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.
exosphere (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.
[ek-soh-sfeer] 
soʊˌsfɪər