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achondrite  A stony meteorite lacking chondrules.
Amor asteroid  A member of a class of asteroids having orbits that cross the orbital distance of the Earth.
Apollo asteroid  A member of a class of asteroids having orbits that cross the orbital distance of the Earth.
asteroid  A small, planetlike solar system body. Most asteroids are rocky in makeup and have orbits of low eccentricity and inclination.
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.
Aten asteroid  An asteroid having an orbit with semimajor axis smaller than 1 AU.
C-type asteroid  One of a class of very dark asteroids whose reflectance spectra show no absorption features due to the presence of minerals.
carbonaceous chondrite  A stony meteorite that contains carbonrich material. Carbonaceous chondrites are thought to be primitive samples of material from the early solar system.
chondrite  A meteorite containing chondrules.
chondrule  A small, spherical body embedded in a meteorite. Chondrules are composed of iron, aluminum, and magnesium silicate rock.
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.
comet  A small, icy body in orbit about the Sun. When a comet is near the Sun, it displays a coma and a tail.
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.
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.
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.
iron meteorite  A meteorite composed primarily of iron and nickel.
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.
long-period comet  A comet with an orbital period of 200 years orlonger.
M-type asteroid  One of a class of asteroids that have reflectance spectra like those of metallic iron and nickel.
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.
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.
meteorite  The portion of a meteoroid that reaches the Earth’s surface.
meteoroid  A solid interplanetary particle passing through the Earth’s atmosphere.
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.
minor planet  Another name for asteroid.
new comet  A comet that has entered the inner solar system for the first time.
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.
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.
plasma tail  A narrow, ionized comet tail pointing directly away from the Sun.
radiant  The point in the sky from which the meteors in a meteor shower seem to originate.
reflectance spectrum  The reflectivity of a body as a function of wavelength.
S-type asteroid  One of a class of asteroids whose reflectance spectra show an absorption feature due to the mineral olivine.
short-period comet  A comet with an orbital period shorter than 200 years.
solidification age  The amount of time that has passed since a meteorite solidified from the molten state.
stony meteorite  A meteorite made of silicate rock.
stony-iron meteorite  A meteorite made partially of stone and partially of iron and other metals.
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.
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.
V-type asteroid  The asteroid Vesta, which is unique in having a reflectance spectra resembling those of basaltic lava flows.
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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