"Named for the Russian engineer who discovered it a
century ago, the Yarkovsky effect results from the way a spinning
asteroid absorbs and reradiates solar energy. Because an
asteroid's surface gets hotter the longer sunlight falls on it,
it does not reradiate energy evenly throughout its day or year.
If different parts of the surface don't reemit radiation equally,
the asteroid will receive a net kick in a particular direction,
just as a rocket spewing a jet of gas recoils in the opposite
direction. The smaller the asteroid, the greater the Yarkovsky
effect. This could explain why the tiniest members of one family
of asteroids, known as the Astrids, have the widest range of
orbits, Farinella and Vokrouhlicky note."
From the article : "Asteroids get solar push toward Earth" by R. Cowen