Up until about the middle of the twentieth century, astronomers still possessed an imperfect notion of how and where asteroids and comets originally formed. The theory that asteroids were the remnants of the explosion of an ancient planet still prevailed. Also, astronomers believed that the vast majority of asteroids inhabited the asteroid belt between Mars and Jupiter. As for comets, scientists assumed they were floating collections of sand and pebbles that formed and moved randomly through the solar system. When one of these objects happened to stray too close to a planet, the larger body's gravity pushed the comet into the inner solar system (the region inhabited by Mercury, Venus, Earth, and Mars). There the comet grew a tail and became visible to observers on Earth.

Eventually, however, astronomers were forced to revise their theories about the origins of asteroids...