exosphere

American  

[ek-soh-sfeer] / ˈɛk soʊˌsfɪər /

noun

1. 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.

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exosphere British  

/ ˈɛksəʊˌsfɪə /

noun

1. the outermost layer of the earth's atmosphere. It extends from about 400 km above the earth's surface

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exosphere Scientific  

/ ĕk′sō-sfîr′ /

1. 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.

2. See also mesosphere stratosphere thermosphere troposphere See illustration at atmosphere

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Other Word Forms

• exospheric adjective
• exospherical adjective

Etymology

Origin of exosphere

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

Example Sentences

The astronauts on the International Space Station receive a dose of around 240 to 480 X-rays on a six-month jaunt up in the exosphere.

From Slate • Mar. 1, 2026

The escaping helium originates from the planet's extended upper atmosphere, known as the "exosphere."

From Science Daily • Dec. 10, 2025

For 48 hours, a French instrument mounted on the lander will study the origin and dynamics of the lunar exosphere, a wispy layer of gases that hugs the surface.

From Science Magazine • Apr. 23, 2024

There are 1.2 million LED “pucks” that make up the 580,000-square feet exosphere, each of which can display more than 1 billion colors.

From Seattle Times • Apr. 19, 2024

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

From The Scarlet Lake Mystery by Goodwin, Harold L. (Harold Leland)