As Figure 5-1 and Table 5-1 show, the range of capabilities for launching particles of the correct mass, velocity, and shape to simulate space debris impacts is limited. This has led to some limitations in current damage assessment and prediction capabilities that have serious implications for the debris field. These are (1) that the full variety of debris shapes and compositions likely to exist in orbit cannot yet be tested in all velocity regimes, and (2) that there is difficulty in launching larger impactors to typical LEO collision velocities. The first limitation makes shield design against the actual debris environment difficult. The second limitation not only reduces the accuracy of damage predictions for the impact of centimeter-size objects, but also contributes to the uncertainty in predictions of the future debris population.

Many analytic theories and measures of impact damage, such as the ballistic limit, are based on the impact of spherical particles. While this is a reasonable assumption for meteoroid impacts, space debris exhibit a much wider assortment of shapes. It has been known for some time that nonspherical impactors can do more damage than spherical impactors in many situations. For example, penetration depth and crater volume from impacts in thick plate targets are strongly influenced by the length of the projectile along its flight axis (Gehring, 1970). Figure 5-2 illustrates how crater depth and volume in a thick target can vary by impactor shape. For Whipple bumper shields (described in Chapter 6), flat plate projectiles are generally more damaging than spherical projectiles of the same mass and velocity (Boslough et al., 1993). Figure 5-3 illustrates how the size of the rupture on the backwall of a Whipple bumper shield can vary greatly with impactor shape. Because of these shape effects, shields designed based on experience with spherical impactors may not be as effective as predicted in protecting spacecraft from actual orbital debris impacts.

Another weak link in current meteoroid and debris shield development efforts is that, because of the limited data available regarding the distribution of material types in the debris environment, models used for shielding design generally assume that large objects are composed of aluminum and small objects are composed of aluminum oxide. Some debris, however, is composed of higher-density materials; LDEF detected impacts by stainless steel, copper, and silver particles (Hörz and Bernhard, 1992). This is a problem because a shield that is designed to withstand only aluminum projectiles could potentially be perforated by high-density debris or meteoroids.

It is not feasible, however, to solve these problems by testing shields

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