The past decade has heralded the advance of both scholarship and exploration of small bodies in the solar system. Once relegated to subsidiary status, asteroids, comets, and now Kuiper Belt objects (KBOs) have come into their own as planetary scientists have become aware of their importance for understanding the origins of the solar system. Because these objects are believed to be debris remaining from the earliest periods of planet formation, they may offer clues to the physical conditions and architecture of the early solar system.
The key role impact processes have played in the evolution of the Earth and other planets has also been recognized. Earth-crossing objects have caused mass extinctions in the past, and comets are now believed to have delivered significant amounts of volatiles and prebiotic material to the inner solar system. NASA has supported this blossoming field of study by sending several missions to comets and asteroids, and by establishing special programs to study them.