Back in school, generally in earth science in whatever grade one took that course in, a lot was taught about the makeup of our atmosphere. Like an onion that has layers, any remember that the atmosphere also comes in layers. These layers are the troposphere, the stratosphere, mesosphere, thermosphere, and the exosphere. As a refresher, let's take a brief look at the exosphere.
The exosphere is the outer most boundary of our atmosphere, starting about three hundred and ten miles above the earth's surface and ending about six hundred and twenty miles out. This distance is dependent on solar activity. The gases, including the chief heat trapping gases such as carbon dioxide that make up the exosphere are light gasses that also include hydrogen, helium and atomic oxygen. This is significant because the extra carbon that we generate that is put in to the atmosphere, this is where it goes, effectively thickening up our atmosphere with its heat trapping properties, and causing unnatural global warming.
The exobase, the lowest part of the exosphere, is also called the critical level and is defined in a formal sense in two ways. One, it's the height above which there are negligible atomic collisions between particles and two, the height above which constituent atoms are on purely ballistic trajectories. In other words, these particles don't collide as often because of more space between them and they are left to wander where they will in the exosphere.
The upper boundary of the exosphere is theoretically defined by the altitude about one hundred and ninety thousand kilometers, or one hundred and twenty thousand miles, (which is about half the distance to the moon) where the influence of solar radiation puts pressure on atomic hydrogen velocities exceeding the earth's gravitational pull.
The part of the exosphere that is observable from space is called the geocorona. It has been seen to extend at least one hundred thousand kilometers, or sixty two thousand miles, from the surface of earth to its end. In fact it is said that the exosphere is the transitional zone between earth's atmosphere and interplanetary space.
The exosphere is also the second part of the thermosphere. The thermosphere is actually made up of the exosphere and the ionosphere, which marks the upper limit of our breathable atmosphere ends and the need for protection from the upper levels of the atmosphere begin, as this part of the atmosphere would kill us otherwise.