The exosphere (from the Greek words exo = out(side) and sphaira = ball) is the uppermost layer of the atmosphere. On Earth, its lower boundary at the edge of the thermosphere is estimated to be 500 km to 1000 km above the Earth's surface, and its upper boundary at about 10,000 km. It is only from the exosphere that atmospheric gases, atoms, and molecules can, to any appreciable extent, escape into outer space. The main gases within the exosphere are the lightest gases, mainly hydrogen and helium, with some atomic oxygen near the exobase. A layer is the following: In abstraction, a layer is an abstract place conceived as having depth. ... Layers of Atmosphere (NOAA) Earths atmosphere is a layer of gases surrounding the planet Earth and retained by the Earths gravity. ... Earth, also known as Terra, and Tellus mostly in the 19th century, is the third-closest planet to the Sun. ... The word Boundary has a variety of meanings. ... The thermosphere is the layer of the Earths atmosphere directly above the mesosphere and directly below the exosphere. ... To help compare orders of magnitude; this page lists lengths between 100 and 1,000 km (105 and 106 m). ... To help compare different orders of magnitude, this page lists lengths starting at 107 m (10,000 km). ... A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... Layers of Atmosphere - not to scale (NOAA) Outer space, also called just space, refers to the relatively empty regions of the Universe outside the atmospheres of celestial bodies. ... A gas is one of the four main phases of matter (after solid and liquid, and followed by plasma), that subsequently appear as a solid material is subjected to increasingly higher temperatures. ... General Name, Symbol, Number hydrogen, H, 1 Chemical series nonmetals Group, Period, Block 1, 1, s Appearance colorless Atomic mass 1. ... General Name, Symbol, Number helium, He, 2 Chemical series noble gases Group, Period, Block 18, 1, s Appearance colorless Atomic mass 4. ... General Name, Symbol, Number Oxygen, O, 8 Chemical series nonmetals Group, Period, Block 16 (VIA), 2 , p Density, Hardness 1. ...

The atmosphere in this layer is sufficiently rarified for satellites to orbit the Earth, although they still receive some atmospheric drag. A satellite is any object that orbits another object (which is known as its primary). ... In physics, an orbit is the path that an object makes, around another object, whilst under the influence of a source of centripetal force, such as gravity. ... Atmospheric drag is a form of drag, which is the force that opposes an object moving through a liquid or gas. ...

Exobase, also called the critical level, the lowest altitude of the exosphere, is defined in one of two ways:

1. The height above which there are negligible atmospheric collisions between the particles and
2. The height above which the constituent atoms are on purely ballistic trajectories.

The exact altitude at which the exosphere ends and space begins is not well-defined, and attempting to attach a specific value to it is not particularly useful.

See also

• Edge of space

Layers of Atmosphere (NOAA) The Karman Line is an internationally designated altitude commonly used to define outer space. ...

External link

• http://www.mardiros.net/atmosphere/exosphere.html