Thermal dissipation force modeling with preliminary results for Pioneer 10/11

The dissipation of thermal energy can produce disturbance forces on spacecraft surfaces if the energy is not dissipated in a symmetric pattern. This force can be computed as the quotient of the radiated power and the speed of light for a plate surface element. Depending on mission and spacecraft design the resulting surface forces have to be included into the disturbance budget. At ZARM (Center of Applied Space Technology and Microgravity) a raytracing algorithm was developed that allows the computation of the resulting force for complex spacecraft geometries. The method is based on the modeling of the spacecraft geometry in finite elements (FEs). Using an FE-solver the surface temperatures of the satellite can be derived with geometry and material parameters using heat sources/sinks as constraints. The outgoing radiation force is computed including reflectivity and absorption between all elements of the model. As an example for the method a test case model of the radio isotope thermal generators (RTGs) of Pioneer 11 is processed with this force computing method. The results show that detailed thermal modeling for the whole craft is necessary as the simplified test case results in a force that is non-negligible with respect to the pioneer anomaly.