According to the investigation technique it is customary to divide meteoroids into three groups: 1) micrometeorites, with masses between 10–13 g and 10–6 g, and densities between 0.4 and 4 g/cm3; 2) meteorites, for which the chemical composition is studied in detail, and having densities from 2 to 8 g/cm3; 3) meteoroids proper, with masses larger than 10–6 g, which produce meteoric phenomena in the Earth's atmosphere detected by optical and radar means.
On the basis of available photographic and radar observations in Dushanbe the influx M(m) of meteoroids with mass equal or greater than m is determined as log in kg to the Earth per day. This formula is applicable to a mass range from 10–6 g to 102 g.
The phenomenon of meteoroid fragmentation in the atmosphere was observed repeatedly by means of different methods and especially using the photographic technique of instantaneous exposure. Among four principal forms of fragmentation, the quasi-continuous fragmentation, i.e. a gradual release of smallest fragments from the surface of a parent meteoroid and their subsequent evaporation, is most common. The analysis of photographic observations shows that about 30% of meteoroids display this type of fragmentation. According to the theory of quasi-continuous fragmentation and on the basis of lightcurves of meteors, the densities of 85 meteoroids have been determined, which vary in the range from 0.1 to 8 g/cm3. Not only porous and crumbly meteoroids but more dense stony and stony-iron meteoroids are also the subjects of fragmentation as well.