Page "learned" Paragraph 96

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 Af 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.