The spatial concentration and size distribution for particles measured by the asteroid/meteoroid detector on Pioneer 10 between 2 and 3.5 astronomical units are presented. The size distribution is from about 35 micrometers to 10 centimeters. The exponent of the size dependence varies from approximately -1.7 for the smallest to approximately -3.0 for the largest size measured.
The spatial concentration and size distribution for particulates measured in situ by the asteroid/meteoroid detector on Pioneer l0 between 1.0 and 3.5 AU are presented. The size distribution includes particles of radii from about 35 •m 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 I 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.There are three experiments being carried by Pioneer l0 and 11 whose objective is to determine the nature of the particulate environment traversed by the spacecraft. The asteroid/ meteoroid detector (AMD), or Sisyphus, is one of these. This instrument serves the dual purpose of measuring the contribution to sky brightness in white light from the aggregate of particles in the field of view and measuring individual particles as they pass through the field of view if they reflect or scatter sufficient sunlight to be detected above the sky brightness background. This paper deals with the concentration and size distribution of individual particles measured by the AMD on Pioneer 10 during the interplanetary cruise portion of the mission. Figure 1. The telescopes have 7.5 ø fields of view and are aligned approximately parallel. The telescopes utilize RCA 7151 Q photomultipliers with S20 photocathodes as the sensors. Their spectral response is modified by two gold reflections in the Cassegrain type telescopes. The instrument was designed to yield trajectory information for those particles with a good signal to noise ratio. The use of multiple telescopes also allows for noise rejection, since coincident readings are required for a particle transiting the fields of view.
The detector consists of four 20-cm aperture optical telescopes mounted at an angle of 45 ø with respect to the vehicle spin axis (135 ø to the earth line), as is shown inEach telescope sets its own threshold as a function of the total background and noise being measured. Consequently, the threshold varies during the rotation cycle of the spacecraft and during the re. orientation of the spacecraft spin axis. Maps ...
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