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.