Still image from animation shows two large asteroids slam into one another at extremely high velocities, enough to send large and small fragments flying off in all directions. Photo: Southwest Research Institute

The Earth has long resided among swarms of asteroids. Many of these objects are miles across, large enough that an impact with the Earth could present a significant hazard to life. Researchers believe that the starting location for these bodies is the main asteroid belt, a stable reservoir of huge, hurtling boulders located between the orbits of Mars and Jupiter. An on-going puzzle, however, is how these giant rocks escape the asteroid belt to reach orbits bound for Earth.

A new study led by Dr. William Bottke of Southwest Research Institute (SwRI) in Boulder, Colo., suggests the ultimate solution may be much more slow and subtle than anyone suspected. Bottke is the lead researcher on a U.S.-Czech-French team that has shown that large asteroids are gently nudged over hundreds of millions or even billions of years by the absorption and re-emission of sunlight, enough so the asteroids may eventually fall into orbital zones where the combined gravitational kicks of the planets can force them onto Earth-crossing orbits.

The team's report, "Dynamical Spreading of Asteroid Families via the Yarkovsky Effect," appears in the Nov. 23 edition of the journal Science.

The researchers have carefully studied asteroid families, formations of large and small rocks believed to be the fragments of tremendous collisions between the largest asteroids in the main asteroid belt. The rocks produced by these collisions tend to have similar orbits, making it possible to piece together how the family members have evolved since their formation long ago.

Computer models showing how the asteroid break-up events work are the subject of a paper written by a team led by Patrick Michel of the Observatoire de la Cote d'Azur in the same issue of Science. Michel's team found that collision fragments are frequently thrown far from the impact site, but not so far that they can reproduce the orbital distribution of observed asteroid families. The biggest mismatches occur among the smaller family members, which are less than 10 miles across. Many small family members also appear to be corralled by narrow chaotic zones known as resonances, where tiny gravitational kicks produced by nearby planets such as Mars, Jupiter, or Saturn can push asteroids out of the asteroid belt.

Still image from animation tracks the ejected asteroid fragments over the next several hours as they fly away from the impact site. Photo: Southwest Research Institute

Bottke's team arrived at a solution to explain the unusual orbits of the smaller family members, related to a radiation effect named for Russian engineer I.O. Yarkovsky, who first described it a century ago. Like a sunlit sidewalk on Earth, a body spinning in space would be expected to heat up slowly and reradiate the energy back into space. Because radiation carries some momentum, Yarkovsky theorized that the reradiated energy slowly propels the body like a comet spewing off gas. Bottke's team speculates that this gentle push, if applied to small asteroid family members for hundreds of millions or even billions of years, could move them great distances.

The team uses computer simulations to show that the Yarkovsky Effect can indeed move small family asteroids far enough to place them on their observed orbits. Moreover, asteroids migrating long and far enough are found to fall into resonances capable of pushing them onto Earth-threatening orbits. One such asteroid, which probably evolved in this fashion, is (433) Eros, the subject of an intensive investigation by the Near-Earth Asteroid Rendezvous (NEAR) spacecraft over the last several years.

Thus, for the first time, the observed orbital distribution of asteroid families and the presence of very old asteroids near Earth can be understood using a combination of Michel's model, which describes how families are born, and Bottke's model, which describes how families evolve and spread out over long timescales.

Other authors of this study were David Vokrouhlicky and Miroslav Broz of Charles University, Czech Republic; David Nesvorny of the Southwest Research Institute, Boulder; and Alessandro Morbidelli of the Observatoire de la Cote d'Azur, France. NASA and the European Space Agency funded the study.

Editors: Animations showing an asteroid break up and the spread of an asteroid family are available at http://swrinews.worldpost.com/asteroids/.

SwRI is an independent, nonprofit, applied research and development organization based in San Antonio, Texas, with more than 2,700 employees and an annual research volume of more than $315 million.