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The main focus of this work is to highlight the main parameters driving the future evolution of the debris environment, in presence of the planned LEO mega constellation of satellites. The goal is to define the best management practices, able to minimize the debris production in the future. First, in order to identify the most important parameters that are actually driving the evolution of the environment and in an effort to discrimiate between possibly equivalent scenarios, we applied tools from the statistical sciences, namely the Wilcoxon signed rank test, i.e., a nonparametric test which, without assuming normality in the data, allows us, given two samples, to assess whether their population mean ranks differ. Then, building on the index described in [6] and [3] and on the analysis of the simulations performed in the aforementioned studies, by means of a simplified model of the constellation building and managing, we define an index able to quantify the environmental impact of the mega constellations. The index takes into account the physical and orbital characteristics of the constellation satellites, along with the mitigation practices adopted for each constellation. Based on the expected collision risk and the capability of avoiding impacts, the operational and non-operational satellites and the related upper stages, present in each constellation, enter the index computation with different relative weights to properly account for the global constellation effects on the environment. An averaged, time evolving, index computed on altitude shells can give quantitative and visually effective indication of the effectivenes of the mitigation practices applied by a constellation operator. The model and the associated index, along with other metrics described in [6] and [4], allows us not only to highlight the prominence of some of the parameters entering in the definition of a satellite constellation but also to "predict" the influence that a change in that particular parameter is going to produce on the long term evolution of the environment. The metrics and the index are applied to the results of a sub-sample of the long term simulations, showing the relative importance of different design parameters and allowing a better ranking of possible mitigation measured. In the simulated scenarios, the parameters playing a major role in the effect that a mega-constellation cause on the environment are: the mass and the area of the satellites, the failure rate in the operational orbit and the collision avoidance success rate.

The stability of the Low Earth Orbit environment is investigated by simulating a common scenario with several different models developed worldwide by the different space agencies.

Opportuaities provided by small satellites in low Earth orbit (LEO) are anticipated to make a significant impact on the space economy through the delivery of important and innovative services. However, with plans by some companies to operate large constellations of small satellites in LEO, and with many small satellite launches forecast in coming decades, there is concern that existing debris mitigation measures will not be sufficient to counteract the impacts of this increased space activity on the LEO environment. Within this context, a team comprising engineers from industry, academia and the European Space Agency have performed an assessment of the potential impact of small satellites and large constellations on the space debris environment. This paper provides an overview of the work undertaken and the results that emerged.

This paper is focused on the long-term simulation part of a study, funded by the European Space Agency, aimed at assessing the net effect of using sails for passive deorbiting at the end of life on the future debris population around the Earth. The aim of the "environmental feedback" task of the study was to perform simulations on the overall debris population considering the cases in which solar and drag sails are used for deorbiting. This is done in three steps, first the requirements in terms of sail area for deorbiting in a desired time are found, and then these requirements are compared with the current capabilities of sail technology. As a third step the revised sail area, considering technological constraints, are used for each spacecraft employing a sail in the debris simulation in SDM. The paper discusses the results of the long-term simulations.

Large constellations of satellites in Low Earth Orbit (LEO) will provide important communications and broadband internet services to large parts of the world where the necessary terrestrial infrastructure is lacking. These services are also likely to have a significant effect on the space economy. At the same time, there is considerable potential for negative impacts on the space environment and other space users unless appropriate space debris mitigation measures are incorporated into the design and operation of the constellations. Recent modelling work by the authors has shown that the robust implementation of post-mission disposal (PMD) to orbits with short lifetimes, in combination with other constellation and satellite design decisions, can help to reduce the negative impacts of large constellations operating for decades in LEO. However, further modelling work is needed to understand the possible impacts emerging from the simultaneous operation of multiple constellations at relatively similar altitudes in LEO. In addition, it is likely that these constellations will move away from ideal Walker-type geometries in an effort to reduce the collision risk between satellites in the same constellation. Consequently, a new simulation study has been performed, with the aim of improving the understanding of the impacts arising from the operation of several large, non-typical constellations in LEO. This new study was focused primarily on the arrangement of orbital planes in two large constellations. In particular, the separations in altitude and inclination were evaluated for their influence on the collision risk to other same-constellation satellites and non-constellation objects, and to assess the potential for collisions between satellites from the different constellations. The DAMAGE evolutionary model was used to evolve the population of objects >= 10 cm in LEO from 1 January 2013 over a 200-year projection period with an optimistic assumption for the behaviour of the constellations and the background orbital population with respect to the commonly adopted debris mitigation measures. The results show that the effect of these measures was to reduce the number of objects and collisions in the LEO environment compared with previously reported results, which represented the implementation of the existing and commonly adopted debris mitigation guidelines. These benefits persisted even for a scenario with two constellations operating in relative proximity. The effects of separating the orbital planes of large constellations were not clear and further work is needed to ensure that the evaluation of large constellations is achieved in a robust fashion.

The number of artificial objects in orbit continues to increase and, with it, a key threat to space sustainability. In response, space agencies have identified a set of mitigation guidelines aimed at enabling space users to reduce the generation of space debris by, for example, limiting the orbital lifetime of their spacecraft and launcher stages after the end of their mission to at most 25 years in low Earth Orbit (LEO). In recent years there has been a considerable increase in space traffic due to the use of small satellites by non-traditional space operators, and due to a significant change of mission scopes and mission orbits in LEO. Many indications point to a further increase in the space traffic in LEO in the near future, including proposals for large constellations in LEO to provide broadband internet services. Several recent studies have shown that, nowadays, the guidelines for the LEO protected region are insufficiently applied by space systems of all sizes. Therefore, a substantial increase in the launch rate, whether through the large- scale release of small satellites or through the construction and maintenance of large constellations, is a potential concern for the environment and particularly if the current End of Life (EOL) behaviour prevails in the future. Even with perfect behaviour with respect to the 25 year lifetime "rule", the new traffic might lead to an extremely challenging environment trends. Furthermore, a 90% requirement for the reliability of the disposal function (ISO- 24113 standard), when weighted by the reliability of the entire system, or a failure rate of 10%, might not be sustainable if the LEO launch rates increase considerably even if these are found to be generally acceptable under current space traffic conditions. In this study, we report the results of a European effort to analyse in detail the effects that such an increase in the launch rate and number of objects in LEO would have on the space debris environment. With the use of long-term environment simulation tools, the main parameters driving the evolution of the environment under the presence of a large number of small satellites and with mega-constellations being deployed have been identified. Further analyses have been performed to identify the sensitivity of the environment to different stress parameters.