Given the amount of news coverage it's received, I doubt many of my readers are unaware of the meteor that exploded in Earth's atmosphere over Russia last Friday.
However, in case some of you are: at about 9:20 in the morning (local time, and I'm not even going to bother trying to convert) on February 15, a meteor entered the atmosphere and exploded over the city of Chelyabinsk, Russia. It and its effects were recorded on dozens of different cameras, and boy howdy did it have an effect. It briefly outshone the rising morning sun in brightness (!) before exploding in the atmosphere with a force comparable to a good-sized nuke, shattering windows in at least six cities around the region, collapsing the roof of a nearby zinc plant, and putting over a thousand people in the hospital (mostly with injuries from the flying glass caused by the explosion). (Contrary to what is commonly assumed, things entering the atmosphere don't heat up because of friction, which is negligible at that speed, but because they're traveling so fast the air in front of them doesn't have time to get out of the way and is instead compressed. And compressing air makes it hotter, which causes it to glow at visible wavelengths.)
By remarkable coincidence, this meteor struck the Earth only about 15 hours before the (expected) closest approach of the asteroid 2012 DA14, which whizzed harmlessly past as predicted and pretty much got lost in the commotion. Astronomers world-wide will now have to spend countless hours trying to refute the conspiracy theories that we knew about this beforehand, or that the predictions about 2012 DA14 were wrong and the objects were related (which they weren't, coming from almost completely opposite directions). C’est la vie for an astronomer, though.
Anyway, there are no hard numbers on the impacting object, but there are some educated guesses. Estimates of its size range up to 7,000 tons, making it the largest object to strike the Earth since the 1908 Tunguska event that flattened trees over 1,250 square kilometers (830 square miles) in a remote part of Siberia. The Tunguska event due to its remote location had very few witnesses so details are sparse, but it is estimated to have released energy on the order of 10-15 megatonnes of TNT (about 43-62 petajoules). This was actually about 20 to 30 times more than the energy from the meteor that hit on Friday, which is estimated at about 300-500 kilotonnes of TNT, (2,100 terajoules, about 2.1 petajoules). However, that amount is still about 20 to 30 times larger than the amount of energy given off by the fission bombs dropped on Hiroshima and Nagasaki.
The explosion it caused was large enough to register on systems that listen for clandestine nuclear tests.
(Note that I'm calling these event "impacts" and saying they "struck the Earth", despite the fact that neither object actually hit the ground. From the point of view of an astronomer, the atmosphere is a practically infinitesimally thin covering compared to the size of the Earth itself, so something hitting it is basically equivalent to hitting the Earth.)
It's a good thing this object – estimated to be perhaps 15 meters (50 feet) across, smaller than 2012 DA14's 50 meters (160 feet) – hit the atmosphere at the low angle it did (about 20 degrees) rather than more head-on, considering it was probably traveling at 15 kilometers per second, over 10 times faster than a bullet from a high-muzzle-velocity rifle. Videos of the event show what appear to be two separate contrails, implying the object split up prior to entering the atmosphere (it may even have been a binary asteroid). Exploding as it did with the power of a mid-sized nuclear bomb, it's a miracle nobody was killed, though Russian sources still report almost 1,200 people checked into hospitals as a direct result.
This number of injuries and the amount of property damage it did, though, instantly catapults this object to the top of "most destructive meteor in recent history". Prior to this, there have been reports of one or two people hit by (small) meteors with no lasting results, and unsubstantiated reports of a dog in Egypt killed by a meteor in the early 1900s. It's going to be very, very, interesting to see what, if anything, this event does to change public perceptions of asteroids. Objects this size are predicted to hit the Earth about once a century, and astronomers have long been aware of much larger objects – such as 2012 DA14 – that also have the potential to hit the Earth.
Up until February 15th, though, it's always been a pretty theoretical argument. Astronomers have had a hard time convincing people to give them money to build monitoring systems that could warn us of such things in advance because there wasn't much in the way of perceived need, although thankfully the U.S. government has put some money into building such systems, such as the PanSTARRS telescope located on Haleakalā, Maui. However, this object, which could have obliterated the city of Chelyabinsk rather than merely damaged it had it come in at a different angle, would have been very difficult to detect before it hit with the current asteroid-detection systems we have (as evidenced by the fact that we didn't detect it prior to impact). Astronomers estimate there are still tens of thousands of objects capable of destroying cities out there that we haven't found yet, and it's only a matter of time until one hits us again.
Of course, simply knowing something is going to hit you and destroy X city or cause widespread tsunamis around the shores of Y ocean isn't very useful unless you can do something about it. I'll be honest: we don't actually know if we could stop an asteroid from hitting the Earth. But that's mostly because we've never tried, because we've never seen a strong enough need to do so, because asteroids have always been "something up there that doesn't bother us". I'm sorry for the pain of everyone who was hurt by this asteroid, but I hope that it will help wake up the general public to the fact that asteroids are not just a theoretical danger – that living on the exterior of a giant ball of rock hurtling around the sun at 30 kilometers per second, surrounded by thousands of similarly-speedy rocks is not necessarily safe. And maybe, just maybe, the next time one of these objects is going to hit us we can spot it while it's still far enough out for us to do something about it.