In search of pioneering research, Rossi turned his attention to cosmic rays, which had been discovered by Victor Hess in manned balloon flights in 1911 and 1912. In 1929, Rossi read the paper of Walther Bothe and Werner Kolhörster, which described their discovery of charged cosmic ray particles that penetrated 4.1 centimetres (1.6 in) of gold. This was astonishing, for the most penetrating charged particles known at the time were electrons from radioactive decay, which could penetrate less than a millimetre of gold. In Rossi's words, it