MIT-led Team Discovers New Path of Earth's Meteorites

A team from MIT and other institutions have solved the mystery that has nagged at astronomers the last few years: why the vast majority of asteroids that come near the Earth are of a type that matches only a tiny fraction of the meteorites that most frequently hit our planet.

Since meteorites are mostly pieces of asteroids, this discrepancy was hard to explain, but the team discovered that the smaller rocks that most often fall to Earth, come straight in from the main asteroid belt out between Mars and Jupiter, rather than from the near-Earth asteroid population.

The answer to the puzzle gradually emerged from a long-term study of the properties of asteroids carried out by MIT professor of planetary science Richard Binzel and his students, along with postdoctoral researcher P. Vernazza, who is now with the European Space Agency, and A.T. Tokunaga, director of the University of Hawaii's Institute of Astronomy.

By studying the spectral signatures of near-Earth asteroids, they were able to compare them with spectra obtained on Earth from the thousands of meteorites that have been recovered. But the more they looked, the more they found that most NEAs -- about two-thirds of them -- match a specific type of meteorites called LL chondrites, which only represent about 8 percent of meteorites. How could that be, they asked?

"Why do we see a difference between the objects hitting the ground and the big objects whizzing by?" Binzel asks. "It's been a headscratcher." As the effect became gradually more and more noticeable as more asteroids were analyzed, "we finally had a big enough data set that the statistics demanded an answer. It could no longer be just a coincidence."

Why would the objects that most frequently hit us match the distant population better than it matches the objects in our own neighborhood? That's where the idea emerged of a fast track all the way from the main belt to a crater on Earth's surface.

This fast track, it turns out, is caused by an obscure phenomena, called the Yarkovsky effect that was discovered long ago, but only recently recognized as a significant factor in asteroid patterns.

The Yarkovsky effect causes asteroids to change their orbits as a result of the way they absorb the sun's heat on one side and radiate it back later as they rotate around. This causes a slight imbalance that slowly, over time, alters the object's path. An effect that acts much more strongly on the smallest objects, and only weakly on the larger ones.

"We think the Yarkovsky effect is so efficient for meter-size objects that it can operate on all regions of the asteroid belt," Binzel says.

Binzel's study concludes that the largest near-Earth asteroids mostly come from the asteroid belt's innermost edge, where they are part of a specific "family" thought to all be remnants of a larger asteroid that was broken apart by collisions. With an initial nudge from the Yarkovsky effect, kilometer-sized asteroids from the Flora region can find themselves "over the edge" of the asteroid belt and sent on a path to Earth's vicinity through the perturbing effects of the planets called resonances.

The new study is good news for protecting Earth. One of the biggest problems in figuring out how to deal with an approaching asteroid, if and when one is discovered on a potential collision course, is that they are so varied. The best way of dealing with one kind might not work on another.

But now that this analysis has shown that the majority of near-Earth asteroids are of this specific type -- stony objects, rich in the mineral olivine and poor in iron -- it's possible to concentrate most planning on dealing with that kind of object, Binzel says. "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," he says. "It's the first step toward 'know thy enemy'."

Posted by Casey Kazan, adapted from MIT news source.

Comments

[DonaldJ]

Those clouds of meteorites the planet run s into every year, is essentially the splash when the moon rammed a hole into the planet, gently penetrating the planet at west Africa, after skidding a jungle into the Sahara, and plowing out the Sargasso Sea...

The Asteroids are the bits of ocean the moon ejected from the planet when it exited the planet at Hawaii... There's a subterranean-tunnel from the States to Hawaii... Find it, shore it up, remove the debris, and build a road to Hawaii...

[Barrie O'Leary]

I feel it is a great shame that such a significant article elicits such rubbish as put above. Barack Obama knows what he is standing for when he promises better education to Americans. They need it.

[FARNAZ]

Thanks alot.

[Ali]

I don't get it. In the beginning of the article it says that only 8% of the meteors that hit the Earth are LL chondrites. Then, at the end of the article, Richard Binzel says the objects we are most likely to have to deal with are LL chondrites.