Laboratory simulation of the physical processes occurring on and near the surfaces of comet nuclei

Laboratory comet simulation experiments are discussed in the context of the oretical models and recent ground-based and spacecraft observations, especi ally the Giotto observations of P/Halley. The set-up of various comet simul ation experiments is reviewed. A number of small-scale experiments have bee n performed in many laboratories since the early 1960s. However, the larges t and most ambitious series of experiments were the comet simulation experi ments known as KOSI (German = Kometen Simulation). These experiments were p rompted by the appearance of Comet P/Halley in 1986 and in planning for the European Space Agency's Rossetta mission that was originally scheduled to return samples. They were performed between 1987 and 1993 using the German Space Agency's (DLR) space hardware testing facilities in Cologne. As with attempts to reproduce any natural phenomenon in the laboratory, there are d eficiencies in such experiments while there are major new insights to be ga ined. Simulation experiments have enabled the development of methods for ma king comet analogues and for exploring the properties of such materials in detail. These experiments have provided new insights into the morphology an d physical behavior of aggregates formed from silicate grains likely to exi st in comets. Formation of a dust mantle on the surfaces and a system of ic e layers below the mantle caused by chemical differentiation have been iden tified after the insolation of the artificial comet. The mechanisms for hea t transfer between the comet's surface and its interior, the associated gas diffusion from the interior of the surface, and compositional, structural, and isotopic changes that occur near the surface have been described by mo deling the experimental results. The mechanisms of the ejection of dust and ice grains from the surface and the importance of gas-drag in propelling g rains have also been explored.