Exosphere

The exosphere is the outermost layer of Earth's atmosphere (i.e. the uppermost layer of air). It extends from the top of the thermosphere to about 10,000 km (6,200 mi) above the surface of the planet.
The exosphere is very thin and contains very little air. Most of the particles in the exosphere are in a state of free fall, meaning they are constantly falling towards Earth's surface. The exosphere is also where most of the satellites orbit.

The exosphere was first discovered in the late 19th century by British astronomer Norman Lockyer. He observed that the spectral lines of some elements were shifted to the red end of the spectrum when they were observed from high altitudes. This led him to conclude that there must be a layer of the atmosphere that was very thin and very hot.