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nebula
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An immense, diffuse body of interstellar gas and dust that has not condensed into a star.
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nebular hypothesis
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A theory of the formation of the planets that states that a rotating nebula contracted and was then torn into fragments by centrifugal forces, with planets condensing from the fragments.
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planetesimal
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Bodies in the solar nebul, formed from accumulation of dust grain. They vary greatly in size from a few microns up to the size of small planets.
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proto-sun
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A large cloud of dust and gas gradually coalescing into a star under the force of gravity.
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meteorite
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A stoney or metallic object from inter-planetary space that penetrates the atmosphere to impact on the surface.
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micrometeorite
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A meteorite less than 1 millimeter in diameter.
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chondrite
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A stony meteorite having chondules and composed almost entirely of the silicate minerale olivine, pyroxene, plagioclase which are the most abundant constituents of the earth's crust. The age of chondrictic meteorites uniformly falls close to 4.6 billion years, nearly as great as the solar system itself. The chonditric meteorites may be close in composition to the terrestiae planets beforc they formed a core.
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carbonaceous chondrite
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A stony meteorite that contains carbon compounds.
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ejecta blanket
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Circular apron of debris surrounding an impact crater.
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ray
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A linear landform of the lunar surface emanating from a large crater and extending as much as 100 kilometers outward, probably consisting of fine ejecta thrown out by the impact of a meteorite.
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impact crater
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Circular, rimmed depression produced by the impact of a solid body traveling at more than a few kilometers per second.
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limb, celestial
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"The edge of a celestial body as it appears to an observer; the edge of the sun's disk, for example."
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bolide
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An extraterrestrial body in the 1-10-km size range, which impacts the earth at velocities of literally faster than a speeding bullet (20-70 km/sec = Mach 75), explodes upon impact, and creates a large crater. Bolide is a generic term, used to imply that we do not know the precise nature of the impacting body . . . whether it is a rocky or metallic asteroid, or an icy comet, for example.
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orbit
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The elliptical or hyperbolic path traced by a planet or meteorite or satellite in the presence of a more massive body.
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