Exosphere

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.

The atmosphere in this layer is sufficiently rarified for satellites to orbit the Earth, although they still receive some atmospheric drag.

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
• Journey to the Exosphere

very smart people

External link

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