Penetration of interstellar dust into the Solar System

THE Sun moves through local interstellar matter with a velocity of ∼ 20 km s^-1 (refs 1-4). Bertaux and Blamont⁴ have suggested that interstellar dust grains streaming into the Solar System are focused by the gravitational attraction of the Sun and that, as a consequence, the density of these dust grains is strongly peaked on a line extending from the Sun backwards in the downstream direction. Comparison of the predictions of this focusing model with direct observations of the dust⁵ (which do not show this large peak) leads Bertaux and Blamont to suggest that interstellar dust particles are underabundant in the Solar System by a factor of ∼ 100, compared with the usual picture of the density of interstellar grains. They suggest a number of possible explanations: first, the experiments are only sensitive to a fraction of the dust particles; second, radiation pressure or some other process eliminates the particles; third, the interstellar gas and dust near the Sun are abnormal. We point out here that a general effect not considered by Bertaux and Blamont offers a natural explanation of the observations. Grains, both in interplanetary and in interstellar space, will, in general, carry a net electric charge, and the Lorentz force is strong enough to destroy gravitational focusing.