A B C D E F G H I K L M N O P R S T V
accretion: The accumulation of material, under the influence of gravity, to form a galaxy, star, planet, or moon.
achondrite: A stony meteorite without chondrules.
anorthosite: An igneous rock made up almost entirely of plagioclase feldspar.
asteroid: A small rocky or metallic body that orbits a star.
asteroid belt: The region of the solar system where most of the asteroids orbit. It lies between the orbits of Mars and Jupiter.
astronomy: The scientific study of the universe.
astrophysics: The branch of astronomy that deals with the physics of astronomical objects and phenomena.
atmosphere: The gaseous envelope surrounding a star, planet, or satellite and bound to it by gravity.
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basalt: A fine-grained, dark-colored igneous rock composed primarily of plagioclase feldspar and pyroxene; other minerals, such as olivine and opaques, are usually present.
bleb: A small, usually rounded inclusion of one material in another.
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calcium-aluminum inclusions (abbreviated CAIs): Small rocks composed of calcium and aluminum, the first to condense in our solar system.
chondrite: A stony meteorite containing chondrules imbedded in a fine-grained matrix of pyroxene, olivine, and nickel-iron.
chondrule: A small, rounded body found embedded in certain meteorites that formed when clumps of dust grains drifting in the solar nebula melted and solidified rapidly, forming small crystals.
comet: A small solar system body made of ice and dust that moves in an elliptical orbit around the Sun. A typical comet has a solid nucleus a few kilometers in diameter. When it nears the inner solar system, the ices evaporate and form an extended and diffuse atmosphere that is blown away from the Sun by the solar wind and radiation pressure to form a prominent tail of gas and dust.
condensation: The formation of a liquid or solid from the gaseous state.
core: The central region of a planet or moon, frequently made of different materials from the surrounding regions (mantle and crust); the Earth is thought to have a core of metallic iron and nickel.
crater: A bowl-shaped depression on a planet or moon created from above by the impact of an extraterrestrial body or from below by a volcanic eruption.
crust: The outermost solid layer of the Earth or of similar bodies.
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density: The amount of matter in a prescribed volume of material.
differentiation: The chemical zonation caused by differences in the densities of minerals; dense materials sink, less dense materials float.
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ejecta: Material thrown out from and deposited around an impact crater.
element: A substance composed of atoms having the same number of protons in each nucleus.
extraterrestrial: Located or originating outside the Earth and its atmosphere.
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fall: The designation given to a meteorite that was observed as it came through the Earth's atmosphere and that was retrieved soon afterward.
find: The designation given to a meteorite that was found, but not observed to fall.
fusion crust: The dark glassy coating on the surface of a meteorite, caused by heating as the meteorite entered the atmosphere.
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gas-giant planet: A giant planet with a massive and deep atmosphere that surrounds a relatively small rocky core.
gravity: The force of attraction acting between any two masses (according to Isaac Newton); the curvature of space by matter (according to Albert Einstein).
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hydrogen: The lightest, simplest, and most abundant element in the universe.
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impact: The forceful striking of one body, such as a meteorite, against another body, such as a moon or planet.
impact crater: The hole or depression formed by a meteorite colliding with a surface.
inclusion: A fragment of one rock enclosed in another rock.
iron meteorite: A meteorite consisting of metallic iron and nickel.
isotope: Elements having an identical number of protons in their nuclei but differing in their number of neutrons.
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kilometer (abbreviated km): A unit of length equal to 1,000 meters, or .62 miles.
kinetic energy: The energy inherent in a body due to its motion; with greater speed and mass, the kinetic energy increases.
Kuiper Belt: A doughnut-shaped region of comets in orbit beyond Neptune, assumed to be the oldest surviving remnant of the original solar nebula and the source of short-period comets.
Kuiper Belt objects (abbreviated KBOs): The comets that populate the Kuiper Belt.
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law of gravitation: The law stating that any two bodies attract each other with a force that increases in proportion to their masses and decreases in proportion to the square of the distance between them (discovered by Isaac Newton).
light-year: The distance that light travels in one year (63,000 astronomical units, or 9.46 trillion kilometers), a convenient unit of measurement for interstellar distances.
long-period comet: A comet with an orbital period exceeding 200 years. Such long-period comets have very elongated elliptical orbits, and can have an orbital period of more than a million years. They originate from the Oort cloud in the outermost reaches of our solar system.
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mantle: The part of the Earth (or other rocky body) lying between the outer crust and the central core, consisting mostly of iron and silicate minerals.
matrix: The smaller-sized grains in a rock when the rock consists of large grains or fragments surrounded by smaller grains.
meteor: A bright streak of light produced by a small fragment of rock or metal that burns up as it enters the atmosphere.
meteorite: A fragment of rock or metal that has landed on the Earth from interplanetary space. Most meteorites come from asteroids, but a few are from other planets or satellites.
meteoriticist: A person who studies meteorites.
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nebula: An immense cloud-like mass of interstellar gas and dust, generally in the spiral arms of a galaxy.
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Oort cloud: A spherical cloud of trillions of comets extending about halfway to the nearest stars and weakly bound by the Sun's gravity. Long-period comets originate from the Oort cloud.
orbit: The path of one celestial body moving around another under the force of gravity.
orbital period: The time interval for a body to complete one orbit around another.
oxygen: An element consisting of atoms with eight protons. Two oxygen atoms combine to make molecular oxygen (O2), and three combine to make ozone (O3). The Earth's atmosphere is 21 percent molecular oxygen.
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physics: The study of matter and energy, and the forces and fields by which they interact in space and time.
planet: An astronomical body with enough mass for its gravity to make it spherical but not enough to generate nuclear energy. Planets have non-intersecting orbits around stars or drift freely in space.
planetesimal: One of the family of asteroid-sized bodies that first condensed out of the disk of the solar nebula and later collided to form the planets.
plutino: A subclass of Kuiper Belt objects which, like Pluto, orbit the Sun twice during every three orbits of Neptune.
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regmaglypt: Any of various small indentations or pits on the surface of meteorites.
resonance: One of the natural states of oscillation in a physical system, such as the periodic swing of a pendulum or vibration of a spring.
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satellite: A body that orbits around a larger body.
short-period comet: A comet with an orbital period of less than 200 years, the most famous example being Halley's comet, which appears every 76 years. Short-period comets come from the Kuiper Belt and typically orbit the Sun in the same direction as the planets.
solar nebula: The cloud of gas and dust that formed the young Sun and the surrounding planets.
solar system: The Sun and all the objects bound to it by gravity (planets, satellites, asteroids, comets).
spectroscope: An optical instrument designed to spread out light into the spectrum of its component colors.
star: A self-luminous body held together by gravity and with a central temperature sufficient to liberate energy by nuclear fusion.
stony-iron: A class of meteorites composed mostly of a mixture of silicates and iron metal.
sublime: To evaporate directly from the solid to the vapor phase. For example, the ice caps on Mars sublime and re-condense with the seasons.
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tektites: The small fragments of melted and aerodynamically shaped rock that were ejected from a large impact crater.
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vaporize: To change something from a liquid or a solid to a gaseous state, as in rock that is completely changed to gas during large impacts.
velocity: The speed and direction of an object's motion.
volatile: Able to vaporize at relatively low temperatures. Ices of water, methane, carbon dioxide, and ammonia are volatile.
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