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This paper presents an analytical model for collision probability assessments between de-orbiting or injecting space objects and satellite constellations. Considering the first to be subjected to a continuous tangential acceleration, its spiraling motion would result in a series of close approaches in the proximity of a constellation. The proposed methodology involves the integration of the collision probability density function on the encounter plane, from which two analytical formulas, one for the number of close approaches and one for their respective average collision probability, are obtained. The mathematical description of the crossing dynamics relies on the assumption of circular orbits and independent collision probabilities, but does not require to propagate the satellites' orbit. A comparison with a conventional propagation method has been performed for validation purposes, proving its accuracy also in case of elliptical crossing orbits. The model developed has been used to assess the risk connected to constellation's satellites replacement, once they have reached their programmed End-of-Life. The environmental impact of the full replacement of 12 approved constellations is analysed by means of average collision probability. In particular, it is shown that the key features for space exploitation sustainability are the maximum propulsion available from the thruster, the selection of an optimal crossing orbit and the true anomaly phases between constellations' and crossing satellites. The consequences of an in-orbit collision are also investigated by assessing the collision risk generated by the formation of a debris cloud. The results corroborate the need for international standards for space traffic management as an exponentially increasing satellites population could trigger a chain reaction of collisions, making LEO inaccessible for decades.