  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.
