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Abstract: Many parameters influence the evolution of the near-Earth debris population, including launch, solar, explosion and mitigation activities, as well as other future uncertainties such as advances in space technology or changes in social and economic drivers that effect the utilisation of space activities. These factors lead to uncertainty in the long-term debris population. This uncertainty makes it difficult to identify potential remediation strategies, involving active debris removal (ADR), that will perform effectively in all possible future cases. Strategies that cannot perform effectively, because of this uncertainty, risk either not achieving their intended purpose, or becoming a hindrance to the efforts of spacecraft manufactures and operators to address the challenges posed by space debris. One method to tackle this uncertainty is to create a strategy that can adapt and respond to the space debris population. This work explores the concept of an adaptive strategy, in terms of the number of objects required to be removed by ADR, to prevent the low Earth orbit (LEO) debris population from growing in size. This was demonstrated by utilising the University of Southampton’s Debris Analysis and Monitoring Architecture to the Geosynchronous Environment (DAMAGE) tool to investigate ADR rates (number of removals per year) that change over time in response to the current space environment, with the requirement of achieving zero growth of the LEO population. DAMAGE was used to generate multiple Monte Carlo projections of the future LEO debris environment. Within each future projection, the debris removal rate was derived at five-year intervals, by a new statistical debris evolutionary model called the Computational Adaptive Strategy to Control Accurately the Debris Environment (CASCADE) model. CASCADE predicted the long-term evolution of the current DAMAGE population with a variety of different ADR rates in order to identify a removal rate that produced a zero net growth for that particular projection after 200years. The results show that using an adaptive ADR rate generated by CASCADE, alongside good compliance with existing mitigation measures, increases the probability of achieving a constant LEO population of objects greater than 10cm. This was shown to be 12% greater compared with removing five objects per year, with the additional advantage of requiring only 3.1 removals per year, on average. [Copyright &y& Elsevier]

Abstract: In the last decade, space debris modelling studies have suggested that the long-term low Earth orbit (LEO) debris population will continue to grow even with the widespread adoption of mitigation measures recommended by the Inter-Agency Space Debris Coordination Committee. More recently, studies have shown that it is possible to prevent the expected growth of debris in LEO with the additional removal of a small number of selected debris objects, through a process of active debris removal (ADR). In order to constrain the many degrees of freedom within these studies, some reasonable assumptions were made concerning parameters describing future launch, explosion, solar and mitigation activities. There remains uncertainty about how the values of these parameters will change in the future. As a result, the effectiveness of ADR has only been established and quantified for a narrow range of possible future cases. There is, therefore, a need to broaden the values of these parameters to investigate further the potential benefits of ADR. A study was completed to model and quantify the influence of four key parameters describing launch and explosion rates, the magnitude of solar activity and the level of post-mission disposal compliance on the effectiveness of ADR to reduce the LEO debris population. Each parameter's value was drawn from a realistic range, based upon historical data of the last 50 years and, in the case of post-mission disposal, a current estimate of the level of compliance and a second optimistic value. Using the University of Southampton's Debris Analysis and Monitoring Architecture to the Geosynchronous Environment (DAMAGE) model, the influence of each parameter was modelled in Monte Carlo projections of the ≥5cm LEO debris environment from 2009 to 2209. In addition, two ADR rates were investigated: five and ten removals per year. The results showed an increase in the variance of the size of the LEO population at the 2209 epoch compared with previous ADR modelling studies. In some cases, the number of LEO debris objects in the population varied by a factor greater than ten. Ten removals per year were not sufficient to prevent the long-term growth of the population in some cases, whilst ADR was not required to prevent population growth in others. [Copyright &y& Elsevier]

Abstract: Since the end of the 20th Century there has been considerable effort made to devise mitigation measures to limit the growth of the debris population. This activity has led to the implementation of a “25-year rule” by a number of space-faring nations for the post-mission disposal of spacecraft and orbital stages intersecting the Low Earth Orbit (LEO) region. Through the use of projections made by computer models, it was anticipated that this 25-year rule, together with passivation and suppression of mission-related debris, would be sufficient to prevent the unconstrained growth of the LEO debris population. In the last decade both the LEO debris environment and the debris modelling capability have seen significant changes. In particular, recent population growth has been driven by a number of major break-ups, including the intentional destruction of the Fengyun-1C spacecraft and the collision between Iridium 33 and Cosmos 2251. State-of-the-art evolutionary models indicate that the LEO debris population will continue to grow in spite of good compliance with the commonly adopted mitigation measures and even in the absence of new launches. Consequently, this has led to considerable interest in the development of remediation measures and, especially, in debris removal. In this paper, we present a new and large study of debris mitigation and removal using the University of Southampton''s evolutionary model, DAMAGE, together with the latest MASTER model population of objects ≥10cm in LEO. Here, we have employed a concurrent approach to mitigation and remediation, whereby changes to the PMD rule and the inclusion of other mitigation measures have been considered together with multiple removal strategies. In this way, we have been able to demonstrate the synergy of these mitigation and remediation measures and to identify potential, aggregate solutions to the space debris problem. The results suggest that reducing the PMD rule offers benefits that include an increase in the effectiveness of debris removal and a corresponding increase in the confidence that these combined measures will lead to the stabilisation of the LEO debris population. [Copyright &y& Elsevier]