The Thermosphere
where the Space Shuttle cruises ...
The thermosphere, named after the Greek θερμός (thermos) for heat, begins about 80 - 90 km above the Earth and extends to 500 - 600 km altitude. At these high altitudes, the residual atmospheric gases separate in different layers according to their molecular mass. The mean free path of individual gas particles here is several kilometers, so that there is hardly any energy exchange between the particles due to rare contact. Thermospheric temperatures increase with altitude due to absorption of high energetic solar radiation by the small amount of molecules still present. Absorption of solar radiation causes the air particles in this layer to become ionised and thus electrically charged, enabling radio waves to be reflected and received beyond the horizon. The thermosphere is thus part of the ionosphere.
Temperatures are highly dependent on solar activity, and can rise to more than 1700°C. Due to the low density and the associated low heat exchange, this is of no importance for astronauts, for example. The Space Shuttle and the International Space Station (ISS) orbit the Earth within the thermosphere, at altitudes between 320 and 380 kilometers. Although the air density is extremely low, aerodynamic drag acts on ISS lowering its orbit steadily. Without being periodically raised to a higher orbit, ISS would sooner or later fall back to Earth.
The polar lights also occur in the thermosphere. The dynamics of the lower thermosphere (below about 140 km, see also: Das Wetter in 140 km Höhe) has been studied by the CRISTA experiment.
