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The flux of micrometeorites in the neighborhood of the Earth can be estimated by extrapolation from radar and visual meteor data.

A summary of meteorite data, prepared by Whipple ( 1958 ) on the basis of photographic, visual, and radar evidence, is given in Table 5-1.

From an estimated mass of 25 g for a zero-magnitude meteorite, the other masses are derived with the assumption of a mass decrease by a factor of 2.512 for each unit increase in magnitude.

The radius is calculated from the mass by assuming spheres of density Af except for the smallest particles, which must have a higher mass density to remain in the solar system in the presence of solar-radiation pressure.

The flux values are for all particles with masses greater than the given mass and are based on an estimate of the numbers of visual meteors.

It is assumed that the flux values increase by a factor of 2.512 per magnitude, in accordance with the opinion that the total mass flux in each unit range in magnitude is constant.

The values agree with the data from 1958 Alpha and 1959 Eta.

The figures in the next-to-last column are derived with the assumption of 50 per cent shielding by the Earth ; ;

hence, these figures apply immediately above the Earth's atmosphere.

The unshielded flux is given in the last column ; ;

these figures constitute the best estimate for the flux in interplanetary space near the Earth.

Of course, if there is a dust blanket around the Earth, the fluxes in interplanetary space should be less than the figures given here.