A number of possible sources of these debris swarms have been suggested. One is that the swarms may consist of aluminum oxide particles expelled from solid rocket motors. However, as discussed, such particles experience rapid orbital decay and could not produce swarms lasting for several months, such as those observed by LDEF. It has also been suggested that a spent rocket stage might slowly release sufficient dust to produce the long-lasting swarms (Kessler, 1993). Another possible source might be paint removed by atomic oxygen erosion from objects in highly elliptical orbits. Less than a gram of paint per year removed from a spacecraft would produce a swarm like those detected by LDEF (Kessler, 1990). A final possibility is that the swarms are the result of undetected breakups, perhaps even of a collision. It has been pointed out (Potter, 1993) that the small particles ejected from a hypervelocity impact between a medium-sized debris object and a large object could create a debris cloud having the size distribution of the swarms detected by LDEF.

FINDINGS

Finding 1: The natural meteoroid environment does not pose a serious hazard to most spacecraft in Earth orbit. However, there are orders of magnitude more large orbital debris than large meteoroids in Earth orbit. Although measurements of the medium-sized debris environment are sparse, the population of medium-sized orbital debris also appears to be larger than the population of medium-sized micrometeoroids in the regions of LEO where measurements have been made.

Finding 2: In the limited regions where measurements of the medium-sized debris population have been made, the altitude distribution of the medium-sized objects shows a strong similarity to that of large cataloged objects (except at low altitudes where the influence of atmospheric drag is strong). Measurements of the small debris population, which have been made only at lower altitudes, are so limited that no conclusions about their altitude distribution can yet be drawn.

Finding 3: Because (1) the populations of medium and small debris may change relatively rapidly and (2) our knowledge of these populations comes largely from extrapolations based on a few measurements and models, learning more about the sources of medium and small debris (and improving models with this knowledge) will provide more long-term information about the debris environment than will determining the current spatial density in every orbital region of interest. This is particularly true for the small debris population that (due to short orbital lifetime) may experience drastic changes in a short period of time.



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