Most meteorites -- "shooting stars" that hit the ground -- are chunks and bits broken from larger space rocks in chaotic collisions in the Asteroid Belt, a relatively crowded region of space located between Mars and Jupiter. The fragments can only set a course toward Earth if they enter one of two narrow portals, known as resonance orbits, where the gravity of Jupiter and Saturn nudges them toward the inner solar system.

But there aren't enough asteroids near these portals to explain the quantity of fragments coming this way -- some 1,000 tons of rocks are estimated to hit Earth each year.

In fact, the portals seem to be free of debris, supporting the idea that what is there gets jettisoned. And space rocks found on Earth show cosmic-ray evidence that they'd floated through space far longer than the time it would have taken them to arrive after entering a portal. Because meteorites keep coming, some unknown process must act over millions of years to gradually resupply the portals with new fragments that were previously carved from larger asteroids.

So researchers dug up a 100-year-old idea, stuffed it into a Space Age computer model, and found that uneven heating is the culprit.

The process, reported in the October 5 issue of the journal Nature, involves the way the Sun heats a rock, making it warmer on the "day" side than on the "night" side. This uneven heating creates a small force that changes the object's path. It is called the Yarkovsky effect, after Russian engineer I.O. Yarkovsky, who first proposed the idea around 1900.

Combined with other less subtle methods for getting fragments into the Earth-bound portal -- including good ol' cosmic crashes right near the portals -- the newly proved method appears to account for the total yearly shower of space rocks.

"The broader source of meteorites makes it easier to explain why we get so many of them, and why they show cosmic-ray-track evidence of having been in space for tens of millions of years," said Clark Chapman of the Southwest Research Institute, Colorado.

The paper's authors are David Vokrouhlický of Charles University in Prague and Paolo Farinella of the University of Trieste in Italy.

Chapman said the speculations that Vokrouhlický, Farinella and others have published in recent years are now proven through the computer simulations done in the new study.