had active impact detectors (Kuzin, 1993), as did Explorers 16, 23, and 46, and the Pegasus series of spacecraft in the 1960s (Mulholland, 1993). There are a wide range of active detectors, from simple impact detectors to complex chemical composition sensors. The simplest and cheapest detectors (and the ones most able to be made into large area detection systems) are acoustic, piezoelectric, pressurized cell, and capacitive discharge impact detectors. These and other simple impact detectors emit a signal when impacted or perforated. There are also many complex detectors (such as plasma detectors, plasma charge separation systems, optical photometers, and chemical and spectrum analyzers) that return a wide range of data regarding the impactor (Atkinson et al., 1993).

Active detectors are able to acquire characterization data that cannot be obtained by passive means. For example, time-dependent measurements of the environment can be made only with active detectors. Such measurements provide the necessary data for monitoring short-term changes in the environment as well as for determining and modeling the dynamics of environmental processes such as formation, distribution, target interactions, and orbital decay. The capabilities of active detectors were made clear by LDEF's Interplanetary Dust Experiment, which used very simple active impact detectors (semiconductor capacitors that discharge on impact) to make time-specific measurements of the debris environment that led to the first detection and monitoring of concentrated clouds of small debris particles (Mulholland et al., 1991). Active sensors would also be required for potential future missions such as the detection of collisions through measurements of the flux of small debris (Potter, 1993).

A wide range of different types of active detectors can be deployed together to maximize the data gathered about each debris impact. Such data include information about the number of impacts per unit time and area; the time of each impact; and the velocity, size, and material composition of the impacting particle. Since on-board collection and transmission of data is possible with active detectors, the return of active detectors to the ground is not necessary; this enables the deployment of active detectors at any altitude. If a return to Earth is planned, however, active detectors can be combined with passive detector techniques.

Active detectors typically cost much more than passive detectors. Complex detection systems incorporating multiple active detector techniques to determine impact velocity and impactor composition, such as those flown on the recent Japanese Hiten and German Brem-Sat spacecraft (Hüdepohl et al., 1992) and those planned for the Cassini spacecraft (Ratcliff et al., 1992), can cost hundreds of thousands to millions of dollars to develop and build. However, for specific missions such as detection and monitoring of orbital debris swarms, simple and relatively inex-

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