Atmospheric density has an important influence in predicting the positions of satellites in low Earth orbit. For long-term predictions of satellite ephemerides, any density trend in the thermosphere would be a valuable input, not only to satellite operators, but also to studies of the future low Earth orbit environment in terms of space debris. A secular thermospheric density trend has not yet been definitively proven but predictions by Ramesh and Roble , along with evidence by Emmert et al. , strongly suggest the existence of such a phenomenon. With the ultimate goal of deriving a long-term empirical model of thermospheric cooling and contraction, the primary focus of this paper is to present preliminary results obtained to support the existing evidence for such a thermospheric contraction.There are many ways of determining atmospheric density, but inferring thermospheric density from satellite drag data is a relatively cost-effective way of gathering in-situ measurements. Given an initial satellite orbit, one approach is to use an orbital propagator to predict the satellite's state at some time ahead and then to compare that state with Two-Line Element (TLE) data at the same epoch. The difference between the semi-major axis of the satellite from the initial orbit and that after the orbit propagation is then integrated to obtain an estimate of global average density. This is the approach adopted in our new work, using a bespoke, orbital propagator that includes perturbations due to atmospheric drag, gravitational anomalies, luni-solar gravity effects and solar radiation pressure. The methods used to derive precise estimates of the ballistic coefficient of each satellite for use in the propagator are outlined, as this information is not contained explicitly in the TLE sets. In validation of the orbital propagator used in this study, Saunders et al.  ran simulations to predict satellite re-entry dates with satisfactory results. Now, historical satellite data from the past 50 years have been used to infer thermospheric density values over the same period. A comparison of these values with those derived from an empirical standard atmospheric model, the US NRLMSISE-00 (Naval Research Laboratory's Mass Spectrometry and Incoherent Scatter Radar up to the Exobase, released in the year 2000), is the method by which the long-term trend is established. More recent atmospheric models have not been used due to their requirement concerning atmospheric state indices (e.g. the Disturbance Storm Time index used with the latest Jacchia-Bowman models) which are not available for the complete historical time period.  Roble, R. G., Ramesh, D. S., (2002), "Cooling Mechanisms of the Planetary Thermospheres: The Key Role of O Atom Vibrational Excitation of CO2 and NO", CHEMPHYSCHEM 2002, 3, 841-843 1 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.  Emmert, J. T., J. M. Picone, J. L. Lean, and R. R. Meier, (2008), "Thermospheric global average trends, 1967-2007, derived from orbits of 5000 near-Earth objects", J. Geophys. Res., 35, L05101, doi:10.1029/2007GL032809.  Saunders, A., Lewis,H. G., Swinerd, G. G., (2009), "A New Tool for Satellite Re-entry Predictions", 5th European Conference of Space Debris, Darmstadt, Germany, March-April 2009.
Advanced Maui Optical and Space Surveillance Technologies Conference
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