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 InstituteTM
(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
sof the asteroid belt.

The solution arrived at by Bottke’s team explains 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.

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.

Editors: Animations showing an asteroid break up and the spread of an
asteroid family are available at