| achondrite | A stony meteorite lacking chondrules.
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| Amor asteroid | A member of a class of asteroids having orbits that cross the orbital distance of the Earth.
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| Apollo asteroid | A member of a class of asteroids having orbits that cross the orbital distance of the Earth.
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| asteroid | A small, planetlike solar system body. Most asteroids are rocky in makeup and have orbits of low eccentricity and inclination.
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| asteroid belt | The region of the solar system lying between 2.1 and 3.3 astronomical units (AU) from the Sun. The great majority of asteroids is found in the asteroid belt.
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| Aten asteroid | An asteroid having an orbit with semimajor axis smaller than 1 AU.
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| C-type asteroid | One of a class of very dark asteroids whose reflectance spectra show no absorption features due to the presence of minerals.
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| carbonaceous chondrite | A stony meteorite that contains carbonrich material. Carbonaceous chondrites are thought to be primitive samples of material from the early solar system.
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| chondrite | A meteorite containing chondrules.
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| chondrule | A small, spherical body embedded in a meteorite. Chondrules are composed of iron, aluminum, and magnesium silicate rock.
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| coma | A spherical gaseous region that surrounds the nucleus of a comet. The coma of a comet may be 100,000 kilometers or more indiameter.
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| comet | A small, icy body in orbit about the Sun. When a comet is near the Sun, it displays a coma and a tail.
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| cosmic ray exposure age | The length of time that has passed since a meteorite broke off from a larger body and became exposed to radiation damage from cosmic rays.
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| dust tail | A comet tail that is luminous because it contains dust that reflects sunlight. The dust in a comet tail is expelled from the nucleus of the comet.
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| fireball | An especially bright streak of light in the sky produced when an interplanetary dust particle enters the Earth’s atmosphere, vaporizing the particle and heating the atmosphere.
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| iron meteorite | A meteorite composed primarily of iron and nickel.
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| Kuiper belt | A region beyond Neptune within which a large number of comets are believed to orbit the Sun. Short period comets are thought to originate in the Kuiper belt.
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| long-period comet | A comet with an orbital period of 200 years orlonger.
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| M-type asteroid | One of a class of asteroids that have reflectance spectra like those of metallic iron and nickel.
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| meteor | A streak of light produced by a meteoroid moving rapidly through the Earth’s atmosphere. Friction vaporizes the meteoroid and heats atmospheric gases along the path of the meteoroid.
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| meteor shower | A temporary increase in the normal rate at which meteors occur. Meteor showers last for a few hours or days and occur on about the same date each year.
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| meteorite | The portion of a meteoroid that reaches the Earth’s surface.
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| meteoroid | A solid interplanetary particle passing through the Earth’s atmosphere.
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| micrometeorite | A meteoritic particle less than 50 millionths of a meter in diameter. Micrometeorites are slowed by atmospheric gas before they can be vaporized, so they drift slowly to the ground.
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| minor planet | Another name for asteroid.
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| new comet | A comet that has entered the inner solar system for the first time.
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| nucleus | The massive, positively charged core of an atom. The nucleus of an atom is surrounded by one or more electrons. A nucleus missing one or more accompanying electrons is called an ion.
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| Oort cloud | The region beyond the planetary system, extending to 100,000 AU or more, within which a vast number of comets orbit the Sun. When comets from the Oort cloud enter the inner solar system, they become new comets.
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| plasma tail | A narrow, ionized comet tail pointing directly away from the Sun.
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| radiant | The point in the sky from which the meteors in a meteor shower seem to originate.
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| reflectance spectrum | The reflectivity of a body as a function of wavelength.
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| S-type asteroid | One of a class of asteroids whose reflectance spectra show an absorption feature due to the mineral olivine.
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| short-period comet | A comet with an orbital period shorter than 200 years.
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| solidification age | The amount of time that has passed since a meteorite solidified from the molten state.
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| stony meteorite | A meteorite made of silicate rock.
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| stony-iron meteorite | A meteorite made partially of stone and partially of iron and other metals.
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| terminal velocity | The speed with which a body falls through the atmosphere of a planet when the force of gravity pulling it downward is balanced by the force of air resistance.
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| Trojan asteroid | One of a group of asteroids that orbit the Sun at Jupiter’s distance and lie 60° ahead of or behind Jupiter in its orbit.
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| V-type asteroid | The asteroid Vesta, which is unique in having a reflectance spectra resembling those of basaltic lava flows.
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| zodiacal light | The faint glow extending away from the Sun caused by the scattering of sunlight by interplanetary dust particles lying in and near the ecliptic.
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