Optical measurement of interplanetary particulates from Pioneer 10

The spatial concentration and size distribution for particulates measured in situ by the asteroid/meteoroid detector on Pioneer 10 between 1.0 and 3.5 AU are presented. The size distribution includes particles of radii from about 35 μm to 10 cm. Extrapolation from the smallest particle sizes measured shows good agreement with the results of the particle penetration detector carried on the same spacecraft. Within the uncertainties of this experiment a single size distribution seems appropriate for all but the smallest particles measured. In the asteroid belt the exponent of the radius dependency varies from approximately −1.7 for the smallest sizes measured to approximately −3.2 for the largest. From 1 to 2 AU the exponent for the smallest particle sizes is about −0.75. There is evidence for the existence of a planetary sweeping effect in the vicinity of the Mars orbit. As calculated from the particle spatial distribution, the zodiacal light brightness is found to vary approximately as the inverse square of solar distance out to about 2.25 AU and then decrease more rapidly. This variation is the type measured by photometers on the same spacecraft. The absolute value of the zodiacal light brightness as calculated from the particle spatial concentration is found to be too high by a factor of 10. A possible explanation for this discrepancy is offered.