[ 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

First recorded in 1950–55; exo- + -sphere

Related forms

ex·o·spher·i·cal [ek-suh-sfer-i-kuh l, -sfeer-] /ˌɛk səˈsfɛr ɪ kəl, -ˈsfɪər-/, ex·o·spher·ic, adjective
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2019

Examples from the Web for exosphere

  • 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


/ (ˈɛksəʊˌsfɪə) /


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

Science definitions for exosphere


[ ĕksō-sfîr′ ]

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 © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.