The risk on the ground and in the airspace posed by uncontrolled re-entries: should the growth observed in recent years be considered worrying?
- Paper ID
97823
- DOI
- author
- company
ISTI-CNR
- country
Italy
- year
2025
- abstract
Since the launch of the first Starlink constellation satellites in 2019, there has been a significant increase in uncontrolled re-entries of human-made space objects. In 2024, there were more than 720 sizable uncontrolled re-entries, 81\% accounted for by satellites, 15\% by orbital stages, and 4\% by large debris. The annual returned mass increased from 139 metric tons in 2019 to 412 in 2024. It was mainly concentrated in orbital stages, but with an increasing satellite contribution in recent years. Considering that the risk posed by uncontrolled re-entries, when no hazardous chemical or radioactive matter is on board, is related to the returned mass, a considerable increase in this mass implies a growth in the risk. Therefore, if space objects are not designed to demise completely, macroscopic fragments could survive and reach the airspace and the ground, with consequences connected to their casualty area and impact energy, as well as to the population – or aircraft – density in the range of latitudes flown over and the year in which the re-entry occurs. Assessing the risk associated with the many uncontrolled re-entries that occur every year presents several challenges, mainly due to the lack of knowledge of the construction details of the objects and of their fragmentation process upon re-entry. Consequently, alternative methods are needed to calculate representative parameters to characterize the re-entry risk. These parameters include the casualty area associated with fragments capable of surviving re-entry with sufficient kinetic energy to cause ground casualties, and the number of pieces with mass and composition that could result in catastrophic aircraft failures. With this in mind, three relationships were adopted to calculate the total debris casualty area for each re-entry event, and two formulas were obtained to roughly estimate the number of fragments that, if hitting an aircraft, can lead to its catastrophic failure. Starting from this information, the annual cumulative casualty expectancy was calculated for satellites, orbital stages and large debris re-entered without control from 2019 to 2024. The expectancy of an aircraft fatal impact and the casualty probability for the passengers on board were also estimated during the same period. Even though the risks found are still rather small, they increased considerably, in every respect, over the past six years, both for the ground and the airspace, highlighting the need to monitor the situation closely and promoting, as soon as possible, the wide adoption of appropriate and effective mitigation strategies.