Theoretical description of spacecraft flybys by variation of parameters

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