Until the 1980s, most theories about the composition and other physical characteristics of asteroids and comets could not be verified because no one had ever seen one of these bodies up close. Astronomers who studied them had to rely on im- ages taken by ground-based telescopes. Even the largest telescopes revealed no distinct visual details, mainly because both asteroids (with the exception of Ceres) and comets are relatively tiny objects. And of course, they are also very far away. An astronomer on Earth trying to make out details on a one-mile- wide asteroid in the asteroid belt faces the same challenge as a person in New York trying to read words painted on a basketball in California.

Clearly, scientists realized, the only way they could solve the long-standing mysteries surrounding comets and asteroids would be to send spacecraft...