Abstract: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. Planned, large constellations of satellites in low Earth orbit (LEO), though addressing the lack of basic internet coverage in some world regions, may disrupt the sustainability of the space environment enabled by these mitigation practices. We analyse the response of the space object population to the introduction of a large constellation conforming to the post-mission disposal guideline with differing levels of success and with different disposal orbit options. The results show that a high success rate of post-mission disposal by constellation satellites is a key driver for space sustainability.

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Collision avoidance is a topic of increasing importance. The number of satellites in Earth orbit is steadily growing and with the high amount of space debris, either crossing through or resident in orbit, collision probabilities between two such objects can become critical. Small satellite missions usually operate with limited capabilities when it comes to locating potential collision occurrences and deriving the associated collision probability. Accordingly, they have to rely on external organizations, such as the Joint Space Operation Center (JSpOC) and their information system to plan for contingency operations. This paper reviews the benefits of using such an external service for a small satellite constellation. It analyses the relevant data for use in daily operations and shows collision avoidance approaches based on the available data. Conjunction summaries for the RapidEye satellite constellation are evaluated and their influence on the planning of collision avoidance maneuvers is shown.

The present distribution of intact objects is a good proxy to quantify the catastrophic collision risk and consequences in the coming decades. The results of a large number of long term simulations of the LEO environment perturbed by the collisional fragmentation of massive objects are used to identify the main driving parameters of the long term collisional evolution of the debris population and measure the danger represented by "typical" classes of space objects. An evaluation norm, able to highlight the differences between comparative long term evolution scenarios and to give a quantitative measure of the effects of specific parameters affecting the evolution, is devised. It is shown how, for collisional fragmentations in LEO, due to the highly stochastic evolution of the LEO environment, even the fragmentation of a massive spacecraft might not be able to alter the long term evolution of the LEO population beyond the intrinsic statistical variability associated * Corresponding author Email address: a.rossi@ifac.cnr.it (A. Rossi) May 26, 2015 with the Monte Carlo procedure. Among the parameters determining the long term effects of a collisional fragmentation in LEO, a combination of mass and altitude of the event appears to be the driving factor. In GEO, the situation is different, and the addition of a massive fragmentation lives a signature on the environment that is detectable throughout the investigated time span, with the mass being the only factor important to asses the long term consequences of a collisional fragmentation. Preprint submitted to Advances in Space Research