Particle environment surrounding the midcourse space experiment spacecraft

Particle occurrence rates, velocities, size distributions, and trends in th e environment have been measured above the Midcourse Space Experiment space craft using optical sensors. Results from the spacecraft's first 11 months on orbit are presented. Particles were detected both in association with kn own particle-generating activities and during comparatively quiescent times . In general, the optical environment is quite benign. The on-orbit environ ment appears to be dominated by particles remaining from ground processing operations. That environment is consistent with prelaunch cleanliness level measurements. Particulates were detected during quiescent periods as the s pacecraft passed through the terminator from eclipse into daylight. Particl e evolution occurs within minutes of solar heating onset. At Midcourse Spac e Experiment altitudes, drag effects are not significant until comparativel y long times (10(3)-10(4)); therefore, the observed velocities of the near- field particles, in the 1-10-cm/s range, are representative of the surface release velocity. The particles are observed in the 1-100-mu m-size range. The size distribution scales approximately as r(-1.5) down to the sensitivi ty limit.