Introduction

Mitigation of any hazard begins with a comprehensive understanding of the forces to be reckoned with. Reckoning – taking measure – is being done with great efficiency on one front: the first-order census of kilometer-scale near-Earth objects (NEOs) may be complete in the next few decades (Jedicke et al. 2003). But the most basic physical properties of these bodies remain unknown: how they are assembled, how they respond to tidal and impact stress, and how they will respond to the artificial perturbations that will one day be required.

This chapter provides an introduction to comets and asteroids and their geophysical evolution, and concludes with recommendations for theoretical and laboratory effort and spacecraft reconnaissance. Little is known for sure. Comprehensive introductions to the rapidly evolving science of comets and asteroids are found in the University of Arizona Press review volumes Asteroids III (Bottke et al. 2002) and Comets II (Festou et al. 2004).

Comets

Not long ago, interplanetary space near Earth was believed to be far emptier than it now appears. The only luminous entities besides the Moon were the passing comets, whose comae and tails can form some of the most extensive structures in the solar system, and which have been scrutinized since the dawn of astronomy. While these centuries of observation have led to an understanding of the dynamics and compositions of cometary envelopes, cometary nuclei – compact objects ranging from a few hundreds of meters to a few hundreds of kilometers diameter – remain a tight-wrapped mystery (Jewitt 1999; Meech et al. 2004).