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.
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