What Shoemaker and his colleagues found was that there was more risk out there—a great deal more—than anyone had ever imagined.
Asteroids, as most people know, are rocky objects orbiting in loose formation in a belt between Mars and Jupiter. In illustrations they are always shown as existing in a jumble, but in fact the solar system is quite a roomy place and the average asteroid actually will be about a million miles from its nearest neighbor. Nobody knows even approximately how many asteroids there are tumbling through space, but the number is thought to be probably not less than a billion. They are presumed to be planets that never quite made it, owing to the unsettling gravitational pull of Jupiter, which kept—and keeps—them from coalescing.
When asteroids were first detected in the 1800s—the very first was discovered on the first day of the century by a Sicilian named Giuseppi Piazzi—they were thought to be planets, and the first two were named Ceres and Pallas. It took some inspired deductions by the astronomer William Herschel to work out that they were nowhere near planet sized but much smaller. He called them asteroids—Latin for "starlike"—which was slightly unfortunate as they are not like stars at all. Sometimes now they are more accurately called planetoids.
Finding asteroids became a popular activity in the 1800s, and by the end of the century about a thousand were known. The problem was that no one was systematically recording them.