1912 In a balloon, at an altitude of 5,000 meters, in 1912 Victor Hess, the father of cosmic ray research, discovered "penetrating radiation" coming from space. In the days before particle accelerators existed, physicists climbed mountains and went up in balloons to observe cosmic rays, naturally-occurring high-energy particles in the atmosphere. Today, particle physicists still study cosmic rays to explore unanswered questions about the structure of matter and the universe.