Comets as Building Blocks

Recent results, many but not all from flybys of comets by spacecraft, particularly the results from Deep Impact, have dramatically improved our understanding of the physical properties of cometary nuclei. Characteristic features are modest size (R<20 km), high porosity, low strength, and heterogeneity. There is also evidence that can be interpreted as showing the original cometesimals in a cometary nucleus and suggesting radial migration of macroscopic cometesimals during the aggregation of nuclei, which in turn aggregated into the cores of the giant planets.