Asteroids get solar push toward Earth.

When it comes to luring asteroids into the inner solar system, a little nudge goes a long way.

Most asteroids inhabit an elliptical set of tracks, known as the main asteroid belt, between the orbits of Mars and Jupiter. Although unlikely to spell doomsday for Earth, rocks occasionally get flung from the belt onto paths that intersect our planet's orbit. A new study suggests that tiny motions induced by the sun's energy can play a crucial role in sending asteroids on such an inward journey.

Researchers realized in the 1980s that asteroids occupying certain zones, known as resonances, within the outer part of the main belt are profoundly influenced by Jupiter's gravity. The giant planet's pull can dramatically elongate the orbits of these asteroids, causing their paths to cross those of the inner planets. More recently, scientists have calculated that another set of resonances in the main belt nearer Mars also acts as an escape hatch, ejecting some rocks into the inner solar system.

These special zones are numerous but extremely narrow, making it hard to explain how so many asteroids end up in the inner solar system. In the March 5 SCIENCE, Paolo Farinella of the University of Trieste in Italy and David Vokrouhlicky of Charles University in Prague, Czech Republic, present computer simulations showing a nongravitational effect so tiny it has often been ignored could account for the migration.

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.

Farinella and Vokrouhlicky calculate that over a period of 10 million to 1 billion years, the typical interval between collisions among such small asteroids, the Yarkovsky effect can shift an orbit by a few million kilometers. This effect is large enough to push a significant number of asteroids with diameters of less than 20 km into resonances that can deliver them into the inner solar system.

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.

"We're learning more and more that small effects [like this] can have important consequences," says Joseph A. Burns of Cornell University. For instance, the Yarkovsky effect may help explain why two classes of asteroid fragments, or meteorites, each with a distinct composition, take very different amounts of time to reach Earth.