1. A method for determining a collision probability between a subject object and a target object, the method comprising the steps of, propagating initial positions and initial velocities and initial error covariances of the subject object and the target object over a propagated trajectory duration having propagated trajectory time steps for providing respective propagated positions and propagated velocities and propagated error covariances in respective initial reference frames, transforming the propagated error covariances of the target object and subject object into scaled error covariances in a scaled reference frame for each of the propagated trajectory time steps, transforming the propagated positions and propagated velocities and a conflict volume into scaled positions and scaled velocities and scaled conflict volume in the scaled reference frame, the scaled velocities are relative velocities between the subject object and target object, aligning the scaled positions and scaled velocities and scaled error covariance matrix and scaled conflict volume into an encounter reference frame having an encounter plane orthogonal to the relative velocities, the scaled conflict volume becoming a keep-out box in the encounter plane, and computing a conflict probability using contour integration over an integration path about the keep-out box and over a probability density.