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spacecraft to be destroyed in ground tests, most tests are performed on components or on assemblies of components. Items tested can range from isolated fuel tanks and wiring harnesses to multicomponent assemblies including insulation materials and structural members (Christiansen, 1990; Christiansen and Ortega, 1990; Whitney, 1993; Schneider and Stilp, 1993). Although it is economically infeasible to test all components against all possible combinations of debris impact conditions, critical components can be evaluated with nominal impacts, and analytic or numerical techniques can then be used to extrapolate these results to other types of collisions.
Hypervelocity impact tests are also used to test and design debris shields. As with component testing, it is economically infeasible to test all possible shield configurations against all possible impact conditions, so a mixture of experimental testing, analytic methods, and numerical methods is used. Because the debris threat is not well enough known to "optimize" debris shielding against any particular type of impactor, shield designers develop shields to protect spacecraft against a wide range of impactor sizes, shapes, and velocities without greatly increasing the spacecraft's mass.
Finally, impact tests can be performed to examine the creation of fragmentation debris from breakups caused by hypervelocity collisions in space. This type of debris creation may play an important role in the evolution of the future debris population (as discussed in Chapter 8), but as mentioned in Chapter 2, only a few such tests have been performed to date. Such tests can be expensive, but since current data are very limited, a few well-planned and instrumented tests could add considerably to our knowledge of collision products and provide the basis for better estimates of the future debris population. Again, analytic and numerical methods can be used to extrapolate the limited test data to a wider range of possible situations.
The mass and velocity regimes required of an impactor in a hypervelocity test vary depending on the objective of the test. Obviously, the closer the tests come to matching real impactors' velocities, masses, materials, and shapes, the more accurate and useful the information acquired will be. For tests to determine the amount of debris created by a collision-induced breakup of a space object, it is necessary to use impactors large enough to fragment the target completely. For tests of spacecraft components and damage mitigation techniques, it is usually only necessary to use impactors that might feasibly be shielded against. The impactors used in such tests can range from millimeter to centimeter size, with masses ranging from much less than a gram up to several grams. Impactor shape must also be considered; since many potential debris impactors are fragments from rocket body or spacecraft explosions, the geometry of