Introduction

``The Sun, Mercury, Venus, Earth, Mars, Jupiter, ... '' Nearly all of us learned in grade school to rattle off the names of the major bodies of our solar system. But very few, if any, learned the name of even one minor planet. It's not surprising. So small, dark, and distant from the Earth, they are invisible to our unaided eyes, the minor planets seem like just so much fluff left over from the formation of the planets. Briefly a cause for concern whenever a spacecraft passed through the asteroid belt, these crumbs of creation seemed eminently negligible. Until recently.

When it was recognized that many craters and other curious features on the Earth's surface are actually the signatures of impacts by minor planets, and not just volcanic features or sinkholes, suddenly the swarm of dark boulders orbiting silently between Mars and Jupiter assumed an aura of menace. In due course, various telescopic surveys revealed that the minor planets are not confined to the distant ``main belt'', but that hundreds, possibly thousands, are in orbits crossing those of Earth and other terrestrial planets. Finally, the discovery of the vast subterranean Chicxulub crater at the northern edge of the Yucatan peninsula seemed to settle the question of what caused the mass extinctions which marked the end of the Cretaceous period. Minor planets were not only numerous, nearby, and threatening, but major impacts had brought about disastrous consequences for life forms on Earth, and were very likely to do so again. Witnessing comet Shoemaker-Levy 9 crashing into Jupiter in the summer of 1994, no one could fail to realize that minor planets and comets are ignored at our peril.