The ionosphere is a very important part of the Earth's upper atmosphere, and according to the Karman line definition (everything above 100km), most of it lies in space. Certainly some spacecraft in low Earth orbit, such as the Space Shuttle and the International Space Station orbit through the ionosphere.

The ionosphere is important because it affects radio waves. It makes long distance high frequency radio communication possible, and it affects satellites communications that pass through it. This note is devoted to the milestones of ionospheric discovery.

1839Gauss first speculated that the upper atmosphere might contain ionized regions
1878Stewart postulated the existence of a conductive layer in the upper atmosphere to explain variations observed in the Earth's magnetic field.
1901Marconi succeeded in crossing the Atlantic with a radio transmission at a frequency of 300 kHz
1902Kennelly & Heaviside both independently suggested that Marconi's signal was propagated by reflection of signals from an ionised layer in the upper atmosphere.
1910Austin determined a formula for long wave transmission and suggested 1.5 MHz as the useful upper limit for distant radio communication.
1912Eccles stated that the ionized layer was maintained by solar radiation.
1919Watson mathematically derived the experimental formula of Austin.
1920-24The phenomenon of long distance short wave (1.5 - 30 MHz) transmission was discovered.
1924Larmor developed a theory of refraction of short waves by free electrons.
1925Appleton & Barnett proved the existence of the ionized layer and found its height by doing a phase comparison of two signals from a transmitter, one the ground wave, and the other reflected from the ionosphere.
1926Breit & Tuve used a pulse technique to measure the heights and critical frequencies of a number of ionospheric layers. Theirs was the first true ionosonde, and may have been the first radar.
1931Chapman presented a theory for the formation of an ionized layer due to the action of solar ultraviolet radiation.
1932Appleton developed an extensive set of equations used to describe the propagation of radio waves in the ionosphere. These included absorption effects and modification due to the Earth's magnetic field.

The theoretical work on ionospheric radio wave propagation also set the stage for and is useful in calculating radio wave behaviour in any space plasma, including planetary atmospheres encountered in deep space missions. In fact, many planetary missions use these interactions to compute the nature of the planetary atmosphere itself. No special sensors are required, the experiments using the normal signal that the spacecraft uses to communicate back to Earth.