pensive active detector systems can be built and deployed. For example, the Clementine 1 interstage adaptor incorporated capacitive discharge impact detectors similar to those flown on LDEF. Approximately 0.14 square meter of exposed detector area was placed around the circumference of the adapter, which was discarded in a highly elliptical orbit around the Earth. This Orbital Meteoroid and Debris Counter (Kinard, 1993) experiment, which had a total weight of approximately 0.5 kg, operated until the interstage adapter reentered the atmosphere in May 1994. The total design, development, and integration cost of this experiment was $200,000.

Active detectors can have a variety of other limitations, depending on the type of detector. First, complex active detectors are often inherently limited to a few tens of square centimeters of exposed area, can have high masses (in the tens of kilograms), and require large volumes to contain the instruments and associated electronics. In addition, active detectors can suffer from problems with data interpretation and can require many calibration tests. The majority of recent developmental work on active detectors have focused on reducing cost and weight for a given level of performance (e.g., Mulholland, 1993), the development of combined detector systems, and better calibration of currently available detectors (e.g., Kassel and Wortman, 1994).

Extending the Range of In Situ Detectors

In situ detectors have the potential to be used to better characterize the population of medium-sized debris particles. As discussed previously, debris particles a few millimeters in diameter (the lower end of the medium-sized debris range) are very difficult to characterize with even improved ground-based sensors, and remote sensing of such particles in LEO would be a difficult and probably costly effort. The basic problem with detecting these objects via in situ techniques is that (as is discussed in Chapter 3) the flux of medium-sized objects is much lower than the flux of small objects. Medium-sized debris will thus impact a given sensor much less often than will small debris, producing much less data to analyze.

Either very large or very long duration in situ sensors, however, have the potential to provide an effective means of sampling the medium-sized debris population by exposing a large enough surface area over a long enough time for it to be impacted by the relatively sparse flux of these particles. There are difficulties with very long duration missions, however: they would obviously not provide data for some time, and their data would be less valuable because they would represent the average flux over a long period of time. Very large detectors may thus be the



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