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[ek-soh-sfeer] /ˈɛk soʊˌsfɪər/
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 of exosphere
1950-55; exo- + -sphere
Related forms
[ek-suh-sfer-i-kuh l, -sfeer-] /ˌɛk səˈsfɛr ɪ kəl, -ˈsfɪər-/ (Show IPA),
exospheric, adjective Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2015.
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Examples from the Web for exosphere
Historical Examples
  • The atoms of the exosphere were lonely, uncrowded, isolated particles.

    First on the Moon Jeff Sutton
  • His instruments told him they were breaching the exosphere, where molecular matter had almost ceased to exist.

    First on the Moon Jeff Sutton
  • In the shed on the left is Orion, which is a two-stage rocket for deep penetration into the exosphere.

    The Scarlet Lake Mystery Harold Leland Goodwin
British Dictionary definitions for exosphere


the outermost layer of the earth's atmosphere. It extends from about 400 km above the earth's surface
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition
© William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012
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exosphere in Science
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.

The American Heritage® Science Dictionary
Copyright © 2002. Published by Houghton Mifflin. All rights reserved.
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