1. www.southampton.ac.uk/engineering | email: hglewis@soton.ac.uk Astronautics Research Group, Faculty of Engineering and the Environment University of Southampton, Southampton SO17 1BJ, United Kingdom IAC-14-A6.P.58 Hugh G. Lewis & Aleksander Lidtke Active Debris Removal: Consequences of Mission Failure Motivation There is widespread belief that Active Debris Removal (ADR) is needed to reduce the hazard posed by an increasing space debris population in low Earth orbit (LEO). Evolutionary models have demonstrated that the sustained removal of a few large debris objects can have a beneficial effect on the LEO debris environment. Following these studies, there has been a drive towards the development of technologies and concepts for ADR, with several on-orbit demonstration missions now proposed. However, the ubiquitous assumption made in modelling studies and the subsequent concept development has been that ADR always results in a successful mission outcome, i.e. the ultimate removal from orbit of the targeted debris and the chaser vehicle used. This is unrealistic, given that traditional space missions can, and do, fail on-orbit. Of real concern, therefore, is the potential for ADR to add to the space debris hazard. Three mechanisms exist through which the benefits of ADR may be lost: key debris targets may not be retrieved, new debris may be added in the form of failed chaser vehicles, or collisions involving the former objects may occur. The last mechanism is of particular concern because debris targets and failed chaser vehicles will be in highly populated regions of LEO. This research aimed to address the concern, through simulations of these mechanisms using the University of Southampton’s debris evolutionary model DAMAGE. In so doing, recommendations for ADR mission reliability and programmatic approach were identified. LEO population growth 2013-2113 Fraction of all Monte Carlo runs showing population growth Fraction of all collisions involving a failed ADR chaser vehicle Fraction of all removal missions targeting a failed ADR chaser vehicle Method Two ADR mission approaches were studied: a “single-removal” chaser vehicle that removed only one debris target before de-orbiting itself, and a “multiple-removal” chaser vehicle that removed five debris targets before de-orbiting. In the latter case, it was assumed that the chaser vehicle attached a de-orbit device (e.g. drag sail, tether or solid rocket motor) to each debris target before performing an orbit transfer to the next debris target and repeating the procedure. Consequently, a failure of the multiple-removal chaser vehicle would only have consequences for debris targets yet to be visited. Launch vehicles associated with the ADR chaser vehicles were assumed to de-orbit immediately following the payload separation. Five different chaser vehicle failure rates were simulated: 0%, 5%, 15%, 30% and 45% per year, with vehicles selected randomly, and three removal rates: 5, 10 and 15 debris objects per year (with the addition of a baseline case with no removals used for comparison). In addition, it was assumed that a mission failure resulted in the complete loss of control of the ADR chaser vehicle, which remained on orbit. ADR chaser vehicles successfully completing their mission were removed from orbit immediately. Failures were assumed to occur in one of three positions, selected randomly, relative to the target debris: (1) in a drift phase 100 km below, (2) in an inspection phase 50 m below, or (3) in a contact phase. The probabilities associated with each position were 50%, 25% and 25%, respectively. Failed missions were not repeated in the same year. Projections of the LEO debris population ≥ 10 cm from 2013 through 2113 were performed using DAMAGE, with 50 Monte Carlo (MC) runs used for each scenario. Conclusions A “multiple-removal” chaser vehicle provided substantial benefits in terms of robustness to failure, compared with the “single-removal” chaser vehicle approach. However, such missions are inherently more complex and involve potentially greater risk of failure, given the need for multiple proximity operations. Further work is required to understand the effect of this trade-off. The “single-removal” chaser vehicle approach resulted in a significant effort (up to 35% of removal missions) focused on removing failed ADR chaser vehicles, and up to one-in-four collisions involved an ADR chaser vehicle (27% of all collisions). The results also showed that debris removal rates of more than five objects per year were needed to prevent, or limit, the growth of the LEO population over 100 years. At the same time, a higher number of removals required a corresponding decrease in the individual mission failure rate (i.e. an increase in reliability) to achieve the same outcome, for both types of mission. The effect of higher failure rates was an increasing probability that the LEO debris population would grow in number. Future efforts in ADR must move beyond the assumption of immunity to failure: consideration needs to be given to ADR technologies and concepts that are robust to failure and result in a benign impact on the environment in case of failure. Without such a move, there is a possibility that a remediation method, widely perceived as the only solution to the growing space debris hazard, could exacerbate the problem. Effective number of objects (LEO, ≥ 10 cm) Year Population density as a function of time for failure scenarios involving single-removal ADR missions launched at a rate of 10 per year No failures; No removals No failures; Target: 10 removals per year 5% failures; Target: 10 removals per year 15% failures; Target: 10 removals per year 45% failures; Target: 10 removals per year Results Charts showing the population density, LEO population growth (compared with the 2013 population), number of MC runs in which population growth was observed, fraction of collisions involving failed ADR chaser vehicles and fraction of removal missions that targeted failed ADR chaser vehicles, are reported here. Single-removal Multiple-removal Single-removal Multiple-removal Single-removal Multiple-removal Single-removal Multiple-removal Acknowledgements Thanks to Holger Krag, Head of the ESA Space Debris Office, for permission to use the MASTER reference population within DAMAGE, and colleagues from IADC WG2 for comments on an earlier report on these results.