Aiaa 98-4287 Investigations into the Unexpected Delta-v Increases during the Earth Gravity Assists of Galileo and Near

Unexpected energy increases during Earth flybys of both the Galileo and Near Earth Asteroid Rendezvous (NEAR) spacecraft have drawn evidence of spacecraft trajectory modeling errors, an unknown perturbing force or failure of Newtonian gravity. This paper will investigate the gravity field of Earth as a possible source of these anomalous AVs. Other possible sources of errors have been considered including: the mathematical models representing the perturbing forces acting on the spacecrafts while in the sphere of influence of Earth such as relativistic effects, tidal effects, Earth radiation pressure and atmospheric drag. However, most of these perturbations such as atmospheric drag can be ruled out because the imparted acceleration upon the spacecraft is several orders of magnitude less than observed. Since the oblateness effect is several orders of magnitude greater than the non-gravitational perturbations, errors in the spherical harmonic representation of Earth's gravity field will be examined. Other sources that have already been examined and tentatively dismissed include numerical round-off, integration errors, spacecraft antenna phase center offset and spacecraft antenna switching during encounter.