The kinetic casualty risk of uncontrolled re-entries before and after the transition to small satellites and mega-constellations

Over the last 11 years (2010-2020), more than 600 intact objects larger than 1 m2 have re-entered without control into the Earth's atmosphere. The total returned mass was approximately 1100 metric tons, roughly corresponding to the re-entry of nearly 100 metric tons per year, mostly concentrated (79%) in rocket bodies. Objects with a mass greater than 500 kg re-entered every about 8 days, those exceeding 2000 kg every about 2 weeks and those heavier than 5000 kg one or twice per year. The total casualty expectancy associated with uncontrolled re-entries over the past 11 years was of the order of 1.4 x 10-1, that in 2020 was almost 1.7 x 10-2, corresponding to a probability of having had at least one victim of approximately 13% and 2%, respectively. Unlike the alert threshold of 10-4, linked to single re-entry events, no cumulative risk limit exists for satellite re-entries over one year or more. However, the casualty probability, although still relatively small, cannot be considered negligible, even in view of the launches of mega-constellations planned in the coming years. For instance, the addition of 4000 spacecraft re-entering annually would increase the probability of having at least one victim to nearly 30% per year, while 20,000 more satellites would boost it to almost 80%.