On force-field models of the spacecraft flyby anomaly

Recently, Anderson et al. published an empirical prediction formula for the so far unexplained parts of the velocity changes of spacecrafts during Earth flybys. In the framework of a perturbational approach, we show that there is no velocity-independent force field of the Earth - in addition to its Newtonian gravity field - that is to reproduce this formula. However, we give examples for fields modeling exactly the flyby anomaly which are quadratic functions of the velocity of the spacecraft.