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Building on work from previous studies a strong case is presented for the existence of a long‐term density decline in the thermosphere. Using a specially developed orbital propagator to predict satellite orbit evolution, combined with a new and accurate method of determining satellite ballistic coefficients, a long‐term thermospheric density change has been detected using a different method compared to previous studies. Over a 40‐year period between the years 1970 and 2010, thermospheric density has appeared to reduce by a few percent per decade. However, the results do not show the thermospheric density reduction to vary linearly with time. Therefore, by analyzing the derived density data over varying solar activity levels, as well as performing a Fourier spectral analysis to highlight any periodicities, connections with physical phenomena, where possible, are proposed. Key Points Long‐term thermospheric density decline Ballistic coefficient determination using TLE data Validation of existing theories

Historically, computer simulations of the near‐Earth space debris environment have provided a basis for international debris mitigation guidelines and, today, continue to influence international debate on debris environment remediation and active debris removal. Approximately 22,500 objects larger than 10 cm are known to exist in Earth orbit, and less than 5% of these are operational payloads, with the remaining population classed as space debris. These objects represent a significant risk to satellite operations because of the possibility of damaging or catastrophic collisions, as demonstrated by the collision between Iridium 33 and Cosmos 2251 in February 2009. Indeed, recent computer simulations have suggested that the current population in low Earth orbit (LEO) has reached a sufficient density at some altitudes for collision activity there to continue even in the absence of new launches. Even with the widespread adoption of debris mitigation guidelines, the growth of the LEO population, in particular, is expected to result in eight or nine collisions among cataloged objects in the next 40 years. With a new study using the University of Southampton's space debris model, entitled DAMAGE, we show that the effectiveness of debris mitigation and removal strategies to constrain the growth of the LEO debris population could be more than halved because of a long‐term future decline in global thermospheric density. However, increasing debris remediation efforts can reverse the impact of this negative density trend. Key Points Thermospheric decline reduces debris mitigation efficacy Thermospheric decline reduces efficacy of debris removal Increasing removal rate restores benefit of debris removal

Predicting the positions of satellites in Low Earth Orbit (LEO) requires a comprehensive understanding of the dynamic nature of the atmosphere. For objects in LEO the most significant orbit perturbation is atmospheric drag, which is a function of the local atmospheric density from a layer in the atmosphere called the thermosphere. For long-term predictions of satellite orbits and ephemerides, any density trend in the thermosphere is a necessary consideration, not only for satellite operators, but also for studies of the future LEO environment in terms of space debris. Numerous studies of long-term thermospheric density change have been performed. Predictions by Roble & Ramesh (2002), along with evidence by Keating (2000), Emmert et al.(2004), Marcos et al. (2005), Qian et al. (2006) and Emmert et al. (2008), strongly suggest the existence of such a phenomenon. Therefore, the objective of the research presented in this thesis is to provide a novel method to evaluate quantitatively thermospheric density change. Satellite drag data is an effective medium through which one can investigate local thermospheric density and changes thereof. 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. To do this, knowledge about a satellite’s physical properties that are intrinsic to atmospheric drag is required. A study by Saunders et al. (2009) highlighted problems with estimating a satellite’s physical properties directly from data given explicitly by Two-Line Element (TLE) sets. This prompted an investigation into ways to estimate ballistic coefficients: a required satellite parameter associated with drag coefficient and area-to-mass ratio. A novel way of estimating satellite ballistic coefficients was derived and is presented in this thesis. Additionally, novel consideration of atmospheric chemical composition was applied on long-term drag coefficient variability. Using a quantitative estimate of a ballistic coefficient one can propagate numerically a satellite’s orbit and predict the effects of atmospheric drag. Given an initial satellite orbit from TLE data, 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 TLE data at the same epoch. The difference between the semi-major axes of the initial orbit and that after the orbit propagation is then integrated and can be used to estimate the global average density. The method employed in this study utilises this process. To achieve this, a specially developed, computer-based, numerical orbital propagator was written in the programming language C/C++. The underlying theories and implementation tests for this propagator are presented in this thesis.