We’ve known about gamma rays in space since 1967, from data collected by the Vela military satellites. Vela’s mission was to look for nuclear explosions in the atmosphere or in space from behind the moon. Scientists were aware that space had high energy radiation moving through it, but Vela was looking for sudden bursts of high energy radiation that might indicate that a nuclear bomb was detonated.

I’ve through almost all of this history. By the time I came to Santa Cruz, it was only 10 years after the discovery of cosmic gamma rays was first announced, in 1973. When I joined the field, people like Stan Woosley and Stirling Colgate were the leading proponents in understanding the source of gamma ray bursts. We thought they were in the galaxy, and they were probably phenomena related to neutron stars. This is quite a long time ago. In the meantime we’ve learned they are of cosmological origin and we understand now that the long duration bursts are related to supernovae.

Before we knew about cosmic gamma radiation we discovered it through experiments carried out by Paul Villard and Ernest Rutherford. Villard was doing radioactivity research in Paris at the same time as Marie and Pierre Curie. Villard and Rutherford collaborated on separating radiation into alpha, beta, and gamma components, based on the rays' penetration of objects and ability to cause ionization. Radium experiments showed that alpha-rays are stopped by paper, beta-rays are stopped by glass or aluminum, but gamma-rays pass through all substances except lead, concrete and steel – and the Earth’s atmosphere. Villard also noticed that gamma rays traveled in a straight line and were unaffected by magnetism. This is because gamma rays are photons – particles with no mass and no charge – so they can’t be affected by a magnetic field.

Of course these experiments were done in extremely short distances compared to the vastness of space. Cosmic gamma-rays travel billions of light years. As they approach Earth they are weakened by the height and density of the atmosphere, which absorbs the gamma-ray photons. This is why gamma rays don’t reach the Earth’s surface. We have to fly balloons or satellites to collect data on them – this is also true for X-rays. If you want to study higher energy phenomena, you have to go above the atmosphere because this type of radiation does not make it to the ground – directly. But more about that later.