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Solar Radiation
The source of the thermosphere's heat is radiation emitted by the sun. The thermosphere absorbs much of the radiation that Earth receives from the sun, leaving only a fraction to actually reach the surface. Ultraviolet radiation, visible light, and high-energy gamma radiation are all absorbed by the thermosphere, which causes the few particles present to heat up considerably. The temperature of the thermosphere fluctuates by hundreds of degrees between night and day, and even more widely between the maximum and minimum points of the solar cycle.
Heat and Pressure
The extremely low pressure of the thermosphere also contributes to its high temperature. Heat is defined by the amount of energy possessed by the individual molecules of a material. In a warm gas, the particles will move around much more quickly than in a cool gas. At sea level, energetic particles will very quickly start to collide with other particles, losing energy with each collision. This loss of energy cools the gas unless more heat is constantly added. Low pressure means that not many particles are around to collide with, which leads to slower energy loss. Thus, a low-pressure gas takes much less energy to heat than a high-pressure gas.
Heat and Quantity
Even though the thermosphere is extremely hot, its low density means that it cannot efficiently convey that energy to objects moving through it. It has high heat, but low quantity. A mercury thermometer suspended within the thermosphere would read a temperature below freezing, as heat loss would exceed any energy the scattered particles of the thermosphere could transmit to the mercury. It is similar in concept to the heat generated by a candle flame, which is extremely hot at some points within the flame but is incapable of heating objects more than a few inches away. It is producing a high temperature, but a low quantity of heat.
Effects of the Thermosphere on Space Travel
The low quantity of the heat-bearing medium of the thermosphere frees objects traveling through it from being significantly affected by the high temperatures. Satellites, astronauts, and spacecraft experience the thermosphere as a very cold place, as the tremendous heat of the thermosphere cannot be efficiently transferred to solid objects. The heat associated with atmospheric re-entry is contributed to by the thermosphere, but this is an effect of friction rather than the temperature of the atmosphere itself.