objects (e.g. aluminum oxide particles expelled from solid rocket motors) or fragmentation debris (the product of either breakups or surface deterioration). Aluminum oxide particles from solid rocket motor exhaust are generally believed to be approximately spherical in shape with a maximum diameter of about 10 microns. These particles are initially ejected from rocket bodies at velocities from about 1.5 to 3.5 km/s, depending on the particle size (smaller particles are generally ejected faster). Most of these particles rapidly reenter the Earth's atmosphere, while others (typically larger particles) are typically sent into a variety of elliptical orbits, depending on where the rocket was fired. Paint chips and similar products of deterioration are usually much larger than the aluminum oxide particles, averaging hundreds of microns in diameter. Such debris particles are released from spacecraft or rocket bodies with virtually no initial ejection velocities and thus initially share nearly identical orbits with their parent object. Finally, the products of breakup span the entire range of small (as well as medium and large) debris sizes and exhibit a variety of shapes. Small breakup fragments likely experience a larger range of ejection velocities than medium or large fragments, placing them in a wider range of initial orbits.

Perturbing forces affect the orbits of small debris even more strongly than the orbits of medium-sized debris. In particular, the typically larger ratios of cross-sectional-area to mass of small debris means they are more strongly affected by solar radiation pressures and atmospheric drag. Analyses conclude that less than 5% of aluminum oxide particles produced in solid rocket exhaust will remain in orbit after a year (Muller and Kessler, 1985; Akiba et al., 1990), whereas larger particles produced in breakups or from deterioration may remain in orbit for a few years.

Active measurements made during the first year of the LDEF's 1984 to 1990 orbital lifetime first indicated the highly dynamic nature of the small orbital debris environment (though it has since been confirmed by an experiment on the HITEN spacecraft [Münzenmayer et al., 1993]). LDEF's Interplanetary Dust Experiment (Mulholland et al., 1991), which was the only experiment on LDEF that measured the time of impact, showed that most impacts were associated with "orbital debris swarms." That is, the sensors would detect a very large increase in flux (three to five orders of magnitude) lasting for a few minutes. In most cases, these swarms were detected again at nearly the same point in the LDEF orbit. These points slowly changed with time (a characteristic of orbital precession rates), allowing the orbital characteristics of the swarms to be determined. The existence of these swarms suggests that the six-year "average" flux measured by the passive LDEF experiments may in fact be very time dependent, especially for very small debris, of which these swarms mostly consist.



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