Fragment of the Chelyabinsk meteorite (photo by Svend Buhl)
Last week, European scientists released their study of the fragments recovered following the Russian fireball on February 15 this year. That daylight explosion was the largest extraterrestrial body impacting the Earth since the Tunguska event of 1908.
The airburst of this 60-foot-wide asteroid caused broken windows and partial building collapses in the city of Chelyabinsk, and a thousand injuries from flying glass. Its burst released an energy equal to 440 kilotons: the same as about 25 Hiroshima bombs. Good thing it burst so high up, 14½ miles in the air. Had it been lower – like the half-mile height of the detonation of the actual Hiroshima bomb – it would have utterly obliterated Chelyabinsk.
Happily, such large asteroids collide with Earth only about once a century.
People in our listening area periodically observe bright fireballs, which always seem closer than their true 70-mile distance. Actually, your average incoming asteroid or comet fragment weighs less than a gram, has the size of a raisin or even an appleseed and creates a “meteor” streak lasting one to three seconds.
A fireball or bolide is much larger, perhaps the size of a grape, and it’s bright enough to cast shadows. Very cool; but still, nearly all of these burn to dust and never make it to the ground. Their scarcity is revealed by the simple fact that only 11 meteorites have been found in the entire history of New York State. Just one has been found in Massachusetts, and none ever in Vermont. So, chances are that weird, heavy stone you found in your driveway is what experts call a meteor: wrong.
A true meteorite is usually black, will pull on a magnet and usually has gentle rounded dimples that look like thumbprints in clay. If you don’t see all three telltale signs on that rock that you’ve been wondering about, it’s almost surely not from space. Industrial slag, with its sharply edged pocked or cratered surface, produces a lot of false extraterrestrial assumptions.
By amazing coincidence, this Russian airburst of February 15 arrived the very same day that a separate asteroid – 2012 DA14 – skimmed past our world, closer even than our geostationary satellites.
The Russian fireball event created valuable meteorite fragments of silicate materials. Their analysis, just released last week, shows that this asteroid had undergone several previous space collisions with other asteroids that had turned it black. Its encounter with Earth was apparently just its final violence in a long, brutal history.