Proper characterisation of the survivability of an unmanned spacecraft to debris impact must go beyond just a simple assessment of the probability of penetration. Some penetrative damage may be survivable, particularly if critical internal equipment is arranged judiciously. This type of 'indirect' shielding approach represents a potentially cost-effective and complementary approach to the more traditional method of adding 'direct' shielding mass. To quantify the benefits of both strategies, and identify candidate protection solutions for a typical satellite design, is both complicated and time-consuming. As a step towards addressing this problem, a new model called SHIELD has been developed. Competing protection options are evaluated by assessing their probabilities of survival, and rapid convergence on one or more 'good' designs is achieved using a genetic algorithm search method. Results from the model highlight the importance of 'indirect' shielding in improving spacecraft survivability. It is concluded, therefore, that unmanned spacecraft should implement protection according to the principle of 'design for survivability'. Implicit in this statement is the notion that the possibility of a degree of penetrative damage should be tolerated. This is in contrast to the approach adopted for manned spacecraft, where penetration risks must be minimised as far as possible.
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