Evolution of artificial space debris clouds
Over 120 cases of on-orbit breakups have now been recorded. Many more undetected events are believed to have occurred. Each time an object breaks up, whether by explosion or collision, a cloud of debris is formed. The overall objective of the PhD is to examine the interaction between the debris clouds produced by on-orbit fragmentation events and specific space systems. A breakup event will give rise to concentrations of debris which, for some time after the event, will have spatial densities considerably higher than the background flux. Thus, a detailed knowledge of the extent to which the cloud will grow over a given time period, and an accurate assessment of the risk of collision for a spacecraft passing through it, may prove to be important in mission planning and satellite shielding design. The SDS (Space Debris Simulation) software suite has been developed to carry out the analysis presented in this thesis and now represents the state-of-theart in debris cloud modelling. The integrated structure of the developed software enables a wide variety of analyses to be conducted and simulations of both historic and potential future orbital fragmentation events to be performed. Program BREAKUP uses a combination of empirical and analytical models to simulate catastrophic and non-catastrophic collisions, and also variable intensity explosive fragmentations. Included in BREAKUP is a novel parametric model for producing and controlling non-isotropic fragment spreads. TRAJECTORY acts as a test-bed for orbit propagation techniques, providing the facility for convenient and direct method comparison. EVOLUTION enables the complex dynamics of debris cloud growth to be visualised and in particular the effects of propagation method to be examined. Program TARGET employs a novel implementation of the method of probabilistic continuum dynamics to perform collision hazard assessments for spacecraft which encounter debris clouds. Among the additional new developments included in TARGET are the consideration of atmospheric drag, a direct interface with a non-isotropic cloud model, the use of a cellular target spacecraft representation and impact energy-related damage assessment algorithm, and a built-in satellite constellation analysis facility. A number of case studies are presented to illustrate the modelling capabilities of the SDS software suite, including the simulation of several historic fragmentation events and the debris cloud collision risks to ENVISAT-1 and the Iridium™ satellite constellation. The results produced by the models are validated by comparisons with other simulation software and, wherever possible, with actual breakup event, debris impact and spacecraft, orbit, data.