Removal targets' classification: How time considerations modify the definition of the index

The growth of the near-Earth debris population since the beginning of human space activities is now a fact commonly admitted by space agencies worldwide. Numerous entities have warned about the danger that debris may have over time. Presently mitigation methods such as imposing post-mission disposal time after launch will no longer be sufficient; remediation processes seem necessary to limit the increase. In particular, this phenomenon is attributed to the generation of fragments due to more and more on-orbit collisions. Therefore, investigations on indexes to select potential removal targets were recently conducted, considering solely objects implicated in a collision course. This study also looks at the multiple fragmentation factors, including time through the altitude at time of impact (due to the behaviour of debris re-entering with time). The focal point is here to compare different criteria to select removal targets that enable scenarios in best adequacy with the future in question (long term, mid term or short term). Aware of the uncertainty of evolutionary models, this study also incorporates the simulation method as an impactful factor and tries to overcome the potential randomness of the results. Therefore, this paper presents a way to establish a selection criterion the most adequate for the time period focused on. In order to solve this issue, a “double-check” method is proposed. First, an analytical evolutionary model simulates the environment over 100 years, through 100 Monte-Carlo runs. Then, among the initial population of year 2009, the objects supposed to be at the origin of the debris detected at a given time are tracked back in time into the simulations, using a collision-detecting program. The “given period” above mentioned for the presence of debris is based on a future as such that 2029 be considered a short-term scenario, 2059 a midterm scenario and 2109 a long-term scenario. This step produces three lists of targets for removal (one for each future), and simulations are run once again, through different scenarios involving the removal of particular listed targets in order to verify the appropriateness of the proposed scenarios. The analysis of the results is based both on the mean of the simulations and on the recurrence considering each run. Three studies were conducted one for each term, and a fourth one completed the work by establishing comparison between short, mid and long-term periods. As a result, three main criteria could be established: the altitude of the objects, the number of targets necessary to remove, and the phenomenon of chain collisions. According to the future that was investigated, the most adequate criterion appeared to be different, consisting in the number of objects in the long-term analysis or the ranking position at short term (linked to the close-time consideration). As a main conclusion and further perspectives, it should be more efficient to consider the collision-probability and mass product together with the time-depending generation of fragments. This would help increasing the precision in the prediction of collision impacts. Rather than pinpointing specified targets to be removed, the aim of this study is simply to understand the mechanisms at the origin of the population increase around the Earth. Also to demonstrate that a careful definition of selection criteria would enable to adopt a suitable removal process in the period of action or for the goal to be reached.