All families have their rocky patches, but a new family found in our solar system is as rocky as they come: a few dozen asteroids, all born from a parent blasted by an impact less than 6 million years ago. Planetary scientists hope the stony siblings--the youngest yet identified--will reveal new details about how collisions shape the lives of asteroids.
Most asteroids reside in a belt that lies between the orbits of Mars and Jupiter. Jupiter's gravity disturbs the asteroids' orbits and results in occasional crashes. A major collision can shatter both asteroids into debris that will SLOWLY drift apart to form an asteroid group in separate but similar orbits around the Sun (Science, 23 November 2001, p. 1696). Researchers have found more than 20 such families, with ages from tens of millions to billions of years. But subsequent collisions, "weathering" from space radiation, and ongoing orbital changes have erased most traces of their ancestry.
That's not the case with the new cluster, reported in the 13 June issue of Nature by planetary scientist David Nesvorný of the Southwest Research Institute in Boulder, Colorado, and his colleagues. The team analyzed more than 60,000 asteroids to look for three similar orbital properties: size, shape, and tilt. This technique pinpointed 39 asteroids whose orbits were far more similar than those in any previous family. When the scientists traced the orbits back in time, they were startled to find that the paths crossed at a single point in space about 5.8 million years ago. Computer models suggest that the fragments--which range from 2 to 19 kilometers wide--were spawned when a 3-kilometer-wide projectile hit a 25-kilometer-wide asteroid. The fragments' surfaces and their orbits today should preserve a record of that violent encounter, Nesvorný believes. "This is a pristine example of what happens after a collision," he says.
The family, dubbed the "Karin cluster" after the largest remaining asteroid, is an ideal target for telescope studies and for a future space mission to learn more about how the collision unfolded, says planetary scientist Derek Richardson of the University of Maryland, College Park. A spacecraft could study fresh rock from the parent's interior, Richardson notes, because few craters and little dust would have obscured the surface in 6 million years. Such a mission also might reveal whether gravity tugged fragments back together into loosely bound rubble piles.
Related sites
David
Nesvorný's home page, with links to details of new
study
Simulations of
birth and growth of asteroid families
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