Daniel Barringer, an entrepreneur and mining engineer who had discovered Meteor Crater in 1891, had postulated that it had been caused by the impact of a meteor. About the same time, G. K. Gilbert, the chief geologist of the USGS, examined the crater and announced that it had been created by an explosive venting of volcanic steam. A majority of scientists accepted Gilbert's explanation of the cause of the crater, and it remained the conventional wisdom until Shoemaker's investigations half a century later. For his Ph.D. degree at Princeton (1960), under the guidance of Harry Hammond Hess, Shoemaker studied the impact dynamics of Barringer Meteor Crater. Shoemaker noted Meteor Crater had the same form and structure as two explosion craters created from atomic bomb tests at the Nevada Test Site, notably Jangle U in 1951 and Teapot Ess in 1955. In 1960, Edward C. T. Chao and Shoemaker identified shocked quartz (coesite) at Meteor Crater, proving the crater was formed from an impact generating extremely high temperatures and pressures. They followed this discovery with the identification of coesite within suevite at Nördlinger Ries, proving its impact origin. Astrogeology and Apollo In 1960, Shoemaker directed a team at the USGS center in Menlo Park, California, to generate the first geologic map of the Moon using photographs taken by Francis G. Pease. Shoemaker also helped pioneer the field of astrogeology by founding the Astrogeology Research Program. He was prominently involved in the Lunar Ranger missions to the Moon, joining the television imaging team of Harold Urey and Gerard Kuiper, which turned into a preparatory mission for the future crewed landing. Shoemaker was then chosen to be the principal investigator for the Surveyor program's television experiment, and then the lunar geology principal investigator for Apollo 11, Apollo 12, and Apollo 13. Shoemaker was also involved in the training of the American astronauts. He himself was a possible candidate for an Apollo Moon flight and was set to be the first geologist to walk on the Moon but was disqualified due to being diagnosed with Addison's disease, a disorder of the adrenal gland. Shoemaker would train astronauts during field trips to Meteor Crater and Sunset Crater near Flagstaff. He was a CBS News television commentator on the early Apollo missions, especially the Apollo 8 and Apollo 11 missions, appearing with Walter Cronkite during live coverage of those flights. According to David H. Levy, just