Collision probability assessment for active debris removal missions

Increasingly larger investments are being made world-wide in the technologies necessary to perform Active Debris Removal (ADR) due to a growing concern about the sustainability of spaceflight. In-orbit validation of certain concepts appears likely in the near future with potential implementations to follow shortly afterwards. Little to no attention appears to be given to the risks associated with such missions, however, which raises concerns about the impact of the ADR missions themselves. This study conducts an assessment of the probability of an in-orbit collision occurring between an ADR mission’s target, whilst being de-orbited, and all the objects in the public catalogue published by USSTRATCOM. Such a collision could have disastrous effects as the target is most likely going to be located in densely populated orbital regimes and thus many follow-up collisions could take place. Two impulsive and one low-thrust example ADR mission trajectories that can be associated with particular types of de-orbiting technologies are screened for conjunctions.The importance of extremely close conjunctions that result in most of the total accumulated collision probability (in excess of 99% in certain cases) and the need to avoid those is highlighted. This likely discards ADR technologies that do not enable collision avoidance to be performed. Collision avoidance during debris removal appears of particular importance if business is to be built around ADR in order not to undermine the credibility of the ADR companies if they were to cause catastrophic collisions. It is also suggested that, depending on the removal method, it may be less risky to the debris environment to leave the target object in orbit for longer and perform fewer removals per year but with a lower probability of collision.