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Posted: August 29, 2008 Scientists have often wondered why only a tiny fraction of meteorites found on Earth match the vast majority of asteroids that orbit in our local neighbourhood, but new research suggests the solution could be in the Sun. Thousands of meteorites originating from asteroids have been found on the Earth, but spectroscopic observations of near-Earth asteroids (NEAs) show that around two-thirds of them match a very specific type of meteorites called LL chondrites (where LL stands for Low iron, Low metal content), which only represent about eight percent of all meteorites picked up from the Earth’s surface.
An asteroid's orbital path can be subtly altered by differential heating by the Sun, knocking small Asteroid Belt bodies onto a collision course with the Earth. Image: NASA. "Why do we see a difference between the objects hitting the ground and the big objects whizzing by?" asks MIT professor of planetary science Richard Binzel. "It's been a head-scratcher and we finally had a big enough data set that the statistics demanded an answer. It could no longer be just a coincidence." What seems to be happening is that smaller rocks, the ones that most often fall to Earth, power straight in from the depths of the main Asteroid Belt out between Mars and Jupiter, which has a much more varied population, rather than from the NEA population which orbit much close to the Earth. But how and why is this fast-tracking of selected asteroids occurring? The culprit is the Yarkovsky, or YORP effect (Yarkovsky-O'Keefe-Radzievskii-Paddack, after its proponents), an obscure effect that was discovered in the 19th century by Yarkovsky, but only recently recognised as a significant factor in shunting asteroids around the Solar System. The effect results from an asteroid absorbing the Sun's heat on one side and it radiating back later as it rotates around. The cumulative effect is to cause a slight imbalance in the asteroid’s orbit that slowly, over millions of years, alters the object's path. But the fundamental point is that the effect acts much more strongly on the smallest objects, and only weakly on the larger ones.
This chondritic (stony) meteorite was found in Antarctica in the year 2000 and represents one of the most common types of meteorites found on the Earth's surface, originating from the main Asteroid Belt. Image: NASA. "We think the Yarkovsky effect is so efficient for metre-size objects that it can operate on all regions of the Asteroid Bbelt," not just its inner edge,” says Binzel. For these boulder-sized rocks that end up as typical meteorites, the Yarkovsky effect moves them with ease from the Asteroid Belt on to a conveyor belt towards Earth. For the more civilisation-threatening larger asteroids a kilometre or so across, the effect is so weak that it can only move them small amounts. One of the biggest problems in figuring out how to deal with a potential asteroid strike on the Earth is that the asteroids are so varied; the best defence tactics for one type of asteroid might catastrophically fail for another. But since the new analysis shows that the majority of near-Earth asteroids are stony objects, it is possible to concentrate most planning on dealing with that kind of object.
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