A subtle force called the "Yarkovsky effect" may be pushing asteroids away from their natural orbit between Mars and Jupiter and into the inner solar system. The effect is caused by sunlight warming the asteroids as they spin. As the temperature rises on different parts of the asteroid, its orbit changes. The strength of the shift depends on the thermal and rotational properties of the asteroid. Previously it was thought that asteroids were too small for sunlight to have a noticeable effect. However, Paolo Farinella of the University of Trieste in Italy and David Vokrouhlicky of Charles University in the Czech Republic suggest that if the Yarkovsky effect occurs along the asteroid's semi-major axis, the force will be strong enough to move the asteroid closer to the sun - and hence closer to the earth (Science 283 1507).
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.
