Real-time air-traffic warning during satellite re-entries: challenges and developments.

Abstract

Over time, new market trends are shaped by advances in technological development. The space sector is not exempt from this evolution, as the latest ESA Annual Space Report highlights the corresponding shift towards commercialisation of the space activities, mainly as consequence of miniaturisation technologies with consequent reduction of the average cost for launching a mission. In 2023, launches across all mass and type classes were at their highest level ever. In terms of re-entries, this translates into a rapid increase in the number of re-entering objects, although the absolute number of re-entries in 2023 was lower than the 2022, due to a specific anti-satellite missile test at the end of 2021, which peaked in the previous year . In particular, large constellations are now a major contributor to this space traffic, and concerns about the resulting ground risk have necessitated specific regulations, which have been introduced in the latest update of the ESA's Space Debris Mitigation Requirements [2], following the recent publication of the ESA's Zero Debris Policy.

Between 10% and 40% of the re-entering mass is expected to survive, with up to 300 fragments generated per object with a mass greater than 800 kg [16], which could pose a potential hazard to aircraft, ships and ground populations. In order to reduce this risk, a large number of dedicated activities have been undertaken in recent years to reduce the survivability of space objects that undergo an atmospheric re-entry [5], with some satellite operators already claiming full demisability [18]. Despite these efforts, the uncertainties in predicting the re-entry point for an uncontrolled re-entry are still relevant today, with uncertainties of around 20% of the remaining lifetime for an uncontrolled re-entry from a circular orbit. The theory for the on-ground risk assessment is in a certain measure harmonised within the international context of space agencies, but the assessment of the risk that re-entering objects pose to air and sea traffic is still a major point of discussion today, with methodologies that can vary widely from one country/agency/entity to another. There are several reasons for this, ranging from technical gaps to the lack of clear metrics or centralised coordination point.

This work aims to provide an overview of the current scenario and limitations for the implementation of a real-time risk assessment for air traffic during the re-entry of space objects, proposing possible starting points to reduce the gap between theory and practical implementation.