IADC AI 19.1 on 'Benefits and Risks of using Electrodynamic Tethers to De-Orbit Spacecraft' - Methodology used at ISTI/CNR to Assess the Sever Probability of Space Tethers

The methods used at ISTI/CNR to compute the fatal impact rate of meteoroids and orbital debris on space tethers and to assess the survival probability of electrodynamic tether systems during a de-orbiting mission are presented. The tethers are supposed to be in circular orbit and aligned along the gravity gradient. Two basically different designs have been considered: 1. Single tether: a single wire or a cylindrical multi- line structure; 2. Double tether: two cables are separated each other by a distance significantly larger than their diameter and form N loops, tied together in N + 1 equidistant knots. In order to assess the survivability of double line tethers, a numerical multi-step algorithm has been specifically developed at ISTI. For a tether at a given orbital altitude, it computes: 1. The sever probability of a single cable; 2. The sever/survival probability of both lines of the same tether segment; 3. The sever/survival probability of the whole tether. Concerning an electrodynamic tether for satellite de-orbiting, its altitude changes during the mission, and with it also the space debris flux. In order to take into account the debris flux variation, as a function of the decreasing altitude, in the survivability assessment, the overall altitude range traversed by the tether is subdivided in a number of altitude intervals, where the space debris flux can be assumed constant. Taking into account that the tether - single or double line - survives during the de-orbiting mission only if it survives in each altitude interval, the overall survival probability during the mission is eventually estimated.