Thermosphere and satellite drag

Accurate forecasts of thermosphere densities, realistic calculation of aerodynamic drag, and propagation of the uncertainty on the predicted orbit positions are required for conjunction analysis and collision avoidance decision making. The main focus of the Committee on Space Research (COSPAR) International Space Weather Action Teams (ISWAT) involved in atmosphere variability studies is satellite drag, and this paper reviews our current capabilities and lists recommendations. The uncertainty in the density of thermosphere models is due to the combined effect of employing simplified or incomplete algorithms, inconsistent and sparse density data, incomplete drivers for upper atmosphere heating processes (proxies for solar and geomagnetic activity), and forecast error of said drivers. When calculating drag, the uncertainty is amplified due to the satellite shape and aerodynamic model. The sources of uncertainty are reviewed in this paper, and possible and promising ways forward are proposed. Data assimilation models/approaches have demonstrated superior skill in reproducing the thermosphere's state and are the most promising way forward. However, data to drive the models is generally lacking, and they require significant computational resources. Substantial progress can only be made by means of setting up a full-blown observing system, including not only density and composition measurements, but equally the necessary model drivers