This Thesis presents a low-fidelity approach to model and analyze the decay phase for space objects (space debris) originally orbiting in Low Earth Orbits (LEO). This approach is used to better characterize some of the involved uncertainties, which are propagated through the propagation of the orbit in order to characterize the re-entry time. In particular, the main source of uncertainty lies in the prediction of solar and geomagnetic indices which is performed through time series analysis techniques. The indices influence the Total Mass Density (TMD) in the Thermosphere, i.e. the layer of the Earth atmosphere from about 90 [km] of altitude to between 500 and 1000 [km].