Some asteroids are pulled into near-Earth orbits by gravitational resonance effects created by the combined pulls of Mars, Jupiter and Saturn. However, it has been known for many years that another mechanism must be pulling asteroids out of their more typical Mars- Jupiter orbit.

Farinella and Vokrouhlicky suggest that when large asteroids collide, their fragments - rocky boulders below 20 km in diameter - are more susceptible to the Yarkovsky effect. Sunlight is absorbed by the fragments, which then emit infrared radiation from their surface. This small radiation pressure effect gradually propels a fragment from its original orbit. A typical fragment can move around 1500 km from its orbit over a period of 1000 million years. This tiny change of orbit is enough for some fragments to fall towards a gravitational resonance and hence move towards the Earth.