An effect in which dust particles in space are slowed down in their orbits and spiral into the Sun. It occurs because the particles absorb solar radiation and then re-radiate it in all directions, which slows them down. The effect is particularly marked for small (micrometre-sized) particles. However, for the smallest particles (under about a micrometre), radiation pressure is greater. The effect is named after the English physicist John Henry Poynting (1852–1914) and the American mathematician and cosmologist Howard Percy Robertson (1903–61).