Global modeling of comets : nucleus neutral and ionized coma of comets 67P Churyumov Gerasimenko and 46P Wirtanen ; preparations for the Rosetta radio science investigations Models of the thermal behavior of a cometary nucleus the evolution of the neutral gas coma the ionized cometary coma and of the interaction of the cometary plasma with the solar wind are studied in this work. The general aim is to develop a global model of the comet and its environment in order to characterize the physical conditions around comets 67P Churyumov Gerasimenko and 46P Wirtanen with respect to the heliocentric distance. The results also provide estimates of the effects of the cometary environment on the radio science investigations experiment (RSI) aboard the spacecraft Rosetta. After the launch that is scheduled for February 2004 the Rosetta mission is planned to encounter comet 67P Churyumov Gerasimenko and accompany it on its orbit. Comet 46P Wirtanen has been the original target comet but serves now as back up target due to the postponement of the Rosetta launch in January 2003. The model of the heat diffusion within the cometary nucleus is one dimensional. A grid of one dimensional models is distributed over the nucleus in order to determine the temperature distribution and the sublimation characteristics of the comet on the whole surface of the comet. A heat balance equation is applied as boundary condition on the surface. Many parameters that have to be accounted for in a heat diffusion model are not precisely known to date. The variation of these parameters within reasonable limits yields a wide range of possible results. The heat diffusion within the cometary nucleus is derived from an energy conservation equation that includes heat conduction through the porous cometary material and heat convection due to the transport of latent heat by the gas phase within the nucleus. Model results are evaluated by a comparison of modeled and observed global gas production rates ...