The outermost region of a planetary atmosphere, called the exosphere, is the tenuous region where the mean free path of the particles is much larger compared to the scale height. The altitude over which the atmospheric density decreases by about a factor of 2.7 compared to its previous level is called the scale height. In the upper atmosphere of a planet, this depends on the mass of the species, temperature of the species, and the gravity of the planet.
Comprehensive timeline of ISRO's journey through decades.
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