Exosphere

The exosphere is the region where molecules from the atmosphere can overcome the pull of gravity and escape into outer space. The atmosphere slowly diffuses into the void of space. The exosphere usually begins about 500 km up (notice, this is well off the chart on page 1), but there is no definable boundary to mark as the end of the thermosphere and the beginning of the exosphere. There is no definable upper limit to the exosphere. Even at heights of 800 km, the atmosphere is still measurable. However, molecule concentrations are very small and considered negligible.

In this region of the atmosphere, hydrogen and helium are the prime components and are only present at extremely low densities. This is the area where many satellites orbit the Earth.