are excluded from the heliosphere. Only a spacecraft such as the Interstellar Observatory that travels to the interstellar medium can determine the full cosmic-ray energy spectrum and its contributions to the energy density and ionization state of the interstellar medium. These measurements will allow further study of astrophysical processes such as the acceleration of cosmic rays by supernova shocks, galactic radio and gamma-ray emissions, recent nucleosynthesis, and the heating of the interstellar medium. Finally, the Interstellar Observatory’s direct measurements of the cosmic-ray energy spectrum will determine how the cosmic-ray pressure in the local interstellar medium affects the size and shape of the solar system’s outer boundaries. REFERENCE 1. National Research Council, Exploration of the Outer Heliosphere and the Local Interstellar Medium, The National Academies Press, Washington, D.C., 2004.