In 1952 he began to study Meteor Crater in Arizona, and became convinced that it and other terrestrial and lunar craters—which many believed to be volcanic—were impact features, especially after his discovery of coesite at terrestrial craters. In the 1960s he oversaw geological experiments on NASA’s lunar missions and helped to train the Apollo astronauts. With the American astronomer Eleanor Kay Helin, née Francis, and later with his wife, Carolyn (Jean) Shoemaker, née Spellmann (1929–2021), he began in the 1970s to search for near-Earth asteroids, potential causes of terrestrial impact craters, using the 0.46-m Schmidt telescope at Palomar Observatory. In addition to asteroids, Shoemaker discovered many comets, including Shoemaker–Levy 9, which demonstrated the effects of an impact on a gaseous body by crashing into Jupiter.