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. Exemplary maps of the local temperature distribution and local sublimation rates at particular heliocentric distances are also provided. The neutral gas coma of the comet is modeled with a hydrodynamic approximation. This method is justified within a collision dominated regime. Due to the expected weak gas production of a comet at large heliocentric distances, this hydrodynamic regime might be small and might not enclose the whole nucleus. When the comet approaches the sun and the gas production increases, the hydrodynamic regime extends to cometocentric distances of several hundred or thousand kilometer. The gas mass flux within the coma perturbs an orbiting spacecraft. The acceleration of the spacecraft due to the gas mass flux is evaluated with the model results. The resulting change in velocity can be measured as a Doppler shift of the recorded frequency of the carrier signal. Case studies at several heliocentric distances are carried out. It turns out that even at heliocentric distances of ~3 AU the drag force of the gas can become strong enough to perturb the measurements of the second order gravity coefficients, which is a primary science objective of RSI. The ionized coma of a comet can also have an effect on the carrier signal. Changes of the electron content in the line of sight between spacecraft and observer at earth are in principle observable. A one-dimensional model of the plasma density at the comet-sun axis is developed. The assumption of photochemical equilibrium is not necessarily justified within the coma of weak outgassing comets. The continuity equation of the plasma density has to be solved without this simplifying assumption. A model of the electron temperature profile is also generated. The transition from a regime where electrons are effectively cooled to a region with temperatures of the electron fluid similar to solar wind levels is assumed to set in at the position of the thermal electron collisionopause. The plasma densities obtained from the ionospheric model indicate only minor effects on the carrier signal. The interaction of the cometary plasma with the solar wind is also studied. The respective standoff distances of the bow shock, the cavity surface and the collisionopause of comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen are determined with respect to the heliocentric distance. The variation of the solar wind parameters with heliocentric distance is accounted for. Effects of transient solar events, such as solar flares or coronal mass ejections, are discussed. It can be concluded that the plasma environment of comets 67P/Churyumov-Gerasimenko and 46P/Wirtanen and their interaction with the solar wind will have only a minor effect on the carrier signal. Special scenarios might be needed in order to locate plasma boundaries within the cometary environment with RSI
