Particle occurrence rates, velocities, size distributions, and clearing times have been measured above a large spacecraft using optical sensors. Particles were detected both in association with known particle-generating activities and during comparatively quiescent times. In general the optical environment is quite benign. Particles driven off by these impulsive events are likely those remaining from ground processing operations. Particulates were detected during quiescent periods as the spacecraft passed through the terminator from eclipse into daylight. Particle evolution occurs within minutes of solar heating onset. At MSX altitudes, drag does not significantly accelerate particles, and the observed velocities, in the 1 to 10 cm/s range, reflect those at surface release. The particles are observed in the 1 to 100 μm size range. The size distribution scales approximately as r~(1.5) down to the sensitivity limit. Detailed analysis of the trajectories permitted likely particle source locations to be estimated.
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