The exosphère is the last layer of Terrestrial atmosphere which is located above the Thermosphère. This layer is defined as the area of the atmosphere where the collisions between particles rare, are considered like negligible. The Atome S behave there freely, some escape even in the space.
The base of the exosphère (the exobase) ranges between 350 and 800 km of altitude according to the Température at the Thermopause, which is related to the Solar activity. The Helium and the Hydrogène are the dominating element S there.
It extends until the extreme limit from the atmosphere, that is to say 50 000 kilometers. There only some hydrogen atoms are found.
It is in this layer, that the satellite majority of the S are placed in Orbite.
Interest for the satellites
One of the great interests of the exosphère resides in the exceptional capacity of lifespan of the satellites placed in its highest layers: hundreds, even of the thousands of years, whereas the satellites which evolve/move at a less altitude, about 300 kilometers (Thermosphère), must undergo a considerable permanent braking.
As of 1960, scientists, such French François Barlier, were interested in the trajectory calculation of the satellites. Their work then very quickly concerned the forces of braking due to frictions resulting from the residual atmosphere of the thermosphère and the very high temperature which reigns there - up to 1.000 Kelvin S -, phenomenon in particular of with the absorption of the ultraviolet radiation emitted by the sun.
See too
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