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to protect their spacecraft against debris impact must involve balancing the risk and cost of damage from debris impact against the expense of implementing measures to protect against debris. The final decision will be different for each spacecraft because the hazard, acceptable risk, and cost of protection will vary depending on the spacecraft's orbit, configuration, and particular mission. One factor that will not vary greatly is that the earlier debris considerations are factored into the design process, the less costly will any necessary modifications be. Early in the design process, designers can modify aspects of the design to meet debris-related requirements at a minimum cost; later, however, the many design choices that have already been made and cannot easily be changed constrain further design changes.
Determining the need for (and extent of) protective measures against debris is a three-step process. First, the hazard from debris must be calculated by determining the size-dependent debris flux that the spacecraft is likely to experience and then determining the probability that the flux will damage the spacecraft. Second, the effectiveness of various protective methods (such as shielding or component rearrangement) that could be used to reduce the hazard must be determined. The final step is to look at the results of these two analyses and consider the tolerable level of hazard for the spacecraft, to determine the costs and benefits of implementing protective measures. As illustrated in Box 6-1, the final decision on protecting a spacecraft will vary greatly depending on the spacecraft involved and the level of hazard acceptable to the designers and operators.
DETERMINING THE HAZARD FROM DEBRIS
To quantify the threat of orbital debris to a spacecraft, designers must analyze the particular debris environment in their spacecraft's orbit, as well as the spacecraft's vulnerability to that environment. A number of analytic and experimental tools that can be very helpful in carrying out these tasks are now available to designers. It is important, though, that spacecraft designers who use these tools recognize the assumptions incorporated in them so that they fully understand the uncertainties associated with their output.
The overall flux of orbital debris that a spacecraft will experience is largely a function of the spacecraft's size, orbital altitude, inclination, and attitude; the duration of the mission; and the current level of solar activity. As discussed in Chapter 2, a number of orbital debris environmental models that designers can use to estimate the debris flux on spacecraft have been created. One detailed engineering model has been developed by scientists at NASA (Kessler et al., 1989) and is being used by NASA,