This thesis mainly reports about two topics: the Earth's atmosphere, the empirical models that modelise it with their solar and geomagnetic indices and their forecast methods, first, and then the occultation of planets and stars by the Moon as seen from the spacecraft's telescopes. The two topics are involved in a Thales Alenia Space's internal software, named NODES. It is a numerical orbit determination software used for years inside the company in Fortran code, but recently translated into Matlab language, and used to simulate the spacecraft's orbit when it is influenced by external variable perturbations. The first topic is particularly important for a Low Earth Orbit Mission Analysis, as the drag is one of the main factors that influences the spacecraft's orbit, and causes the orbit altitude decay .