km (not to scale)
Please note: some models of the atmosphere
consider the exosphere to be included within the thermosphere
which extends into the ionosphere
(not shown above). This is because of the differing models for the layers of the atmosphere. One model separates layers by temperature gradients. However, there is a special layer called the ionosphere
that is defined by charged particles rather than temperature gradient. In this case, the exosphere sits on top of the ionosphere.
The exosphere is the last part of the atmosphere before you hit the vacuum of interplanetary space. It isn't quite a vacuum - there are some molecules and atoms in this region though collisions are rare. These particles are in ballistic orbits. The average distance that a particle travels before hitting another one at 500 km is about 100 km (compare this to 1 micron at ground level).
The thermosphere is often said to start at 500km above the surface of the earth, though this fluctuates with the density of the atmosphere below it - as solar activity increases, the top of the thermosphere is pushed out. This point is known as the exobase.
While it is fairly well defined where it starts
by looking at the temperature of the gases within that region (such as the sudden change in temperature between the mesosphere and the thermosphere at about 90km) or charged particles, the 'boundary' between the atmosphere and space is not well defined though is guessed at 960km to 1000km above the surface of the earth. This is partly because the classic definition of temperature loses its meaning and hydrostatic law is no longer in effect. One may say that the exosphere extends to the last atmospheric particle that is held in orbit around the Earth.
The majority of the gas within the exosphere is hydrogen and helium with a some atomic oxygen (rather than molecular oxygen which we breath) near the exobase (beyond 600km, the exosphere is almost completely composed of hydrogen and helium).
Contained within the exosphere is the Clarke orbit (geosynchronous orbit). Other high earth orbit satellites pass through the exosphere, though more well known satellites and spacecraft such as the ISS, the Space Shuttle are in low earth orbit in the thermosphere along with most other satellites.
Because the exact definition of the exosphere is related to the mean free path, it should be noted that this is the only layer of the atmosphere on some planets (Mercury has a sodium exosphere, as does the Moon).