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A cosmic ray is a high-energy charged particle propagating through space. Galactic cosmic-ray electrons emit synchrotron and inverse-Compton radiation, while cosmic-ray nuclei produce gamma rays through collisions with gas.

In modern terminology, a primary cosmic ray arriving at Earth's atmosphere is usually a proton or another atomic nucleus, with an electron component as well. Atmospheric collisions generate secondary cosmic rays and air showers, including muons, electrons and photons. A gamma ray can also initiate an air shower, although it is electrically neutral. Consequently, detecting atmospheric ionization does not establish that an incoming primary cosmic ray passed through the detector. Magnetic fields bend charged particles, while gamma rays provide a more direct directional connection to their sources.^[ref]

The Hess cosmic-ray balloon experiment asks how atmospheric shielding and separation from the ground affect ionization. Its power comes from changing the environment around an enclosed detector instead of identifying each incoming particle.

Awarded in equal shares to Victor Franz Hess for cosmic rays and Carl David Anderson for the Positron. The Hess cosmic-ray balloon experiment and Anderson positron track experiment explain the measurements.^[ref]

Victor Hess investigated penetrating radiation by taking ionization instruments on balloon flights. If the dominant source were radiation from the ground, increasing the distance from that source should generally reduce its contribution.

At high altitude the measured ionization instead increased. Hess's 1912 report used this altitude dependence to argue that part of the radiation entered from above. The experiment inferred an external origin from a changing detector response; it did not by itself identify every component of what is now called cosmic rays.^[ref]

The Hess cosmic-ray balloon experiment discusses the apparatus and the need to distinguish atmospheric ionization from instrument leakage and local background.