Large constellations vulnerability assessment

In the last years, plans for the deployment of large (>100 satellites) Low-Earth orbit constellations have been published from several companies. Above all, there are plans to deploy a constellation of more than 720 operative satellites in a 1200 km of altitude circular orbit, to launch about 4000 spacecraft to a 1100 km orbit, and to upgrade existing constellations. Despite their great advantages in terms of reliability and access time, large constellations must be monitored due to their influence on the space debris environment: the large number of satellites increases collisions probability and deorbiting strategies must be implemented to avoid further crowding highly packed orbits. In this contest, the analysis of the large constellation vulnerability is a powerful tool for defining the hazards for the space environment as well as evaluating the proposed solutions for satellites survivability and end-of-life strategies. In fact, if catastrophic fragmentation threshold is fixed by the EMR criteria (that, for microsatellites, gives a critical debris size of about 5 cm), smaller debris (in the class > 1 mm) may cause partial failures that may influence the EOL deorbiting operations. In this work, a simple tool for large constellations vulnerability assessment is proposed, as well as its application to two reference cases; the results in terms of impact probability are reported with particular attention to their dependence on the constellations main parameters (i.e. operational life, number of satellites, spacecraft cross section). The application of the tool to two reference cases shows that, while the probability of catastrophic impacts is not negligible, reducing the single vehicles operational life by substituting them with newer spacecraft and selecting the critical components protections can be sufficient to respect high post mission disposal success rates (e.g. >90%).