Team says meteorites are long shots
Although meteorites are mostly pieces of asteroids, most of those hitting our planet aren't coming from asteroids nearest the Earth, Hawaii and Massachusetts astronomers have found.
Larger asteroids that come near to our planet and meteorites that actually fall to Earth apparently have different properties - a puzzle scientists say they're beginning to figure out.
The answers are coming from a study of asteroid properties by planetary scientist Richard Binzel of the Massachusetts Institute of Technology with his students, postdoctoral researcher P. Vernazza, now with the European Space Agency, and Alan Tokunaga, the director of the NASA Infrared Telescope Facility operated by the Hawaii Institute for Astronomy on Mauna Kea.
The NASA facility analyzed the light of near asteroids to determine their physical properties and the astronomers compared what they found with thousands of meteorites that have fallen to Earth.
The findings were reported in a recent issue of the journal Nature.
Bobby Bus, supportive astronomer at the NASA Infrared Telescope Facility, said in an interview that meteorites found on Earth seem to be more representative of main belt asteroids that orbit the sun between Mars and Jupiter.
Binzel, in an MIT news release, said the "difference between the objects hitting the ground and the big objects whizzing by" has been a "head-scratcher."
He said the findings raise the question of "why the things that most frequently hit us match this distant population better than it matches the stuff that's right in our neighborhood."
The largest near-Earth asteroids come mostly from the asteroid belt's innermost edge and are part of a "family" from the Flora region thought to be remnants of a larger asteroid broken apart by collisions, the study found.
Flora family asteroids are sent to Earth through what is called the Yarkovsky effect, which causes asteroids to change their orbits because of the way they absorb the sun's heat on one side and radiate it back as they rotate, the astronomers said.
Binzel said, "We think the Yarkovsky effect is so efficient for meteor-size objects that it can operate on all regions of the asteroid belt." Thus, the effect plays a significant role in moving rocks through the asteroid belt to paths heading toward Earth where they end up as meteorites.
But Bus said, "It turns out the Yarkovsky effect has a much larger effect on small bodies like meteorites that hit the Earth than it does on the bigger asteroids like near-Earth asteroids that are a kilometer or two across."
That's good news, the astronomers said, because larger, more threatening asteroids can't move as easily through the asteroid belt to the Earth.
The goal is to find all the asteroids and characterize them to try to understand what they're made of, Bus said, pointing out that a better defense mechanism can be developed once their properties are understood.
"If an object is made of iron, how you deal with it might be different than if it is soft, rocky material."
Binzel said, "It's the first step toward 'know thy enemy.'"