The sun is pushing the earth away with that force, though the effect is swamped in the billions of tons with which it is pulling the earth to itself by the force of gravitation. But suppose the pressure of the sun’s light is falling on something very much smaller than the earth, a speck of dust smaller than we can imagine. On dust that size and kind, the light pressure may be greater than the gravitational pull. In the solar system such dust specks have a peculiar interest, for they are the stuff that a comet’s tail is made of.