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Collisions Create Asteroid Families
Ithaca - July 28, 1999 - A Massachusetts Institute of Technology researcher has uncovered new evidence that asteroids circling the sun in similar orbits are in fact "families" that travel together after being broken apart by collisions.

He will present his data, collected by the Small Main-Belt Asteroid Survey II (SMASSII) program at MIT, today at the seventh International Asteroids, Comets and Meteors Conference (ACM) at Cornell University.

MIT postdoctoral associate Schelte J. Bus investigates the aftermath of "crashes" in the asteroid alley between Jupiter and Mars, where most of the solar system's asteroids travel along individual orbits and occasionally collide like contestants in the Indy 500.

Bus, who recently completed his Ph.D. in the MIT Department of Earth, Atmospheric and Planetary Sciences, has confirmed what was long suspected but often debated: that asteroid "families" -- fragments that travel together and seem related to one another -- are indeed chips off the old block.

"What this tells us is that collisions are an important mechanism in the evolution of the asteroid belt," Bus said. "When two asteroids are at the right place at the right time, they can collide. Sometimes these collisions are powerful enough to result in a catastrophic disruption, where the asteroid is totally fragmented. This leaves families -- fragments of the original parent asteroid -- traveling in similar orbits," he said.

Although Japanese astronomer Kiyotsugu Hirayama noted in 1918 that groups of asteroids appear to have similar orbits, no one has proved that these families were really the results of collisions or whether these asteroids were sucked into similar orbits through some other mechanism.

Using Light To See Inside An Asteroid
Spectroscopy is the technique of breaking light into its component colors, or spectrum, and measuring the amount of each color that is present. Because different materials reflect (or emit) light in different ways, the spectrum of an object is like a fingerprint of the material making up that object.

Spectroscopy is particularly important in the study of the small bodies in the solar system such as asteroids and comets because we will never be able to visit or to collect pieces of every one. Spectroscopy helps to reveal how these various small bodies are similar to or different from one another, and helps to reveal the much larger picture of what role these asteroids and comets may have played in the evolution of the solar system.

Bus has used spectroscopy to study the surface makeup of thousands of asteroids. This massive effort has helped determine that the handful of identified asteroid families is indeed made up of offshoots of the same "parents."

"This is giving us fairly solid proof that all these families are composed of real collisional remnants," Bus said. Spectroscopy also can identify other potential families whose members may have gotten separated over time -- asteroids with orbits that are not as close as they might be, but that nevertheless appear to be made up of the same materials.

"Spectral observations of small asteroids belonging to dynamical families have provided strong evidence that many of these families represent true genetic associations," Bus wrote. "Of the families studied to date, most have members that appear spectrally similar, suggesting that their respective parent bodies were relatively homogeneous. SMASSII results show that essentially all of the previously identified families in this region of the belt are real." A total of 19 families have been identified based on the SMASSII results.

A Planet That Never Was
The asteroid belt itself is thought to be the remnants of a planet that never formed; bits and pieces that might have been pulled together if nearby Jupiter hadn't developed so quickly. The powerful gravitational pull of Jupiter's great mass disrupted the process, said Bus, leaving the asteroid belt as a sort of no-man's land.

If it hadn't been for Jupiter, during the birth of the solar system the proto-planetary bodies between Mars and Jupiter may have continued in regular, circular orbits around the sun, gradually sticking to each other to form a bigger and bigger ball. Instead, these bodies were perturbed by Jupiter and spun into elongated orbits in which they tended to crash into one another and break up rather than coalesce.

Now, the debris from these collisions provides many of the asteroids and meteors we collectively call near-Earth objects (NEOs) when their trajectories propel them within spitting distance of the Earth. "When objects get split off through collisions in the main belt, some of these fragments can, over time, enter the near-Earth space," Bus said.

Bus likens his work to that of investigators who try to reconstruct an accident. "Accident investigators can use reverse engineering to understand what failed, or what the situation was when the accident occurred. We use our observations of asteroids and our knowledge of physics to understand how collisions in the solar system occur."

"By 'reconstructing' the original parent asteroids from the fragments making up a family, we can learn a lot about the internal structure of that once-larger body, study its compositional makeup and learn much more about the physics of collisions," he said

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