For many years, scientists had been puzzled by the levels of
ionizing radiation measured in the atmosphere. The assumption at the time was that the radiation would decrease as the distance from the earth, the then assumed source of the radiation, increased. The
electroscopes previously used gave an approximate measurement of the radiation but indicated that at greater altitude in the atmosphere the level of radiation might actually be higher than that on the ground. He approached this mystery first by greatly increasing the precision of the measuring equipment, and then by personally taking the equipment aloft in a balloon. He systematically measured the radiation at altitudes up to during 1911–1912. The daring flights were made both by day and during the night, at significant risk to himself. His conclusion was that there was radiation penetrating the atmosphere from outer space, and his discovery was confirmed by
Robert Millikan, who gave the radiation the name
cosmic rays. His discovery opened the door to many new discoveries in
particle and
nuclear physics. In particular, both the
positron and the
muon were first discovered in cosmic rays by
Carl David Anderson. In 1936, Hess and Anderson shared the
Nobel Prize in Physics for their discoveries of cosmic rays and the positron, respectively. == Personal life and death ==