The Earth’s atmosphere is broken up into several distinct layers. We live down in the troposphere, where the atmosphere is thickest. Above that is the stratosphere, then there’s the mesosphere, thermosphere and finally the exosphere. The top of the exosphere marks the line between the Earth’s atmosphere and interplanetary space.
The exosphere is the outermost layer of the Earth’s atmosphere. It starts at an altitude of about 500 km and goes out to about 10,000 km. Within this region particles of atmosphere can travel for hundreds of kilometers in a ballistic trajectory before bumping into any other particles of the atmosphere. Particles escape out of the exosphere into deep space.
The lower boundary of the exosphere, where it interacts with the thermosphere is called the thermopause. It starts at an altitude of about 250-500 km, but its height depends on the amount of solar activity. Below the thermopause, particles of the atmosphere have atomic collisions, like what you might find in a balloon. But above the thermopause, this switches over to purely ballistic collisions.
The theoretical top boundary of the exosphere is 190,000 km (half way to the Moon). This is the point at which the solar radiation coming from the Sun overcomes the Earth’s gravitational pull on the atmospheric particles. This has been detected to about 100,000 km from the surface of the Earth. Most scientists consider 10,000 km to be the official boundary between the Earth’s atmosphere and interplanetary space.
You can learn more about the layers of the atmosphere, including the exosphere from this page at NASA.
We have recorded a whole episode of Astronomy Cast talking about the Earth’s (and it’s atmosphere). Check it out here, Episode 51: Earth.