Ballistic orbit perturbations arising from terrestrial gravity and reentry aerodynamic drag

Non-Newtonian gravity, caused by an aspherical earth, and reentry aerodynamic drag result in significant perturbations in the Keplerian elements of a ballistic orbit. The number of terms retained in the geopotential model, coupling between gravity and drag, and selection of initial trajectory parameters are shown to influence prediction accuracy of position and velocity on the actual orbit. It is demonstrated that, with drag, ballistic orbits can be 'captured' by a rotating atmosphere such that, at impact, the local horizontal components of relative velocity are zero and radial velocity is constant. Proximity to 'capture' in velocity space is primarily controlled by the drag coefficient and the initial velocity vector.