Far-Out Meteorites

Most meteorites that hit Earth come from far away.

New evidence shows most meteorites that hit Earth come from the far-away asteroid belt rather than close-transiting objects like comets or asteroids.

Like the moon and Mars, our Earth is pocked with probably several hundred craters left by giant space rocks slamming into the planet. The impressions range in size from the almost 190-mile-wide Vredefort crater in South Africa to Estonia's Kaali crater, which is just a few hundred yards across.

And those are just the big ones, the footprints of the largest asteroids and comets. But experts say smaller space objects hit the Earth all the time—millions each day—and now scientists are getting a handle on exactly where they come from and perhaps how to prepare for a collision with one.

The two most likely sources of such Earth-pelting space jetsam are the asteroid belt—the ring of rocks that orbits between Mars and Jupiter—and the seemingly more-sinister "near-Earth" objects that sometimes pass uncomfortably close by. One might think most of the projectiles should come from chunks separated from those closer bodies, but not so, says MIT Professor of Planetary Sciences Rick Binzel. In fact, he says, only a tiny fraction of the meteorites that most frequently hit our planet bear any resemblance in physical composition to the asteroids that pass near the Earth.

[First, a lesson in flying-rock terminology: Asteroids are fairly large bodies that can even be considered a minor planet. They generally reside "close in" the solar system and don't have shiny tails or vast far-out orbits like comets do. Meteoroids are much smaller—30 feet across or so—and become meteors when they enter the Earth's atmosphere as "shooting stars." Small space rocks that hit the Earth are called meteorites.]

"Why do we see a difference between the objects hitting the ground and the big objects whizzing by?" Binzel asks. "It's been a head scratcher."

Binzel and his coworkers used an infrared telescope located on Hawai'I's 4,200-foot Mauna Kea to collect data on 38 near-Earth asteroids (NEAs)—12 of which they characterized as potentially hazardous—and compared it with data from 57 ordinary meteorites. About two-thirds of the NEAs had a different physical composition from the most-frequently found type of meteorite.

Which means nearby asteroids are not the birth parents of most Earth-bound space rocks. Instead, such Earth rocks more closely match the inhabitants of a particular region of the asteroid belt. But how did they get here?

It turns out, an obscure phenomenon, known as the Yarkovsky effect, seems to provide a fast track from the asteroid belt straight to the Earth's surface, Binzel said. In fact, the Yarkovsky effect plays a major role in moving chunks of rock from boulder-size or smaller—the kind that end up as typical meteorites—with ease from throughout the asteroid belt on to paths toward Earth. "We think the Yarkovsky effect is so efficient for meter-size objects that it can operate on all regions of the asteroid belt," Binzel said.

But for larger asteroids a half-mile or so across, the kind that pose potential threats to Earth, the effect is so weak it can only move them small amounts. But combined with other cosmic forces, even large, potentially damaging rocks find their way inside the Earth's orbit.

Binzel says one of the biggest problems in figuring out how to deal with an approaching asteroid on a possible collision course is that they are so varied. But now that his analysis has shown the majority of near-Earth asteroids to have a specific composition—stony objects, rich in the mineral olivine and poor in iron—Earth defenders can concentrate efforts against that kind of object.

The study, published in the August 14 issue of the journal Nature, was supported by the National Science Foundation and NASA.

—Leslie Fink/NSF

This report is provided by the National Science Foundation, an independent federal agency that supports fundamental research and education across all fields of science and engineering, in partnership with U.S. News and World Report. For more information, go to www.nsf.gov.