A satellite-centered criticality index for post-fragmentation environmental hazard assessment using toroidal regions

Ensuring the operational safety of a space asset requires continuous conjunction assessment and, when necessary, the execution of Collision Avoidance Manoeuvrers based on catalogued tracking data. Fragmentation events, such as explosions or collisions, pose a distinct challenge, as they can suddenly generate thousands of debris dispersed across a broad range of orbital altitudes. If an asset intersects such a cloud, the collision probability may increase dramatically, potentially leading to mission failure and further fragmentations. Risk assessment in the immediate aftermath is particularly challenging due to the high number of untracked fragments and the time required to estimate their trajectories. To address this limitation, the present study introduces the Criticality of Neighbourhood Index (CNI), a novel environmental hazard metric focused on a single asset in Low Earth Orbit. Building upon the Criticality of Spacecraft Index and Shell Criticality, the CNI quantifies the risk associated with remaining on the nominal orbit following a nearby fragmentation and identifies potentially safer orbital altitudes. The starting point is the definition of a toroidal region centred on the asset’s orbit, from which successive contiguous and concentric toroids are introduced at higher and lower orbits. Hereafter, the index is quantified by simulating the fragmentation with the NASA Standard Break-up Model, propagating the resulting debris distribution and computing their effects on each toroid. The proposed approach enables short-term hazard prediction in the vicinity of an asset while requiring limited computational resources and only high-level information about the fragmentation event.