Deep sub-surface exploration of cometary nuclei

The surface regions of cometary nuclei are expected to be extensively modified by cosmic radiation and solar insolation. Induced state changes and associated sublimation and phase-change boundaries are expected to propagate deep into the interior. In the current exploration program a reconnaissance of the highly evolved upper meter of the cometary surface is comprehensively covered. However, the relationship of evolved surface materials to the pristine (i.e., unchanged since emplacement in the comet) materials that may exist at greater depths remains to be explored. Except for the possibility of remote sensing by radar and microwave tomography, the problem of sampling the properties of deep (1-100 m) sub-surface materials has received little attention. We consider a concept for the scientific exploration of these deep sub-surface regions by means of excavation by hypervelocity impact. A massive (approximately 500 kg) impactor is delivered at hypersonic speeds to a comet nucleus to excavate a crater several tens of meters deep and over 100 m in diameter. We give two examples of such missions: one to a suspected dormant comet, 3200 Phaethon, and one to a currently active comet, 9P/Tempel 1. We show how a reconnaissance of the newly excavated crater structure from the flyby delivery spacecraft or an associated lander can yield new information about the physical and compositional structure of the deep subsurface layers and about any lateral inhomogeneities that could be associated with the primordial cometesimal structure of the nucleus. (C) 1999 COSPAR. Published by Elsevier Science Ltd.