The interaction between space tether systems and the orbital space debris environment

There have been a number of proposals for using tethers in space. One such proposal involves deploying an electrodynamic tether, at the end of a host spacecraft's mission, in order to reduce the spacecraft's post-mission orbital lifetime. However, a space tether is particularly vulnerable to the orbital debris environment. When fully deployed, a tether may have a very large cross-sectional area and, unlike most spacecraft, they are very susceptible to small debris impacts. It has been estimated that a debris fragment with a diameter smaller than half the tether diameter can cause a single strand tether system to fail. Therefore, a good understanding of the interactions between the space debris environment and a space tether is of vital importance. The Tether Risk Assessment Program (TRAP) has been developed as an extremely flex- ible tool with state-of-the-art capabilities in tether collision and sever risk assessments. The integrated structure of the developed software enables a wide variety of analyses to be conducted and simulations of both historic and potential future fragmentation events to be performed. The model is also capable of modelling the historic, current and future background orbital debris environment. A highly novel aspect of the TRAP model is the implementation of the Probabilistic Continuum Dynamics (PCD) method, enabling the debris density to vary over the length of the tether system, providing a more realistic collision risk assessment. This method is an improvement over other traditional methods, which simply take an average debris density for the whole tether system. Another novel aspect is its capability to determine debris penetration depths, producing an accurate method of determining tether sever probabilities. This approach is much more advanced than current methods that simply use a lethality coefficient in order to determine a minimum fragment diameter capable of severing the tether. The TRAP model has undergone a vigourous validation programme to assess its accuracy. This has included simulating past tether missions and comparing with actual flight data. The validation exercise has greatly improved the confidence in the TRAP model for the prediction of realistic and accurate collision and sever risk assessments, for both single and double strand tether systems. The validated TRAP model has been used for a number of case studies. The collision and sever probability risks associated with space tether systems arising from the orbital space debris environment have been extensively simulated. These simulations have considered both single and double strand tether systems, of various length and thickness, and should be of considerable interest to tether mission designers.