The two leading perturbations acting on satellites in the orbital decay region of low Earth orbit are gravitational nonsphericity of the Earth and atmospheric drag. A detailed understanding of their properties is required to accurately predict satellite dynamics, such as the orbital lifetime. However, there is still room to improve current perturbation models in this region, especially for the atmosphere. The purpose of this paper is 1) to develop a simple method to reverse engineer the leading perturbations from two-line elements (TLEs), and 2) to apply the method to the 2017 Re-Entry Satellite with Gossamer Aeroshell and GPS/Iridium (EGG) nanosatellite mission as a case study. Because of the low ballistic coefficient of EGG and its low orbital altitude, flight was highly affected by both aerodynamic and geopotential effects. Secular and long-periodic effects are extracted via linearized perturbation theory. The results show that EGG acted as a low-cost passive multiphysics sensor for several phenomena, including the changing solar flux, the diurnal atmospheric bulge, rotating winds, and the zonal geopotential. For example, the second zonal geopotential harmonic coefficient is estimated to within 0.008%. To the authors’ knowledge, this is the first study to produce estimates of both atmospheric and geopotential perturbations via TLEs from a single satellite.