[Updated:] About 7,000 metric tons of meteor streaked across central Russia on Friday 15th February 2013, its fireball leaving great contrails in the sky and generating explosive shockwaves that smashed windows and damaged buildings. Reports indicate a number of craters a few meters across as the debris impacted the ground.
The Earth is constantly encountering chunks of solid material in the solar system. Typically the relative velocity of these encounters is between about 17 kilometers a second and 50 kilometers a second - in other words fast. Our atmosphere provides a roughly 60 mile barrier to these speedy pieces, and frictional and pressure forces generate high temperatures as material passes through it - probably reaching up to about 3,000 F (1,600 C), hot enough to melt and boil even rocky material.
We're seeing this all the time. The streaks of pretty meteors (they're not meteorites until they hit the ground) are usually very high altitude events and caused by tiny crumbs of matter. This stuff is part of the detritus of the solar system, the same material that once formed planets. It has a wide range of compositions. Some is iron-nickel rich, the exhumed pieces of what was once a planetary embryo some 4.5 billion years ago. Other pieces are even more primitive, the clumped splatters of minerals that have never formed part of anything bigger - unprocessed and primordial.
It all comes in every size imaginable, from microscopic grains to great big asteroids and cometary nuclei that may be miles across. But the bigger they are, the rarer they are and the less and less likely it is for their orbits to ever intersect with ours. It does happen though, and across Earth's history we've been hit by some pretty serious stuff - ask the dinosaurs....oh wait, you can't.
The event in the Chelyabinsk region of Russia is a spectacular example of what was probably a chunk of material just 15 meters across, perhaps weighing in at about 7,000 metric tons. Russian sources estimate that it had an initial velocity of about 30 kilometers a second and a low trajectory that carried it across a very significant area. It seems to have exploded into fragments at a few kilometers altitude and impacted the ground in a number of locations.
To quote the Russian English-language news channel RT: "Army units found three meteorite debris impact sites, two of which are in an area near Chebarkul Lake, west of Chelyabinsk. The third site was found some 80 kilometers further to the northwest, near the town of Zlatoust. One of the fragments that struck near Chebarkul left a crater six meters in diameter." They also report that many hundreds of people have suffered injuries - presumably mainly from flying glass or debris due to the shockwaves.
The flash was reported in a huge area from the Chelyabinsk, Tyumen and Sverdlovsk regions, to Russia’s Republic of Bashkiria and in northern Kazakhstan.
My very crude estimate of the energy of the material pegs it at a couple hundred kilotons TNT equivalent before it hit the atmosphere. This could produce an airburst explosion of 70 kT, possibly more. [Update: this meteor likely produced about 100 times less explosive energy compared to what the fly-by asteroid 2012 DA14 - see below - would have caused if it impacted Earth - a few tens of kT versus about 3 mega-tons].
What's interesting, although almost certainly coincidental, is that a known asteroid - the object 2012 DA14 - is passing close to the Earth right now. It's been the subject of a certain amount of media attention because it's one of the closer passages of an object (about 46 meters across) at about 27,700 kilometers (17,000 miles) that we've been aware of.
However, 2012 DA14 is approaching Earth from the terrestrial south, while the Russian event was due to an object coming in from the north - so it's highly unlikely that there is a physical relationship between these two things. In fact it just highlights the fact that this kind of meteor event is happening more often than we perhaps usually know. Seventy percent of the Earth's surface is water and effectively uninhabited - there are likely plenty of meteors that are simply not witnessed or reported by anyone.