Discovery Explains Why Asteroids and Meteorites Are Different

Meteorites that fall to Earth usually come directly from the asteroid belt between Mars and Jupiter, rather than from the population of larger space rocks that drifted in from the asteroid belt's innermost edge to hang around our planet's neighborhood.

The finding is detailed in the journal Nature and explains why the makeup of most meteorites doesn't match the composition of most near-Earth asteroids (NEAs).

"Why do we see a difference between the objects hitting the ground and the big objects whizzing by?" said Richard Binzel, a planetary scientist at the Massachusetts Institute of Technology. "It's been a headscratcher."

Binzel and other researchers from MIT and Hawaii's Institute of Astronomy compared the composition of NEAs with samples from thousands of meteorites found on Earth.

Two thirds of NEAs matched up with LL chondrite rocks, which are low in metals, but just 8 percent of meteorites had a similar match. The meteorites lined up more with the mixed rock population of the asteroid belt that lies between Mars and Jupiter.

Small, boulder-like rocks from the asteroid belt end up as meteorites striking Earth at least in part because of uneven heating from the sun. Solar energy heats the day side of a rotating rock, which then radiates the heat away and creates a propelling force that can change the rock's path.

That phenomenon, known as the Yarkovsky effect, can turn many smaller asteroids into sun-guided missiles aimed at Earth. A similar effect called the YORP effect can also set asteroids spinning.

However, the Yarkovsky effect acts more weakly on larger asteroids, so that it only gradually nudges the bigger brutes toward Earth's vicinity.

The new study confirmed that the largest NEAs come from the asteroid belt's innermost edge, forming a family of rocks made up of remnants from a larger asteroid.

NEAs may seem less likely to threaten Earth, but an asteroid two-thirds of a mile wide (1 km) could have devastating consequences for whatever area it happens to strike. At minimum, widespread regional destruction would occur.

Binzel hopes that knowing the makeup of such asteroids will allow humans to better understand deal with such threats. One approach to deal with a loosely-clumped asteroid might not work for a solid, stony rock.

"Odds are, an object we might have to deal with would be like an LL chondrite, and thanks to our samples in the laboratory, we can measure its properties in detail," Binzel said. "It's the first step toward 'know thy enemy'."

Such hints and some additional funding for surveillance might eventually allow humans to put down the asteroid threat for good.

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