High collision probability conjunctions and space debris remediation
Derelict satellites, rocket bodies, and pieces thereof have been left on orbit. These space debris have been increasing in numbers and simulations of their future evolution have shown that this increase might continue due to collisions between objects. It has been suggested that active debris removal (ADR) i.e. removing objects from orbit by technological means rather than by their natural decay due to drag, might be necessary in order to prevent an excessive increase of the number of debris. Selection of objects to be targeted by ADR is considered important because removal of non relevant objects will unnecessarily increase the cost of ADR. Collision probability of every object should form part of the metric to select appropriate ADR targets. This work examines how the collision probabilities of all the objects in orbit depend on particular conjunctions, which cannot be forecast far in advance due to increasing orbit propagation uncertainty and variations in solar activity. It is found that conjunctions with high collision probabilities contribute more to the collision probabilities accumulated by objects over a period of time than other close approaches. Objects that are not large in mass and size are found to take part in conjunctions with high collision probabilities. Such objects are not likely to be removed from orbit when using existing ADR target selection schemes, and collisions involving them might not be prevented. Thus, the growth of the number of debris might continue in spite of ADR because collision fragments will continue to be generated. A complementary solution to constraining the number of debris in orbit, i.e. prevention of collisions between derelicts (just in time collision avoidance, JCA) is thus investigated. It is found that fewer than ten JCA actions per year could constrain the number of objects in orbit. However, certain objects will repetitively take part in conjunctions with high collision probabilities. Permanently removing such objects from orbit via ADR may be more cost effective than mitigating their collision risk via JCA. The finding that conjunctions with relatively high collision probabilities are the reason why ADR may be insufficient to constrain the number of debris, and analysis of JCA using an evolutionary debris model are the main novel contributions of this work.