The comets of the century - Nuclei of P/Shoemaker-Levy 9 and Hale-Bopp

The currently most central question about comets is if they preserve the material of the presolar cloud in original shape or if this material was subject to sublimation and recondensation in the outer parts of the solar nebula. Closely associated to this is the question of the structural properties and chemical build-up of cometary nuclei. The present contribution is prompted by the occurrence, within a period of just a couple of years, of two famous comets, each of which will bring important information bearing on these issues. Here I summarize the preliminary indications as of early 1996.