The impetus for asteroid exploration is scientific, political, and pragmatic. While the rationales vary widely -- from wanting to learn about the origin of planets, to wanting to divert Earth-approaching asteroids, to wanting to establish research or resource outposts in space -- the present requirements for exploration are largely the same: to discover what they are made of and how they behave.
Learning what asteroids are made of -- meteoritics -- is a celebrated enquiry that has motivated a number of missions including the NEAR rendezvous with 433 Eros. Learning how they behave -- low gravity geophysics -- is less celebrated but equally important. The recent failure of the Japanese Hayabusa mission -- so tantalizingly close to meeting its ambitious goals of surface operations and a sample return -- revealed the surprising complexity of the asteroid environment, which may have closer resemblance to Earth's abyssal plains than to the Moon.
The present epoch of asteroid exploration seems reminiscent of the early 1960's, when the race to the Moon was imminent and we knew so little about lunar formation or the composition of its rocks. I would not be surprised if the next space race was not to Mars but to the handful of near-Earth asteroids that are easiest to approach. These are "hazardous asteroids" by virtue of their proximity, so there is political capital similar to what motivated the race to the Moon -- even if the hazard is truly not that great. What will we find? The journey is just beginning, but we do know that near-Earth asteroids comprise a representative sampling from all the regions of planet formation, from igneous relics of disrupted precursors to terrestrial planets, to primitive chondritic bodies and extinct comets.
These are prefatory comments to a talk that will focus primarily on several scientific aspects of asteroids. I will first talk about their origin, focusing on a new idea (Asphaug et al. Nature 2006) that cryptic differentiated asteroids may have originated in the product of one or more large-scale "hit and run" collisions when terrestrial-sized planets roamed the Main Belt. I will then address their geophysics, focusing on what has been learned from the careful study of craters and larger impact structures on asteroids and comets. I will conclude with some current ideas for asteroid exploration, including radio reflection tomography (the Deep interior mission proposal) and a companion concept to study seismogenic blast experiments which also simulate the natural process of crater formation and reveal the unweathered subsurface.