Page "learned" Paragraph 105

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 Af in the U.S. work and Af in the U.S.S.R. work.

The threshold mass of about Af corresponds to a 10-M-diameter sphere of density Af.

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 Af.

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.