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.

diagram of the asteroid breakup. Credit: Southwest Research Institute

"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.