exosphere

[ek-soh-sfeer]
noun
the highest region of the atmosphere, where the air density is so low that a fast-moving air molecule is more than 50 percent likely to escape from the atmosphere instead of hitting other molecules.

Origin:
1950–55; exo- + -sphere

exospherical [ek-suh-sfer-i-kuhl, -sfeer-] , exospheric, adjective
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Based on the Random House Dictionary, © Random House, Inc. 2014.
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Collins
World English Dictionary
exosphere (ˈɛksəʊˌsfɪə)
 
n
the outermost layer of the earth's atmosphere. It extends from about 400 km above the earth's surface

Collins English Dictionary - Complete & Unabridged 10th Edition
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American Heritage
Science Dictionary
exosphere  [%PREMIUM_LINK%]     (ěk'sō-sfîr')  Pronunciation Key 
The outermost region of the Earth's atmosphere, beginning at an altitude of approximately 550 km to 700 km (341 to 434 mi) and merging with the interplanetary medium at around 10,000 km (6,200 mi). The exosphere consists chiefly of ionized hydrogen, which creates the geocorona by reflecting far-ultraviolet light from the Sun. On the remote edges of the exosphere, hydrogen atoms are so sparse that each cubic centimeter might contain only one atom; furthermore, the pressure and gravity are weak enough that atoms in the exosphere can escape entirely and drift into space. Artificial satellites generally orbit in this region. See also mesosphere, stratosphere, thermosphere, troposphere., See illustration at atmosphere.

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Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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Encyclopedia Britannica
Encyclopedia

exosphere

outermost region of a planet's atmosphere, where molecular densities are low and the probability of collisions between molecules is very small. The base of the exosphere is called the critical level of escape because, in the absence of collisions, lighter, faster-moving atoms such as hydrogen and helium may attain velocities that allow them to escape the planet's gravitational field. Most molecules, however, have velocities considerably lower than the escape velocity, so their rate of escape to outer space is quite low.

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Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.
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Example sentences
Perhaps this filters out all but the tiniest organisms, lofted up to the
  exosphere, evaporated out into space.
Of the phenomena listed, only the ionosphere and the exosphere can be scaled to
  laboratory size.
The increase in temperature stops at this height, beyond which lies the
  exosphere.
The exosphere is the top of the atmosphere above the hetero-sphere.
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