Cometary Nuclei—From Giotto to Rosetta

We will briefly recapitulate the beginning of modern cometary physic. Then we will assess the results of the cometary flyby missions previous to ESA's Rosetta rendezvous with comet 67P/Churyumov-Gerasimenko. Emphasis is given to the physical properties of cometary nuclei. We will relate the results of the Rosetta mission to those of the flybys. A major conclusion is that the visited cometary nuclei seem to be alike but represent different stages of evolution. Coma composition and appearance are not only controlled by the composition of the nucleus but also strongly influenced by the shape and rotation axis orientation of the nucleus and resulting seasons that generate varying surface coverage by back fall material. Rosetta showed that the coma composition is not only varying spatially but also strongly with time during the perihelion passage. Hence past interpretations of cometary coma observations have to be re-considered. Finally, we will try to assess the impact of the cornerstone mission leading to a critical evaluation of the mission results. Lessons learned from Rosetta are discussed; major progress and open points in cometary research are reviewed.