Abstract: A computer-implemented method for determining an orbital effect on a low-Earth orbiting object includes, forming an ensemble of thermosphere-ionosphere state models to represent thermosphere-ionosphere state model uncertainty, assimilating measured thermosphere-ionosphere data into a select thermosphere-ionosphere state model of the ensemble in accordance with a Kalman gain derived from statistics of the ensemble, computing drag on the low-Earth orbiting object from elements of the assimilated select model of thermosphere-ionosphere state models and randomizing elements of the assimilated select thermosphere-ionosphere model in accordance with statistics of the ensemble to generate a new ensemble of thermosphere-ionosphere state models. A system is arranged to perform the method. A computer program product is arranged to cause one or more computers to perform the method.