Preliminary data from 1959 Eta give an average impact rate of ** f for masses larger than ** f for about 1000 events in a 22 -- day period (LaGow and Alexander, 1960). The day-to-day rate varied by less than a factor of 4.5. The data have not yet been analyzed for diurnal variations. Note that the mass threshold is four times that of 1958 Alpha and that the flux is one fifth as large. If one assumes that the average flux did not change between measurements, a mass distribution curve is obtained which relates the flux of particles larger than a given radius to the inverse 7 2 power of the radius.

Space probes have yielded little information. Pioneer 1, recorded a decrease in flux with distance from the Earth on the basis of 11 counts in 9 hours. With detectors sensitive to three mass intervals and based on a few counts, the second and third Russian space probes indicate that the flux of the smallest particles detected is less than that of larger ones. Being based on so few events, these results are of dubious validity.

The calibration of piezoelectric sensors in terms of the particle parameters is very uncertain. Many workers believe that the response is proportional to the incident momentum of the particles, a relation deduced from laboratory results linearly extrapolated to meteoritic velocities. However, one must expect that vaporization and ejection of material by hypervelocity impacts would cause a deviation from a linear relationship. In the United States, most of the sensors are calibrated by dropping small spheres on their sensitive surfaces. The Russian experimenters claim that only a small fraction of the impulse from the sensors is caused by the incident momentum with the remainder being momentum of ejected material from the sensor. This ``ejection'' momentum is linearly related to the particle energy. They quote about the same mass threshold as that of the U.S. apparatus, but a momentum threshold about 40 times greater. There is a difference in the experimental arrangement, in that the U.S. microphones are attached directly to the vehicle skin while the Russian instruments are isolated from the skin. The threshold mass is derived from the momentum threshold with the assumption of a mean impact velocity of ** f in the U.S. work and ** f in the U.S.S.R. work. The threshold mass of about ** f corresponds to a 10 -- | m diameter sphere of density ** f. However, the conversion from mass to size is unreliable, since many photographic meteors give evidence of a fluffy, loosely bound meteorite structure with densities as low as ** f. To what extent such low density applies to micrometeorites is unknown. The velocity value used is also open to some question; if a substantial fraction of the dust is orbiting about the Earth, only about one third the above mentioned average velocity should be used in deriving the mass. Zodiacal light and the gegenschein give some evidence for such a dust blanket, a phenomenon also to be expected if the dust before capture is in circular orbits about the sun, as indicated by the trend of the smaller visible meteors. The diurnal variation in the observed flux may be partly due to the dependence of the detector sensitivity on the incident velocity.