The exosphere is the uppermost layer of the atmosphere. In the exosphere, an upward travelling molecule can escape to space if it is moving fast enough to attain escape velocity; otherwise it will be pulled back to the celestial body by gravity. In either case, such a molecule is unlikely to collide with another molecule due to the exosphere's low density.

Earth's exosphere

The main gases within the Earth's exosphere are the lightest gases, mainly hydrogen, with some helium, carbon dioxide, and atomic oxygen near the exobase. The exosphere is the last layer before space. Since there is no clear boundary with space and the exosphere, the exosphere is sometimes used synonymously with outer space.

Lower boundary

The altitude of its lower boundary, known as the thermopause or exobase, ranges from about depending on solar activity. Its lower boundary at the edge of the thermosphere has sometimes been estimated to be above the Earth's surface. The exobase is also called the critical level, the lowest altitude of the exosphere, and is formally defined in one of two ways:

• The height above which there are the negligible atomic collisions between the particles and

• The height above which constituent atoms are on purely ballistic trajectories.

At the exobase, the mean free path of a molecule is equal to one pressure scale height. As the pressure scale height is almost equal to the density scale height of the primary constituent, and since the Knudsen number is the ratio of mean free path and typical density fluctuation scale, this means that the exobase lies in the region where $\backslash mathrm\{Kn\}(h\_\{EB\})\; \backslash simeq\; 1$.

The fluctuation in the height of the exobase is important because this provides atmospheric drag on satellites, eventually causing them to fall from orbit if no action is taken to maintain the orbit.

Upper boundary

The upper boundary of the exosphere can be defined theoretically by the altitude about , half the distance to the Moon) at which the influence of solar radiation pressure on atomic hydrogen velocities exceeds that of the Earth’s gravitational pull. The exosphere observable from space as the geocorona is seen to extend to at least from the surface of the Earth. The exosphere is a transitional zone between Earth’s atmosphere and interplanetary space.