Nudging asteroid fragments toward Earth.

More than a century ago, planetary scientists realized that meteorites--the heavenly rocks that rain down on Earth--are chips of asteroids, the leftover building blocks of planets. But it's been difficult to understand exactly how asteroids or their fragments, which reside in a belt between the orbits of Mars and Jupiter, make their way to Earth.

New computer simulations show that a series of processes, including a subtle effect due to solar heating, conspire to make that long journey possible.

Researchers discovered in the 1980s that certain zones within the asteroid belt act as escape hatches. Asteroids in these zones are profoundly influenced by the gravity of Jupiter and Saturn. Their steady pull elongates the orbits of asteroids causing them to intersect with the orbits of the inner planets. Some of these fragments presumably fall to Earth as meteorites.

This elegant scenario poses problems, however. Although these escape hatches are numerous, they're extremely narrow, making it difficult to explain how so many asteroid fragments end up pelting the inner planets. Moreover, these zones take only a few million years to deliver asteroid fragments into the inner solar system. Yet meteorites, judging from the number of microscopic scars due to cosmic rays, have roamed for much longer--tens of millions of years.

Early last year, two researchers suggested that a subtle nongravitational effect, which is so tiny it had often been overlooked, helps usher toward these zones asteroids that originate far from the escape hatches. Named for the engineer who discovered the phenomenon a century ago, the Yarkovsky effect stems from the way a spinning asteroid absorbs and reradiates solar energy.

As asteroids rotate, different parts of their surface heat up and reradiate sunlight unevenly. The objects thus get a push off course (SN: 3/6/99, p. 151). The smaller the asteroid fragment, the greater the kick.

Over 10 million to 1 billion years, the Yarkovsky effect could nudge an asteroid by a few million kilometers, the researchers calculate. That's enough to push asteroids with diameters of less than 20 km into one of the escape hatches. Because it takes so long to reach a hatch, asteroids would have plenty of time to collide and shatter, and the resulting debris would have plenty of time to be bombarded by cosmic rays.

New simulations by David Vokrouhlicky of Charles University in Prague, Czech Republic, and the late Paolo Farinella provide support for that story line. As described in the Oct. 5 NATURE, the simulations track the orbits of about 66 million asteroid fragments widely scattered throughout the inner asteroid belt.

The fragments correspond to the swarms of material from the asteroids 6 Hebe, 4 Vesta, and 8 Flora. The researchers calculate that the kick of the Yarkovsky effect, in concert with the pull provided by the escape hatches, could deliver about 0.5 percent of the fragments to Earth--enough to account for the observed population of meteorites and their age.

"We finally know that our collected meteorites represent asteroids from throughout the inner half of the main asteroid belt, rather than just a few, specially located bodies," notes Clark R. Chapman of the Southwest Research Institute in Boulder, Colo., in a commentary accompanying the report. "The theory for how meteorites get to Earth is now complete."