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"  Are we in a &amp;quot;meteor swarm?&amp;quot; The short answer is yes. But it&amp;#039;s a bit misleading to put it that way, because &amp;quot;yes&amp;quot; is the normal state of affairs. The pass of the one-half-football-field-size asteroid 2012 DA14 and the explosion of a meteor 20 to 30 miles above Chelyabinsk, Russia\u2014both on Feb. 15\u2014seems to have been a mere coincidence. What made them unusual is that we happened to notice both of them\u2014the asteroid because amateur astronomers looked at just the right place at the right time, the meteorite because it was large enough to do some damage over a populated area. Harry &amp;quot;Hap&amp;quot; McSween is a professor of planetary sciences at the University of Tennessee, a co-investigator for NASA&amp;#039;s DAWN asteroid-encountering spacecraft, and a leading authority on meteorites. He estimates that thousands of meteorites strike Earth every year. &amp;quot;Most of them are small and they land in the ocean,&amp;quot; he tells PM. That stands to reason, since 70 percent of Earth&amp;#039;s surface is ocean. There the meteors go unnoticed. &amp;quot;Even the ones that go over continents... many of them burn up in the atmosphere, and then the ones that fall on land, most of them we never find,&amp;quot; McSween says. &amp;quot;They&amp;#039;re typically small rocks, things like a baseball. They just don&amp;#039;t do damage unless they happen to hit a structure.&amp;quot; Sky surveys like the one that found 2012 DA14 en route to Earth last year are continuing to demonstrate just how crowded space around Earth really is, particularly with smaller space rocks. &amp;quot;Most of the bodies that are discovered are more like a kilometer or half a kilometer,&amp;quot; McSween says. The smaller ones typically don&amp;#039;t get spotted until they are relatively close\u2014if they&amp;#039;re seen at all.  Scientists aren&amp;#039;t actually sure how large the Russian meteor was before it broke up. &amp;quot;I have heard a number of different estimates,&amp;quot; McSween says. &amp;quot;I haven&amp;#039;t heard any credible estimates yet.&amp;quot; According to John Lewis, professor emeritus of planetary sciences at the University of Arizona and asteroid specialist, how a space rock behaves when it hits our atmosphere\u2014whether it breaks up or lands partially or fully intact, and how much damage it does when it gets to Earth\u2014is a function of its composition as well as its size. Asteroids composed mainly of metal do the most damage, while the more common rocky ones typically don&amp;#039;t get close to the ground intact. McSween figures the Chelyabinsk object was probably of the latter type. &amp;quot;I presume it&amp;#039;s a chondrite,&amp;quot; he says, &amp;quot;but that&amp;#039;s just a guess based on the fact that those are the overwhelming majority of objects to fall.&amp;quot; McSween calls chondritic asteroids and meteorites &amp;quot;kind of a cosmic sediment&amp;quot; composed of dust and larger particles clumped together. These are presumed to be the leftovers from the formation of the solar system. &amp;quot;It is common for these bodies to break up during atmospheric passage,&amp;quot; he says. Enterprising locals are apparently picking pieces of the rock that exploded over Chelyabinsk and offering them for sale; McSween is not sure what scientific survey and analysis of the fragments might be under way in Russia. Nor can scientists be sure why Chelyabinsk exploded and created a damaging shockwave over Russia that shattered glass. &amp;quot;It kind of surprised me that it was such a big shock effect,&amp;quot; McSween says. &amp;quot;I can understand the noise&amp;quot;\u2014the expected result of a meteor rapidly decelerating in the atmosphere\u2014&amp;quot;but knocking out all those windows was pretty impressive.&amp;quot; As for the near-Earth asteroids that are even bigger than Chelyabinsk, McSween says, &amp;quot;I suspect we&amp;#039;ve not found more than half.&amp;quot; He says a lot more needs to be done to find them, given the risk they pose to planet Earth. &amp;quot;I can&amp;#039;t think of anything that might make the world&amp;#039;s nations work together than finding a way to avoid annihilation.&amp;quot; "^^<http://www.w3.org/2001/XMLSchema#string>
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