Evidence for Drag Coefficient Modeling Errors near and Above the Oxygen-to-Helium Transition

Abstract

The ability to model atmospheric drag is critical for precise satellite orbit prediction below 1000 km altitudes. Uncertainty in atmospheric drag comes primarily from estimates of the atmospheric mass density and the spacecraft drag coefficient. Mismodeling the drag coefficient introduces errors in densities derived from atmospheric drag that are then used to predict drag forces and satellite orbits. Drag coefficient models are based on assumptions about the energy and momentum exchange between the atmospheric gas and the satellite surface, and the associated interaction parameters are the main source of uncertainty. In this work, some of these assumptions are evaluated by comparing the aerodynamic drag on satellites of different shapes. Atmospheric densities derived for Gravity Recovery and Climate Experiment (GRACE) and a set of compact satellites within a scale height of GRACE are normalized to an atmospheric model and compared. Offsets in these derived densities can be attributed to drag coefficient modeling assumptions. Results indicate that current drag coefficient models lead to normalized derived density differences of up to in low-pressure, high-altitude atmospheres nearing the oxygen-to-helium transition region. Consequently, current drag coefficient assumptions may lead to drag-derived mass densities that are underestimated by up to 35% at and above 500 km altitudes.