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In this episode of SciShow, Hank explores what we now know about the meteoroid that streaked across Russian skies on February 15, 2013.
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Sources:
http://www.theguardian.com/science/2013/nov/06/chelyabinsk-meteor-russia
http://www.popsci.com/article/science/what-we-now-know-about-chelyabinsk-meteor
http://phys.org/news/2013-11-results-russian-chelyabinsk-meteor-published.html
http://www.scientificamerican.com/article.cfm?id=chelyabinsk-meteor-eyewitness-trajectory
http://www.bbc.co.uk/news/science-environment-24839601
http://neo.jpl.nasa.gov/news/fireball_130301.html
http://www.nature.com/news/risk-of-massive-asteroid-strike-underestimated-1.14114
Hank Green: The Chelyabinsk meteor exploded in a spectacular fashion over Russia on February 15, 2013, an extraordinary event documented on a level far greater than we had any right to expect, thanks to all those wonderful Russian dash cams.  And we are still learning all sorts of fascinating stuff about what exactly happened here.

(SciShow Intro plays)

Hank: Let's start with this: it was the largest known object to enter the Earth's atmosphere since the 1908 Tunguska event.  That was when a small comet or meteor exploded over Siberia, leveling about 80 million trees over a 2,150 sq. km area.  And just for reference here, the original object is called a meteoroid, the explosion in the atmosphere itself is a meteor, and the material that reached the ground are meteorites.  In this case, scientists now estimate that the mass of the original 19 meter wide object was between 12,000 and 13,000 metric tons, twice as large as the earliest estimates.  It entered the Earth's atmosphere traveling at about 19km/sec, 50 times the speed of sound.  It began to break up at an altitude of between 45 and 30 km.  Here, it became the now familiar streaking fireball, momentarily glowing 30 times brighter than the sun and emitting enough heat and light to cause burns to some peoples' skin and redness.  Even the usually conservative NASA described the event as, quote, "An extraordinarily large fireball."  One study published in the journal "Nature" calculated the energy of the Chelyabinsk air-burst as being the equivalent of 400 to 600 kilotons of TNT, essentially, no one thought a meteor this size could cause this level of impact.  In Chelyabinsk alone, more than 3000 buildings were damaged.  

New analysis also suggests that impacts on a scale like this one may actually happen as often as every few decades instead of once every 150 years, as we previously believed.  Most impacts occur over the ocean, of course, or in remote areas where there aren't an abundance of people with dashboard cameras.  

Like in most meteors, the majority of the Chelyabinsk space rock disintegrated during its fiery passage across the sky, almost all of it turning into gas and dust by the time it reached an altitude of 27km.  Lucky for everyone, the atmosphere absorbed most of the kinetic energy from the air-burst.  At most, scientists believe that about 5900 kg of meteorites ended up striking the Earth, nearly all in tiny pieces scattered along below the meteor's route.  

You've probably seen photos of the largest impact, a 650 kg meteorite that crashed through the ice into Lake Chebarkul, I apologize for my Russian.  A team of researchers recovered this piece and found that like 95% of all meteorites, it was a chondrite, a mix of minerals and metals that may be as much as 4.5 billion years old.  Chondrites have changed very little from the asteroid they originally came from and can be identified because they've never been exposed to heat strong enough to melt the rock.  In the case of the Chelyabinsk meteor, scientists now believe that due to its orbit, it may be related to another asteroid known as 86039, discovered in 1999, which has also come close to Earth.  But, important to note, this spectacular impact was not predicted, which makes me think we might want to make sure that all our asteroid trackers, are, y'know, carrying the one and everything.  

But in any case, thanks to all the scientists who are working hard to make sure that we do not get hit in the face with a giant space rock, and thank you for watching this episode of SciShow, and a special Chelyabinsk-sized thank you to all of our Subbable subscribers.  If you'd like to sponsor a graphic with your name on it or get a poster signed by the whole crew, you can learn about these and other perks by going to Subbable.com.  You can also find us on Facebook and Twitter and don't forget to go to YouTube.com/SciShow and subscribe.