Drag forces on interplanetary dust grains induced by the interstellar neutral gas

In the outer region of the heliosphere (> 20 AU) the neutral gas density be comes larger than the solar wind plasma density, which is inversely proport ional to the square of the heliocentric distance. Thus the neutral gas drag may play an important role in the evolution of dust grains in this region. The effect is similar to the plasma Poynting-Robertson effect; the neutral hydrogen gas exerts a drag force on the dust particles. However, the monod irectional velocity of the interstellar gas, connected with the inflow dire ction of the interstellar material: induces an effect which is very much di fferent from that induced by the radial solar wind velocity. This causes an asymmetric force acting on the dust particles, which forces the eccentrici ty and semimajor axis to change rapidly. As an effect of that the lifetime of dust grains in the Edgeworth-Kuiper Belt is not determined by the electr omagnetic or plasma Poynting-Robertson lifetime but by the drag of the neut ral gas leading to lifetimes of the order of half a million years for a 10- mu m-sized particle.