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• First detection
• The Compton Gamma Ray Observatory
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Gamma rays are high-energy subatomic particles formed either by the decay of radioactive elements or by nuclear reactions. The wavelength of a gamma ray is very short—less than the radius of an atom—and the energy they carry can measure millions of electron volts.

Terrestrial gamma rays—those produced on Earth—are the only gamma rays we can observe here. A second class of gamma rays, called cosmic gamma rays, do not penetrate to the surface of Earth because the ozone layer absorbs high-energy radiation. The only way to detect cosmic gamma rays, which are created by nuclear fusion reactions that occur within the core of stars, is by sending a satellite-observatory into space.

First...