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Chapter 8
Pluto, Comets, and Space
Debris
term:
Asteroid
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definition:
A "minor planet," a non-luminous chunk of rock
smaller than planet-size but larger than a
meteoroid, in orbit around a star.
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term:
asteroid belt
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definition:
A region of the Solar System, between the
orbits of Mars and Jupiter, in which most of the
asteroids orbit.
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term:
coma
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definition:
(a) Of a comet, the region surrounding the
head; (b) of an optical system, an off-axis
aberration in which the images of points appear
with comet-like asymmetries.
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term:
Comet
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definition:
A type of object orbiting the Sun, often in a very
elongated orbit, that when relatively near to the
Sun shows a coma and may show a tail.
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term:
dust tail
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definition:
The dust left behind a comet, reflecting sunlight.
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term:
gas tail
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definition:
The puffs of ionized gas trailing a comet.
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term:
head
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definition:
Of a comet, the nucleus and coma together.
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term:
Kuiper-belt object
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definition:
An object in the Kuiper belt; many comets and
probably even the planet Pluto are examples.
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term:
Meteor
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definition:
A track of light in the sky from rock or dust
burning up as it falls through the Earth's
atmosphere.
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term:
meteor shower
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definition:
The occurrence at yearly intervals of meteors at
a higher-than-average rate as the Earth goes
through a comet's orbit and the dust left behind
by that comet.
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term:
meteorite
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definition:
An interplanetary chunk of rock after it impacts
on a planet or the Moon, especially on the
Earth.
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term:
meteoroid
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definition:
An interplanetary chunk of rock smaller than an
asteroid.
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term:
Micrometeorite
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definition:
A tiny meteorite. The micrometeorites that hit
the Earth's surface are sufficiently slowed down
that they can reach the ground without being
vaporized.
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term:
minor planets
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definition:
Asteroids.
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term:
Near-Earth Object
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definition:
A comet or asteroid whose orbit is sufficiently
close to Earth's that a collision is possible or
even likely in the long term. See Apollo
asteroids.
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term:
Nucleus
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definition:
(a) Of an atom, the core, which has a positive
charge, contains most of the mass, and takes
up only a small part of the volume; (b) of a
comet, the chunks of matter, no more than a
few km across, at the center of the head; (c) of
a galaxy, the innermost region.
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term:
Oort comet cloud
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definition:
The trillions of incipient comets surrounding the
Solar System in a 50,000 A.U. (radius) sphere.
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term:
Radiant
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definition:
The point in the sky from which all meteors in a
meteor shower appear to be coming.
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term:
shooting stars
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definition:
Meteors.
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term:
sporadic meteor
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definition:
A meteor not associated with a shower.
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term:
Tail
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definition:
Gas and dust left behind as a comet orbits
sufficiently close to the Sun, illuminated by
sunlight.
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term:
Trans-Neptunian
Object (TNO)
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definition:
One of the sub-planetary objects in the Kuiper
belt; many people feel that Pluto is one, though
it also retains its status as a planet, at least for
the time being.
[ end of Chapter 8 ]
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