The Short Answer:
What is the exosphere?
The exosphere is the outermost layer of Earth’s atmosphere.
The outermost layer
The exosphere is the outermost layer of our atmosphere. “Exo” means outside and is the same prefix used to describe insects like grasshoppers that have a hard shell or “exoskeleton” on the outside of their body.
The exosphere is the very edge of our atmosphere. This layer separates the rest of the atmosphere from outer space. It’s about 6,200 miles (10,000 kilometers) thick. That’s almost as wide as Earth itself. The exosphere is really, really big. That means that to get to outer space, you have to be really far from Earth.
The exosphere has gases like hydrogen and helium, but they are very spread out. There is a lot of empty space in between. There is no air to breathe, and it’s very cold.