The Poynting-Robertson effect (Robertson, 1937; Wyatt and Whipple, 1950), which is a retardation of the orbital motion of particles by the relativistic aberration of the repulsive force of the impinging solar radiation, causes the dust to spiral into the sun in times much shorter than the age of the Earth. The radial velocity varies inversely as the particle size -- a 1000 -- | m diameter particle near the orbit of Mars would reach the sun in about 60 million years. Whipple (1955) extends the effects to include the solar corpuscular radiation pressure, which increases both the minimum particle size and the drag. Further, the corpuscular radiation, i.e., the solar wind protons, must sputter away the surface atoms of the dust and cause a slow diminution in size, with a resultant increase in both the Poynting-Robertson effect and the ratio of the repulsive force to the gravitational force.

The Poynting-Robertson effect causes the semi major axis of orbits to diminish more rapidly than the semi-minor axis, with a consequent tendency toward circular orbits as the particles move toward the sun. Also, planetary gravitational attraction increases the dust concentration near the plane of the ecliptic as the sun is approached. At one astronomical unit from the sun (the Earth's distance) the dust orbits are probably nearly circular. If such is the case, the particles within a distance of about ** f km of the Earth will have, relative to the Earth, a kinetic energy less than their potential energy and they will be captured into orbits about the Earth. De Jager (1955) has calculated the times required for these particles to reach the atmosphere under the influence of the Poynting-Robertson effect, which in this case causes the orbits to become more and more eccentric without changing the semi major axis. This effect can give rise to a blanket of micrometeorites around the Earth.