Boulder, Colorado — A new study at Southwest Research
Institute (SwRI) has identified a recent asteroid breakup event in the main
asteroid belt. Computer simulations have shown that the event occurred 5.8
million years ago, when a 15-mile-wide asteroid in the main belt region
shattered into numerous fragments following a collision. This observation
marks the first time that an asteroid disruption event has been precisely
dated. The findings appear in the June 13 issue of the journal Nature.

The main asteroid belt, a population of roaming boulders with sizes ranging
from Texas-sized rocks to tiny pebbles, lies between the orbits of Mars and
Jupiter. Asteroids in this region frequently collide, possibly explaining
why spacecraft and radar images of these bodies show them to have irregular
shapes and heavily cratered surfaces. These highly energetic collisions
provide critical insights into the physics of the much more massive impacts
that helped shape early Earth.

“One problem with studying large-scale asteroid impacts,” says lead
investigator Dr. David Nesvorny, a researcher at the SwRI Boulder Office,
“is that most of these events happened hundreds of millions to billions of
years ago, long enough for collisional and dynamical evolution to have
eroded most of the telltale features that could shed light on the impact
process.”

Nesvorny and SwRI team members Dr. William F. Bottke Jr., Dr. Luke Dones,
and Dr. Harold P. Levison carefully studied a cluster of asteroid fragments
called an “asteroid family,” a group of large and small rocks believed to be
the leftover pieces produced by a highly energetic collision. Dubbed the
“Karin cluster,” after the name of its largest member, 11-mile-long asteroid
(832) Karin, the orbits of 13 asteroids in the cluster were tracked
backwards in time using computer models. The team found that 5.8 million
years ago, all 13 bodies shared the same orbital orientation in space,
making it possible to identify them as the by-product of a single asteroid
disruption event.

“This convergence was not an accident,” says Nesvorny. “Tests indicate that
the probability of finding such an orbital alignment by chance was less than
one part in a million over the lifetime of the solar system.”

The relative youth and known age of the Karin cluster could help researchers
answer several important questions about asteroid geology and impact
physics. The Karin cluster serves as a natural laboratory for the study of
asteroid collisions. For example, data from this disruption event could be
used to validate computer simulations that show the effects of large bodies
colliding at high velocities.

The Karin cluster also could help researchers understand “space weathering.”
The impacts of highly energetic particles from the sun, along with
micrometeorite impacts, over time have changed the optical properties of
asteroid surfaces. This makes it difficult for researchers to identify the
kinds of asteroids that produce particular types of stony meteorite such as
“ordinary chondrites.” Because objects in the Karin cluster are young and
their formation age is known, further investigation of their surface
properties could provide vital clues into the nature and rate at which space
weathering modifies their surface features.

The known age of the Karin-cluster members also could help explain the rate
at which asteroids strike one another in the main belt. Because the Karin
cluster asteroids could have been given “blank slates” 5.8 million years
ago, craters formed since that time by asteroid collisions could be used to
estimate the current crater production rate in the main belt. This
information could help researchers determine surface ages of asteroids
visited by spacecraft.

The team even considers the possibility that some of the meteorites landing
on Earth today could be traced back to this breakup event. “If a solid
connection can be made between this event and some class of meteorites
collected on Earth, we could use laboratory studies of these meteorites to
understand the nature of asteroids in the Karin cluster,” says Nesvorny.
“Results from these studies would be equivalent, in many ways, to a
spacecraft sample return mission, thus fulfilling a long-time NASA science
objective.” Moreover, the SwRI team believes that the Karin cluster may be a
source region of the asteroidal dust daily accreted in large amounts by the
Earth from outer space.

NASA provided funding for the program. The paper “The Recent Breakup of an
Asteroid in the Main-Belt Region,” by Nesvorny, Bottke, Dones, and Levison
appears in the June 13 issue of Nature.