The few craters that remained evident on Earth (most had been eroded away) were generally attributed to other causes or treated as fluky rarities.
By the time Shoemaker came along, a common view was that Meteor Crater had been formed by an underground steam explosion. Shoemaker knew nothing about underground steam explosions—he couldn't: they don't exist—but he did know all about blast zones. One of his first jobs out of college was to study explosion rings at the Yucca Flats nuclear test site in Nevada. He concluded, as Barringer had before him, that there was nothing at Meteor Crater to suggest volcanic activity, but that there were huge distributions of other stuff— anomalous fine silicas and magnetites principally—that suggested an impact from space. Intrigued, he began to study the subject in his spare time.
Working first with his colleague Eleanor Helin and later with his wife, Carolyn, and associate David Levy, Shoemaker began a systematic survey of the inner solar system. They spent one week each month at the Palomar Observatory in California looking for objects, asteroids primarily, whose trajectories carried them across Earth's orbit.
At the time we started, only slightly more than a dozen of these things had ever been discovered in the entire course of astronomical observation, Shoemaker recalled some years later in a television interview. "Astronomers in the twentieth century essentially abandoned the solar system," he added. "Their attention was turned to the stars, the galaxies."