Almost all of our information about the universe beyond our planet comes from the study of electromagnetic radiation. Gamma radiation is part of the electromagnetic spectrum that includes the familiar visible light and radio waves.

All electromagnetic radiation travels as packets of energy call "photons". Gamma-ray photons have many properties similar to photons of visible light and radio waves: they travel in straight lines; they move at the speed of light; and they are not affected by magnetic or electric fields in space.

However, there are also many differences between gamma-rays and other forms of energy. For example, a gamma-ray photon has one million to one trillion (one million million) times the energy of a photon of visible light. And, unlike photons of radio and visible light, gamma-ray photons cannot penetrate the earth's atmosphere, for they are absorbed by interactions with air molecules ten miles above the Earth's surface. Also, gamma-rays cannot be reflected by mirrors. Thus, normal telescope optics cannot be used to enhance or increase the collection area of a gamma-ray detector.

Because gamma-rays can traverse great distances in space without absorption by intergalactic dust and gas, they can serve as powerful probes of distant regions of the cosmos as well as otherwise obscured regions of our own Milky Way.

Gamma-rays are produced on Earth from the radioactive decay of naturally occurring elements, and oncologists may use such gamma-rays to treat cancer tumors. Particle physicists can produce gamma-rays at accelerators such as Fermilab in Illinois or the Stanford Linear Accelerator in California, where they are used to study the properties of elementary matter. But very-high-energy gamma-rays like those emitted by astronomical bodies do not occur naturally on Earth.