How to assess and manage the risk from uncontrolled re-entries in view of future developments of space activities

Pardini C., Anselmo L.

Uncontrolled reentries  Casualty risk  Evolution of space activities 

We are currently experiencing profound transformations in space activities. The number of operational satellites could increase tenfold over the next decade, radically changing the modalities and scale of space operations. The risk in the airspace and on the ground, due to the uncontrolled re-entry of satellites into the Earth's atmosphere, could increase as well in consequence of the huge number of new payloads placed in low altitude orbits, or disposed from higher orbits in order to comply with space debris mitigation measures. To date the re-entry risk has been assessed and managed on an object-by-object basis, such as assessing whether or not a single event might exceed a certain casualty expectancy threshold, typically set equal to 10-4. However, in view of future developments in space activities, characterized by the launch of numerous small satellites and mega-constellations, it may be necessary to shift to a more holistic approach to the problem, at the very least more system-oriented than object-oriented. As a matter of fact, for instance, even if a single satellite re-entering from a mega-constellation has a casualty expectancy of the order of 10-5, i.e. below the alert threshold of 10-4, 100 of such satellites re-entering annually would have a casualty expectancy of about 10-3, i.e. comparable to that of various past events, e.g. the re-entry of the NASA's spacecraft UARS or the Chinese space station Tiangong-1, which received great international attention. However, so far no collective risk limit has been established for spacecraft re-entries, and even managing that risk according to the US Range Commanders Council (RCC) recommendations, the proposed annual collective risk for the general public has been already marginally exceeded by the re-entry of the Starlink satellites in 2020. A priority action for the next few years will therefore be to at least establish a cumulative risk threshold on an annual basis, so that uncontrolled re-entries can be managed before their associated risk becomes too high to be controlled. The aim of this analysis is first to introduce one or more metrics to characterize and to classify the risk associated to uncontrolled re-entries, occurred during the transition phase between the old and new space activities (2010-2021), then to apply such metrics to possible scenarios, roughly representing space activities in the next few decades. An object-oriented approach is applied to single events in order to identify potentially risky re-entries - based on international regulations - and the re-entry risk evolution over the last 12 years. Successively, a system-oriented strategy is introduced - currently based on the RCC guidelines - to manage the annual collective risk associated with re-entries from mega-constellations. Based on the scenarios simulated, projections of the re-entry risk are carried out over the next 3 decades, also considering the predictions for the world population increase. The possible challenges to be faced in the near future to manage an ever-increasing growth in uncontrolled re-entries conclude this analysis.

Source: COSPAR 2022 44th Scientific Assembly, Athens, Greece, 16-24/07/2022