Here's why:

* Exosphere: The outermost layer, where the atmosphere gradually fades into space. It's too thin for satellites to orbit.

* Thermosphere: This layer extends from about 80 to 600 kilometers above Earth. It's where the International Space Station orbits and where most satellites are found. The air is extremely thin, but there is still enough to cause drag on spacecraft.

* Mesosphere: This layer is below the thermosphere and is too dense for stable orbits.

* Stratosphere: The layer above the troposphere, where the ozone layer is located. Too dense for orbiting spacecraft.

* Troposphere: The lowest layer of the atmosphere, where weather occurs. Far too dense for stable orbits.

While the thermosphere is extremely thin, it's still denser than the exosphere, allowing spacecraft to maintain an orbit.