Gamma-rays are the hardest radiation of the entire electromagnetic spectrum. Luckily, our atmosphere protects us against this radiation, which would otherwise destroy all life on Earth. Gamma-ray observatories on the ground measure secondary or tertiary radiation species, not the primary radiation. To mention just one example, one such experiment is the Milagro detector. In order to gain access to primary gamma-ray photons one needs to launch observatories into space, onboard satellites.