Theoretical description of spacecraft flybys by variation of parameters

The planning of space missions to planets and small Solar-System bodies often requires numerous simulations in order to quantify the relative scientific value of various flyby geometries. Here, using the Born approximation for the unperturbed reference orbit, we describe a new perturbation theory for flyby orbits. Results are expressed in terms of the Doppler shift induced in the frequency of a spacecraft tracking signal. We show how these results can be used to simulate the determination of the body's gravitational field, information that provides important boundary conditions on its interior structure. Covariance simulations for the Galilean satellites and for Titan compare favorably with rigorous results from JPL's Orbit Determination Program (ODP). (C) 1999 Academic Press.