Speaker
Description
Periodic encounters arise from repeated close approaches between the same pair of objects, driven by their relative orbital geometry, and represent one of the main causes of Conjunction Data Messages. Due to their recurrent nature, these events can generate multiple conjunction notifications over extended time intervals, making their early identification particularly important for long term conjunction monitoring.
This study is conducted within the ESA project “Orbital Neighbourhood Awareness”, funded by the ESA Space Safety Office, developed in collaboration with GMV France, GMV Romania, and Politecnico di Milano, which aims to provide an orbital neighbourhood monitoring tool for predicting potential conjunctions several months in advance. Within this framework, periodic encounters constitute a specific and operationally relevant class of events and are the focus of the present work.
The proposed methodology is based on analytical formulations designed to efficiently process Two Line Element sets of objects within a given orbital region, such as Low Earth Orbit. For a selected primary object, a screening procedure compares it with the surrounding population to identify orbital configurations leading to periodic close approaches. The population is classified into active satellites and inactive objects. For active satellites, an orbit maintenance assumption constrains the long-term evolution of orbital elements, enabling more reliable estimates of conjunction recurrence. For inactive objects, the framework is complemented by limited orbit propagation to account for secular drift, at the expense of increased computational cost.
For the screened pairs, the model estimates the expected number of encounters within a given time interval with low computational burden. The methodology is validated using Conjunction Data Messages associated with known periodic encounters. Results show good performance over short and medium time horizons, with a gradual degradation for longer predictions due to cumulative perturbations and modelling uncertainties. The approach is less effective in the Geostationary Earth Orbit regime, where station keeping manoeuvres introduce kilometre scale orbital displacements, significantly altering the orbital geometry and dominating the encounter dynamics
An extension of the methodology is introduced to analyse constellation crossing scenarios, accounting for slowly varying semi major axis profiles associated with low thrust manoeuvres, thus enabling the assessment of encounter dynamics during gradual orbit changes. In addition, a complementary framework is proposed to evaluate the impact of new constellation deployments on the orbital corridor of a target satellite, providing insight into the evolution of the local encounter environment.
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