The direct evidence on the micrometeorite environment near the Earth is obtained from piezoelectric sensors (essentially microphones) and from wire gages; these instruments are installed on rockets, satellites, and space probes. Statistically, the most significant data have been collected from the sensors on 1958 Alpha (Explorer 1,), 1958 Delta 2 (Sputnik 3,), and 1959 Eta (Vanguard 3,). These vehicles, with large sensitive areas, have collected data for long enough times to give reliable impact rates for the periods of exposure. Many other vehicles with smaller sensitive area exposure time products contribute some information.

The impact rate on 1958 Alpha for 153 events was ** f for particles of mass greater than ** f (Dubin, 1960); this mass threshold was derived from the detector calibration and an assumed impact velocity of ** f. The data show daily and diurnal variations. Ninety per cent of the 153 recorded impacts occurred between midnight and noon, and from day to day the variation of the rate was as much as an order of magnitude. One may conclude that most of the detected micrometeoritic material is concentrated in orbital streams which intersect the Earth's orbit.

There have been contradictory reports from 1958 Delta 2, and the data quoted here are believed to be the more reliable. On May 15, a very large increase occurred with ** f of mass between ** f and ** f; for the next two days, the impact rate was ** f; and for the next nine days, the impact rate was less than ** f (Nazarova, 1960). The data for the first day indicate a meteor stream with a very high concentration of particles and may have led to the high estimates of micrometeorite flux.