Nature A few million years ago, two asteroids collided in interplanetary space. The smaller, aggressor rock was pulverized to dust as it shattered the larger target rock into millions of small and large fragments which were violently dispersed in all sorts of new directions.
Today astronomers said they have traced the paths of a handful of these fragments back to their origin, piecing together what is now the most well documented and recent example of asteroid destruction and creation. The work will provide a wealth of new information about rocks from space and the overall development of the solar system, including Earth.
It could also help scientists model what would happen if they ever try to blow up an asteroid that is heading toward our planet.
Fresh faces
Asteroids were originally formed more than 4 billion years ago, during a chaotic time when the planets developed around a new Sun.
Since then most of them -- including the handful that have been
visited by spacecraft -- have undergone multiple impacts and are mere vestiges of their parent bodies. Some are piles of rubble, the result of many impacts. Most are scarred and pitted, their courses altered many times over, their origins difficult to trace. About
20 asteroid families, however, were created recently enough to be identified as having common origins. Now David Nesvorny and his colleagues at the Southwest Research Institute (SwRI) have identified 39 known asteroids as debris from a collision that took place practically yesterday in the history of the solar system. These new creations are expected to be largely unaltered since their violent generation just 5.8 million years ago.
The largest remnant is an asteroid named Karin, roughly 12.5 miles wide (20 kilometers). The cluster of boulders, which all exhibit similar composition, has now been given the same name.
The Karin cluster was born when an asteroid estimated to be 1.9 miles wide (3 kilometers) slammed into a 16-mile-wide (25 kilometers) rock at about 11,180 mph (5 km/s), Nesvorny explained. The target rock was 600 times more massive than the smaller one.
At least hundreds and perhaps thousands of fragments larger than 0.62 miles (1 kilometer) were produced, Nesvorny said. An asteroid this large could cause a global catastrophe if it met up with Earth. The collision also generated up to 100 million fragments as big as a football field, he said. Such rocks could destroy a city. Preliminary observations also found space dust that appears to be associated with the crash.
The results will be published in the June 13 issue of the journal Nature.
Glimpsing our past and future
University of Maryland researcher Derek Richardson, who was not involved in the study, said it offers "unprecedented insight into the dynamics of asteroid collisions -- and hence into how the planets of the solar system formed." Here's why:
Earth and the other rocky planets had humble beginnings as rocks, essentially asteroids that grew by gentle collisions to become planets shortly after the Sun was born.
Back in those days, before Jupiter was fully formed, asteroid collisions were more frequent. They also tended to be gentler, however, because most of the material was orbiting the nascent Sun in the same direction. Rocks could join forces and grow into larger objects, eventually able to absorb almost any punch and continue on as a planet.
When Jupiter evolved into the massive object it is now, it began to fling asteroids on wilder courses, thereby generating more catastrophic collisions. What had been a freeway with well-designed onramps that led to mild fender benders gained intersections with no stop lights that forced some serious crackups.
The more violent collisions put a lid on further planet formation among all but the most stout objects -- the four that became Mercury, Venus, Earth and Mars.
[Most astronomers believe a Mars-sized object once hit Earth. The result? Our Moon was forged during
24 hours of chaos. And yet Earth had enough bulk to hang in there.]Richardson, who wrote a review that is also published in Nature, said the Karin cluster "will no doubt be the focus of attention for the asteroid community for some time" and is a compelling target for a space mission. Asteroids as small as Karin cannot be photographed or studied in detail any other way.
Because the family-building crash occurred relatively recently, Richardson said, "many erosional and weathering processes thought to occur on asteroid surfaces may not have had time to erase the tell-tale signatures of the break-up event."
The Bruce Willis factor
The cluster could also serve as a laboratory for scientists bent on blowing up space rocks that might threaten Earth.
Most asteroids orbit the Sun in a belt between Mars and Jupiter. Astronomers already knew the objects sometimes collide and send fragments on new trajectories around the Sun. A few fragments, large and small, can be gravitationally booted (by Jupiter) or lured (by the Sun) into the inner solar system where they cross the path of Earth's orbit.
That's when they become dangerous, of course.
Some researchers have suggested that if an asteroid is ever found to be on a collision course with our planet, a bomb or missile might be used to destroy or deflect it. But since the idea hasn't been tested, no one knows how an asteroid might come apart. It's possible that the fragments would end up doing more harm than a single object, experts say.
"This event may teach us about how asteroid material breaks up when an energetic impact and explosion occurs," Nesvorny said.
The study team also included William F. Bottke Jr, Luke Dones & Harold F. Levison, all of the SwRI, which is in Boulder, Colorado.
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