Accepted for publication in Planetary and Space Science. February, 2003. For preprint information please contact the first author. 
With the surface properties and shapes of solar system small bodies 
(comets and asteroids) now being routinely revealed by spacecraft and 
Earth-based radar, understanding their interior structure represents the 
next frontier in our exploration of these worlds. Principal unknowns include 
the complex interactions between material strength and gravity in 
environments that are dominated by collisions and thermal processes. Our 
purpose for this review is to use our current knowledge of small body 
interiors as a foundation to define the science questions which motivate 
their continued study: In which bodies do “planetary” processes occur? 
Which bodies are “accretion survivors”, i.e. bodies whose current form and 
internal structure are not substantially altered from the time of formation? 
At what characteristic sizes are we most likely to find “rubble piles”, i.e. 
substantially fractured (but not reorganized) interiors, and intact monolithlike 
bodies? From afar, precise determinations of newly discovered 
satellite orbits provide the best prospect for yielding masses from which 
densities may be inferred for a diverse range of near-Earth, main-belt, 
Trojan, and Kuiper belt objects. Through digital modeling of collision 
outcomes, bodies that are the most thoroughly fractured (and weak in the 
sense of having almost zero tensile strength) may be the strongest in the 
sense of being able to survive against disruptive collisions. Thoroughly 
fractured bodies may be found at almost any size, and because of their 
apparent resistance to disruptive collisions, may be the most commonly 
found interior state for small bodies in the solar system today. Advances in 
the precise tracking of spacecraft are giving promise to high order 
measurements of the gravity fields determined by rendezvous missions. 
Solving these gravity fields for uniquely revealing internal structure requires 
active experiments, a major new direction for technological advancement in 
the coming decade. We note the motivation for understanding the interior 
properties of small bodies is both scientific and pragmatic, as such 
knowledge is also essential for considering impact mitigation.