Orbital kinematics of conjuncting objects in Low-Earth Orbit and opportunities for autonomous observations

As satellites and debris continue to proliferate in Low Earth Orbit (LEO), collision avoidance maneuvers are becoming routine for operational spacecraft, causing increasingly discontinuous operations. Despite the congested environment, collisions are still very unlikely events and most satellites would never actually need to maneuver during their lifetime. Maneuvers are currently performed far more than necessary because of the limited accuracy of the tracking data used for assessing collision risks, which leads to overestimates. A little explored solution is to equip satellites with sensors that enable them, whenever at risk of collision, to autonomously track the hazardous objects and refine their positional knowledge before making a maneuver decision. Unlike what is possible from ground, satellites can collect data and estimate the collision risk even shortly before a potential collision (e.g., one orbit before), improving the final prediction accuracy and reducing false alarm rates. Moreover, while most surveillance data is currently obtained through ground-based sensors, space-based observations can provide unique information as they do not suffer from atmospheric issues (e.g., diffractions, aberrations). Here, the feasibility of observing hazardous objects directly from at-risk satellites before potential collisions is corroborated by demonstrating that objects in LEO usually pass close together several times before the closest approach, providing short observation opportunities. A thorough characterization of these observing windows is performed by reconstructing and analyzing the trajectory evolution for thousands of historical conjunction events, starting from Two-Line Element (TLE) sets and using a Simplified General Perturbations 4 model (SGP4) for the propagation. Statistical analysis is performed to assess the average number and duration of the observation opportunities, the relative distances and velocities involved, and other relevant features. Results show that a satellite with a detection range of 500 km would be able in more than 80% of the cases to observe a high-risk object twice and for at least 10 s before the potential collision