E
Glossary, Acronyms, and Abbreviations


Accretion, accretion disk:

Astronomical objects as diverse as protostars and active galaxies may derive their energy from the gravitational power released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk.

ACE:

Advanced Composition Explorer.

Active cooling:

Cooling achieved by power-consuming heat-management techniques such as the use of Stirling coolers or other refrigeration systems.

Active galactic nuclei (AGN):

A term that refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies that cannot be attributed clearly and directly to individual stars.

Active galaxy:

Certain galaxies emit far more energy than can be accounted for by their stars alone. The central regions of these galaxies harbor a compact, solar-system-sized object capable of outshining the rest of the galaxy by a factor of 100. The ultimate energy source for active galaxies may be the accretion of matter onto a supermassive black hole. Active galaxies can emit strongly across the entire electromagnetic spectrum, from radio waves to gamma rays.

Aerobot:

A robotic aerial vehicle that can be used to remotely investigate planetary surfaces.

Aerocapture:

The technique by which an incoming spacecraft uses a single, precisely determined passage through a planet’s atmosphere to shed sufficient excess velocity to enter a predetermined orbit about that planet.

Alloy:

A combination of two or more elements, at least one of which is a metal, and where the resultant material has metallic properties.

Altimetry:

Determination of the altitude of an object or surface with respect to a fixed level, like sea level.



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Priorities in Space Science Enabled by Nuclear Power and Propulsion E Glossary, Acronyms, and Abbreviations Accretion, accretion disk: Astronomical objects as diverse as protostars and active galaxies may derive their energy from the gravitational power released by the infall, or accretion, of material onto a central object. The combined effects of gravity and rotation often force the accreting material into an orbiting accretion disk. ACE: Advanced Composition Explorer. Active cooling: Cooling achieved by power-consuming heat-management techniques such as the use of Stirling coolers or other refrigeration systems. Active galactic nuclei (AGN): A term that refers to the existence of energetic phenomena in the nuclei, or central regions, of galaxies that cannot be attributed clearly and directly to individual stars. Active galaxy: Certain galaxies emit far more energy than can be accounted for by their stars alone. The central regions of these galaxies harbor a compact, solar-system-sized object capable of outshining the rest of the galaxy by a factor of 100. The ultimate energy source for active galaxies may be the accretion of matter onto a supermassive black hole. Active galaxies can emit strongly across the entire electromagnetic spectrum, from radio waves to gamma rays. Aerobot: A robotic aerial vehicle that can be used to remotely investigate planetary surfaces. Aerocapture: The technique by which an incoming spacecraft uses a single, precisely determined passage through a planet’s atmosphere to shed sufficient excess velocity to enter a predetermined orbit about that planet. Alloy: A combination of two or more elements, at least one of which is a metal, and where the resultant material has metallic properties. Altimetry: Determination of the altitude of an object or surface with respect to a fixed level, like sea level.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Anomalous cosmic rays (ACRs): Cosmic rays that originate from the interstellar space beyond the heliopause and differ from other types of cosmic rays in that they are singly charged, contain more helium than protons, and contain more oxygen than carbon. Arcsecond: A unit of angle corresponding to 1/3600th of a degree; 1/60th of an arcminute. An arcsecond is approximately the size of a dime viewed from a distance of 1 mile. ARISE: Advanced Radio Interferometry between Space and Earth. ASEB: Aeronautics and Space Engineering Board. Astrobiology: Study of the origin, evolution, and distribution of life in the universe. Astronomical unit (AU): A basic unit of distance equal to the separation between Earth and the Sun, about 150 million km. Aurora: A glow in a planet’s ionosphere caused by the interaction between the planet’s magnetic field and charged particles from the Sun. Baseline: The separation between telescopes in an interferometer. The largest baseline determines the finest detail that can be discerned with an interferometer. Bow shock: Also, “bow wave,” where the interstellar flow is slowed, heated, and deflected by the solar wind. In a planetary magnetosphere, the bow shock is the boundary at which the speed of the solar wind abruptly drops as a result of its approach to the magnetopause. Brayton power conversion: An open-cycle power-conversion system that uses a thermodynamic cycle featuring heat addition and rejection at constant pressure. This cycle represents the idealized behavior of the working fluid in a gas turbine engine. Bremsstrahlung: Electromagnetic radiation produced by the acceleration of a charged particle, such as an electron, when deflected by another charged particle, such as an atomic nucleus. C3: The energy per unit mass of a spacecraft once it gets away from Earth’s gravitational field. If C3 > 0, the launch vehicle sends the spacecraft directly to its destination—outside Earth’s sphere of influence. If C3 < 0, the launch vehicle sends the spacecraft into Earth orbit, and the spacecraft then has to use its own propulsion to leave Earth’s sphere of influence. The units of C3 are km2s−2. Celestial mechanics: A term for the application of physics, historically Newtonian mechanics, to astronomical objects such as stars and planets. centiSievert (cSV): One-hundredth of a Sievert, the SI unit of radiation dose equivalent, which indicates what dose of x-rays or gamma rays produce the equivalent damage. Chirality: The right- or left-handedness of an asymmetric molecule. Absence of symmetry on reflection. CIRB: Cosmic infrared background radiation. COBE: The Cosmic Background Explorer, a NASA mission launched in 1989 to study the cosmic background radiation from the Big Bang.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Corona: The luminous “atmosphere” of the Sun extending millions of kilometers into space. Coronal mass ejection (CME): A disturbance of the Sun’s corona involving eruptions from the lower part of the corona and ejection of large quantities of matter into the solar wind. These ejecta sometimes have higher speed, density, and magnetic field strength than is typical for the solar wind. If their speeds relative to the background solar wind are high, they can produce shocks in the plasma that precede them as they move outward. Cosmic accelerator: The process by which matter can be accelerated to speeds far above those available in Earth-based particle accelerators. Cosmic microwave background (CMB) radiation: The radiation left over from the Big Bang explosion at the beginning of the universe. As the universe expanded, the temperature of the fireball cooled to its present level of 2.7 degrees above absolute zero (2.7 K). Blackbody radiation from the cosmic background is observed at radio, millimeter, and submillimeter wavelengths. Cosmic radiation: Atomic nuclei, accelerated to extremely high energies by unknown processes in space, which continually bombard Earth from all directions. Cosmogonic: Relating to the creation or origin of the planets. CR: Cosmic ray (see Cosmic radiation). Cryogenic: Of or relating to the production of very low temperatures. Cryostat: An apparatus for maintaining a constant very low temperature. CSSR: Comet Surface Sample Return. Cyclotron radiation: The characteristic electromagnetic radiation emitted when nonrelativistic charged particles spiral around magnetic field lines. Dark energy: An as-yet-unknown form of energy that pervades the universe. Its presence was inferred from the discovery that the expansion of the universe is accelerating, and these observations suggest that about 70 percent of the total energy density of the universe is in this form. The remaining 30 percent is composed of visible matter (4 percent) and dark matter (26 percent). Such an acceleration would be predicted if the cosmological constant that Einstein included in his general theory of relativity were non-zero. Dark matter: Approximately 87 percent of the matter in the universe may have so far escaped direct detection. The presence of this unseen matter has been inferred from motions of stars and gas in galaxies, and of galaxies in clusters of galaxies. Candidates for the missing mass include brown dwarf stars and exotic subatomic particles. Dark matter was called “missing mass” for many years. However, because it is the light, not the mass, that is missing, astronomers stopped using that term. Decalcification: The removal or loss of calcium or calcium compounds (as from bones or soil). Delta-V: The change in velocity needed by a spacecraft to switch from one trajectory to another. Diurnal: Recurring every day; or of, or relating to, occurring in the daytime. DSN: Deep Space Network.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Dynamo: An electromagnetic process in which the movement of conductive material gives rise to a magnetic field. Eccentricity: A measure of how much an orbit’s shape deviates from a circle. Ecliptic: The plane of Earth’s orbit around the Sun. Einstein rings: A gravitational lens effect in which the image of a remote background point source of radiation, such as a quasar, is distorted into a ring by the gravity of a foreground galaxy. Electromagnetic spectrum: Radiation can be represented as electric and magnetic fields vibrating with a characteristic wavelength or frequency. Long wavelengths (low frequencies) correspond to radio radiation; intermediate wavelengths, to millimeter and infrared radiation; short wavelengths (high frequencies), to visible and ultraviolet light; and extremely short wavelengths, to x-rays and gamma rays. Most astronomical observations measure some form of electromagnetic radiation. Emission line: A bright line in the spectrum of a luminous object caused by the emission of light at a particular wavelength. Emission lines may appear on their own, as in the spectrum of a nebula energized by radiation from a nearby hot star, or they may be superimposed on an absorption spectrum, as happens when a star is surrounded by hot gas. Energetic neutral atom (ENA) imaging: ENA production mechanisms in space plasmas are charge-exchange reaction with atmospheric/exospheric gases, sputtering of planetary atmospheres, backscattering from the planetary atmospheres (ENA albedo), sputtering from planetary surfaces, ion neutralization/sputtering on dust particles, and recombination (CMI). In contrast to charged particles, ENAs are no longer influenced by electromagnetic fields and propagate on straight paths from the source to the observer. Directional detection of ENAs yields a global image of the interaction and by complex inversion techniques properties of the source populations can be deduced. eV: An electron volt, a measure of energy equal to that gained by an electron passing through a potential difference of 1 volt; also a unit of particle mass when divided by the speed of light (c) squared. Electrons have a mass of about 0.511 MeV/c2 (million electron volts); protons have a mass of about 938 MeV/c2 (billion electron volts). Event horizon: The “surface” of a black hole. It is a one-way membrane, allowing matter or signals to flow in but not out. EXIST: Energetic X-ray Imaging Survey Telescope. Fast neutron: A free neutron with a kinetic energy level close to 1 MeV (speed of 14,000 km/s), produced by nuclear processes such as nuclear fission. Field programmable gate arrays (FPGAs): Electronic components of digital circuits that use a grid of logic gates that can be programmed by the consumer (rather than the manufacturer). Fission: A process whereby a large nucleus such as uranium is split into two smaller nuclei. Formation flying: Multiple spacecraft maintaining a constant distance from each other to high precision. Full width half maximum (FWHM): A term commonly used in statistics and telecommunications that refers to the value of the width of a function at which the dependent variable is at half its maximum value.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Gamma ray: Electromagnetic radiation with wavelengths less than 0.00001 micron, more energetic than x-rays. Gamma-ray burst (GRB): Burst of gamma rays from cosmic sources observed by detectors on satellites. Several hundred are detected per year, and they range in duration from fractions of a second to several seconds. Most gamma-ray bursts come from objects at cosmological distances. Gardening (of the regolith): The process of mixing surface materials by impact. GLAST: Gamma-ray Large Area Space Telescope. Gravitational lensing: A consequence of Einstein’s general relativity theory is that the path of light rays can be bent by the presence of matter. Astronomers have observed that the light from a distant galaxy or quasar can be “lensed” by the matter in an intervening galaxy to form multiple and often distorted images of the background object. Gravitational wave: According to the theory of general relativity, a ripple in the geometry of space-time propagating as a wave. Gravity assist: Also known as the sling-shot effect, an important spaceflight technique, already used successfully on a number of interplanetary missions including Voyager, Galileo, and Cassini, whereby the gravitational field of a planet is used to increase the speed and alter the course of a spacecraft without the need to expend fuel. Ground truth: Facts that are found when a location is studied in situ, as opposed to information obtained from remote-sensing observations. Hall fields: A phenomenon thought to play a critical role in changing the large-scale structure of magnetic fields (at Earth, on the Sun, and elsewhere in the cosmos). HCIPE: High Capability Instruments for Planetary Exploration program. Heliosheath: Where the solar wind is heated and slowed beyond the termination shock. Heliosphere: The region of space dominated by the Sun’s magnetic field. It includes the Sun itself as well as the corona and the solar wind, and extends beyond the orbit of Pluto to distances in excess of 50 AU. HEMT: High Electron Mobility Transistor. HEO: High Earth orbit Hot atom: Atom released from a molecule when a nuclear reaction takes place. Such particles are likely to be moving very fast, enabling them to break other bonds and induce further chemistry when they collide with other atoms or molecules. This may induce reactions that could not occur under thermal conditions, leading to production of unusual species that cannot otherwise be generated. HST: Hubble Space Telescope. IMAGE: Imager for Magnetopause-to-Aurora Global Exploration. Inclination: Angle between the plane of the object’s orbit and the ecliptic.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Interferometer: An optical system in which radiation from multiple sources is combined and the resulting interference pattern is used to provide information on the nature and configuration of the sources. Interplanetary Monitoring Platform (IMP)-8: The last of a series of NASA probes, managed by the Goddard Space Flight Center, designed to study Earth’s magnetosphere and plasma (ionized gas) in interplanetary space. Interplanetary shock: The abrupt boundary formed at the front of a plasma cloud (e.g., from a coronal mass ejection) moving much faster than the rest of the solar wind, as it pushes its way through interplanetary space. Interstellar medium (ISM): The material between the stars, consisting of gas, dust, and cosmic rays (high-energy charged particles moving at nearly the speed of light). Interstellar Observatory: A mission detailed in this report that uses nuclear power and propulsion to directly explore interstellar space beyond the heliopause. Interstellar scintillation: An apparent twinkling of the signals from distant point-like radio sources that is due to changes in the density of the interstellar medium through which the signals have passed on their way to Earth. Iogenic: Originating from Io. Ionosphere: The part of the atmosphere that is kept partially (up to 0.1 percent) ionized by ultraviolet light and x-rays from the Sun. It lies immediately above the stratosphere, roughly between altitudes of 50 and 500 km from Earth. IPSTDT: NASA’s Interstellar Probe Science and Technology Definition Team. IRAS: Infrared Astronomical Satellite. ISO: The European Space Agency’s Infrared Space Observatory. JGA: Jupiter Gravity Assist. JIMO: Jupiter Icy Moons Orbiter. JPL: Jet Propulsion Laboratory. JWST: James Webb Space Telescope. KBOs: Kuiper Belt objects (see Kuiper Belt). Kolmogorov spectrum: A concept arising in the theory of turbulence describing how energy is distributed among eddies of different sizes. kpc: Kiloparsec, which is 3.08568025 × 1019 meters. Kuiper Belt: A region of the solar system containing icy planetesimals distributed in a roughly circular disk some 40 to 100 AU from the Sun. Pluto is believed to circumscribe the innermost region of the Kuiper Belt. kW: Kilowatt, a unit of power.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion kWe: Kilowatt-electric, a unit of electrical power (e.g., the total amount of electrical power produced by a nuclear reactor, which is substantially less than the total thermal power because of the inefficiencies in converting thermal energy to electrical energy). L1/L2: Sun-Earth Lagrangian points. L1 and L2 are approximately 1.5 million km from Earth. L1 is located between the Earth and the Sun. L2 is located on the far side of the Earth from the Sun. Lagrangian point: One of five locations in space around a rotating two-body system (such as the Earth-Moon or Earth-Sun) where the pulls of the gravitating bodies combine to form a point at which a third body of negligible mass would be stationary relative to the two bodies. LEO: Low Earth orbit. Liquid metal coolants: Molten metal used as a reactor coolant. LISA: Laser Interferometer Space Antenna. Lithium hydride: A lightweight, high-temperature alloy of lithium that is effective as a neutron shield and can also be used as a liquid metal coolant. Lithosphere: The rigid outer crust of rock of a planetary body. Local Group galaxies: A collection of more than 40 galaxies, spread across a volume of space some 10 million light-years in diameter, of which our own Milky Way Galaxy and the Andromeda Galaxy are the dominant and central members. Magnetopause: The boundary between a planet’s magnetosphere and the magnetic field of the solar wind. Magnetosphere: The region exterior to a planet in which its magnetic field plays the dominant part in controlling the physical processes that take place there. Magnetotail: The part of a planet’s magnetosphere that is elongated away from the Sun by the solar wind. MAO: Mars Upper Atmosphere Orbiter. Maser: A natural or artificial source of very intense, narrow-band, coherent microwave radiation. “Maser” stands for “microwave amplification by stimulated emission of radiation.” Mass spectrometer: Instrument that produces and measures, usually by electrical means, a mass spectrum. It separates ions according to the ratio of their mass to charge, allowing scientists to determine the abundances of each isotope. Mass wasting: Loss of material from a slope due to gravity-driven processes. Maunder minimum: The period from approximately 1645 to 1715 during which there was a substantial reduction in the number of sunspots visible on the solar disc. This phenomenon was first identified by the British astronomer Edward W. Maunder (1851–1928). Metrology: The study of precise measurements.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion MIDEX: Midsize Explorer; a continuing series of highly focused and relatively inexpensive (less than $180 million in total cost to NASA) astrophysics and space physics missions. MMRTG: Multi-mission radioisotope thermoelectric generator. Moore’s law: An empirical observation, made in 1965 by Gordon Moore, Intel cofounder, that the number of transistors per integrated circuit exhibits exponential growth over time, resulting in an approximate doubling of transistors per circuit every 18 months. MSL: Mars Science Laboratory. Nebula: A cloud of gas and dust in space. NEP: Nuclear-electric propulsion, a nuclear propulsion system that uses electricity from a space nuclear reactor to power an electric propulsion system, such as an ion engine. NEPA: National Environmental Policy Act. NERVA/Rover: U.S. nuclear thermal rocket technology program conducted from 1955 to 1973. Neutrino: One of a family of subatomic particles with little or no mass. These particles are generated in nuclear reactions on Earth, in the centers of stars, and during supernova explosions and can give unique information about these energetic processes. Because neutrinos interact only weakly with matter, they are difficult to detect. New Horizons: A NASA mission to explore Pluto and the Kuiper Belt, launched in January 2006. NGST: Next Generation Space Telescope, the original name for the James Webb Space Telescope. Noble gases: The chemical series of elements (found in group 18 of the periodic table) that includes helium, neon, argon, krypton, xenon, and radon. These gases are relatively unreactive, due to their full outer electron shells, and were historically referred to as “inert gases” or “rare gases.” Nova: An extreme example of the cataclysmic variable phenomenon in which a star’s brightness suddenly increases by a factor of a million and then fades over a period of weeks. Novas occur in binary systems of one normal and one white dwarf star, where the normal star transfers matter to the dwarf via an accretion disk. The accreted matter accumulates until such time that it spontaneously ignites in a thermonuclear outburst on the white dwarf’s surface. NRC: National Research Council. NTP: Nuclear-thermal propulsion, a nuclear propulsion system that converts the thermal energy produced by a nuclear reactor directly into thrust, for example, by heating hydrogen, which is then exhausted through a propulsion nozzle. Nucleon: Neutron or proton. Nucleosynthesis: The process by which heavy elements such as helium, carbon, nitrogen, and iron are formed out of the fusion of lighter elements, such as hydrogen, during the normal evolution of stars, during supernova explosions, and in the Big Bang.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Obliquity: The angle between the orbital plane of an object and the plane of its rotational equator. Occultation: The disappearance of the light of a celestial body owing to the intervention of another body of larger apparent size across the line of sight. Outgassing: The emanation of gases from within an object. Parallax: The apparent shift in position of a nearby object relative to a more distant object, as the observer changes position. Using basic trigonometry, it is possible to derive the distance of a star from its parallax as observed from opposite points on Earth’s orbit. PDS: Planetary Data System. Petrology: Field of geology that focuses on the study of rocks and the conditions by which they form. Photovoltaic: A photovoltaic cell is a device that turns light into electrical energy. Planetesimals: The planetary bodies that formed the building blocks of all the solar system’s planets and satellites. Plasma waves: Periodically interacting electromagnetic waves and particles common in ionized plasmas such as the solar wind. Positron: Antiparticle of the electron. Prebiotic: Not yet alive; a chemical system or environment that is a precursor to life. Project Prometheus: A NASA initiative to develop nuclear technology for power and propulsion in space exploration, announced in January 2003. Protostellar disk: The disk of gas, dust, and matter from which planets are formed around other stars. PUI: Pickup ions, produced by charge exchange between interstellar neutral atoms and solar-wind ions. Pulsar: A spinning neutron star that emits radiation in a beam. The sweeping action of the beam causes the object to pulse regularly when viewed by an observer, just as with a lighthouse. Quantum gravity: Quantum gravity is the study of theories that incorporate known gravitational and quantum phenomena in a unified mathematical framework. Quasi-stellar object (QSO): Together with active galactic nuclei, quasi-stellar objects form the group of objects known as active galaxies. Radioisotope: An atom with an unstable nucleus that spontaneously undergoes radioactive decay by emitting gamma ray(s) and/or subatomic particles. Radioisotope power system (RPS): A system that produces electrical power utilizing the nuclear decay of radioactive isotopes, typically plutonium-238 (238Pu). Radiolysis: Molecular decomposition of a substance as a result of the action of radiation.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Rankine power conversion: A closed-cycle power-conversion system commonly found in power generation plants. Heat is added at constant pressure in a boiler, which converts the working fluid (e.g., water) to superheated steam. The steam expands through a turbine at constant entropy before being exhausted at a lower pressure into a condenser, where the steam is cooled. The resulting condensate is compressed at constant entropy so that it can be returned to the boiler. Reactor coolant: A gas or fluid circulated through a heat exchanger, radiator, or other cooling device to maintain the core of a nuclear reactor at the desired temperature. Redox: Chemical reactions in which there is a loss of an electron by a molecule or atom (oxidation) and an uptake of an electron by another molecule or atom (reduction). Such reactions are essential to biological energy exchange and thus to life on Earth. Redshift: Radiation from an approaching object is shifted to higher frequencies (to the blue), while radiation from a receding object is shifted to lower frequencies (to the red). A similar effect raises the pitch of an ambulance siren as it approaches. The expansion of the universe makes objects recede so that the light from distant galaxies is redshifted. The redshift is parameterized by z, where the wavelength shift is given by the factor (z) times the wavelength in the rest frame. Refractory alloy: An alloy with an extremely high melting point, which makes it suitable for use in construction of reactor cores and other high-temperature applications. Regolith: The layer of fragmented, incoherent rocky debris on the surface of a planetary body. Relativistic: Systems with particles moving with velocities close to the velocity of light. RF: Radio frequency. RHESSI: Reuven Ramaty High Energy Solar Spectroscopic Imager. RHU: Radioisotope heater unit. RJ: Distance measure, radii of Jupiter. RTG: Radioisotope thermoelectric generator. SAFIR: Single Aperture Far-Infrared Observatory. SDO: Solar Dynamics Observatory. Shannon’s theorem: A fundamental concept in information theory relating the maximum rate at which data can be transmitted in a communications system to the system’s bandwidth and background noise. Shot noise: Shot noise consists of random fluctuations in a signal (electrical or optical) caused by the discrete nature of the signal carriers (i.e., electrons or photons, respectively). The strength of this noise increases with growing magnitude of the average current or intensity of the light. Signal-to-noise ratio (S/N): The ratio between the magnitude of a signal and the magnitude of background noise.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Small Explorer (SMEX): A continuing series of highly focused and relatively inexpensive (less than $120 million in total cost to NASA) astrophysics and space physics missions. SOHO: Solar and Heliospheric Observatory. Solar Coronal Cluster: A mission of four clustered spacecraft near the Sun that provide basic understanding of the physical drivers of space weather and radiation, as detailed in this report. Solar-electric propulsion: Reaction system that utilizes solar energy to power an ion engine. Solar energetic particles (SEPs): Atoms that are associated with solar flares. SEPs are a type of cosmic ray. Solar flux: A unit of radio emission from the Sun, used as a measured index for solar activity. Solar sail: A device that uses the pressure of sunlight to propel a spacecraft in the same way that a sailing ship uses wind. Solar System Disk Explorer: A mission to explore the outer heliosphere and the disk of dust in the Kuiper Belt and beyond. Solar wind: The stream of charged particles and atoms (mainly ionized hydrogen but actually a mixture of all atoms in the Sun) moving outward all the time from the Sun with low velocities in the range of 300 to 500 kilometers per second. Space Infrared Telescope Facility (SITRF): Now known as the Spitzer Space Telescope, it was launched in August 2003 and detects the infrared energy, or heat, radiated by objects in space between wavelengths of 3 and 180 microns. Specific impulse (Isp): Total thrust provided by an engine divided by the mass flow rate of the propellant used to provide that thrust. In other words, it is the length of time that unit weight of propellant can provide unit thrust. Spectroscopy: A technique whereby the light from astronomical objects is broken up into its constituent colors. Radiation from the different chemical elements that make up an object can be distinguished, giving information about the abundances of these elements and their physical state. Sputtering: The process in which atoms on the surfaces of airless planetary bodies are knocked free by high-speed atomic particles in the solar wind; much-higher-energy cosmic rays can also sputter surface materials. SRG: Stirling radioisotope generator. SSB: Space Studies Board. STDT: Solar Probe Science and Technology Definition Team. STEREO: Solar-Terrestrial Relations Observatory. Stirling power conversion: A closed-cycle power-conversion system that features isothermal (constant temperature) expansion of a gas in a cylinder, followed by an isochoric (constant volume) cooling, followed by isothermal compression back to the original volume and isochoric heating back to the original temperature.

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Priorities in Space Science Enabled by Nuclear Power and Propulsion Stratigraphy: The study of the relationships between stratified rocks. String theory: A new physical theory that is described as being both a consistent quantum theory of gravity and a unified theory of all particles and forces. Sublimation: Phase transformation directly from the solid to gaseous state. Supernova: A star that, due to accretion of matter from a companion star or exhaustion of its own fuel supply, can no longer support itself against its own weight and thus collapses, throwing off its outer layers in a burst of energy that outshines an entire galaxy. In 1987 a star in the Large Magellanic Cloud was observed as a dramatic supernova called Supernova 1987A. Systems for Nuclear Auxiliary Power (SNAP): A program of experimental radioisotope thermoelectric generators (RTGs) and space nuclear reactors flown during the 1960s. Telemetry: Data received electronically from a spacecraft during flight. Termination shock: Where the solar wind is abruptly slowed and heated prior to being deflected by the interstellar medium. Thermoelectric power conversion: Direct, static conversion of heat energy into electric power through the use of special materials that produce a voltage differential when parts of the material or system are maintained at different temperatures. Time-of-arrival triangulation: A method of calculating the location of an object or event (e.g., a gamma-ray burst) by using the difference in detection times at a minimum of three different, widely separated detectors. Torus: A doughnut-shaped surface of revolution generated by revolving a circle about an axis coplanar with the circle. TPF: Terrestrial Planet Finder. Transverse velocity: The component of the velocity of an object, such as a star, that is at right angles to the observer’s line of sight; also known as tangential velocity. Van Allen belts: A donut-shaped region in Earth’s magnetosphere that contains a high density of energetic charged particles trapped in the dipole field of the planet. VISE: Venus In Situ Explorer. Volatiles: Elements that condense from or exist as a gas at low temperatures. Waveband: A band of adjacent frequencies. WMAP: Wilkinson Microwave Anisotropy Probe. Zodiacal dust cloud: A lenticular-shaped dust cloud surrounding the Sun and maintained by fine material from asteroidal collisions and cometary activity. Zodiacal light: A faint glow of light scattered off the zodiacal dust; it can sometimes be seen under very dark sky conditions, along the horizon, either just after dusk or before sunrise.