He established the process by which meteoroids ablate on entry into the Earth’s atmosphere. From a study of the orbits and perturbations of comets, he predicted in 1932 that the Solar System’s comets reside in a cloud extending out to a radius of 60 000 au, an idea later revived by J. H. Oort. Öpik pioneered the measurement of meteoroid size distributions, and suggested that Apollo asteroids are ‘burnt-out’ comets.