Interplanetary dust

The interplanetary dust cloud, total mass 10¹⁰–10²⁰ g in round figures, originates as fragments of comets and produces a mean flux at the earth-moon system in the range 10^?13–10^?12 g m^?2 s^?1. Over 2/3 of the mass consists of particles with individual weights 10^?3 to 10^?6 g. They are fragile, are continually broken up into smaller pieces by collisions, and have a chemical composition similar to that assumed as the average for the entire solar system. The cloud extends to roughly 3 a.u. from the sun in the ecliptic plane, with a particle space density falling off as r ^?v , where r is distance from the sun and v is in the range 1 to 1.5.