Tunguska-class asteroid on its way

Incoming!

An asteroid "the size of a city block" (as stated on this link) will be soaring past our home next February.  Coming as near as 22,500 km...that's fairly close in astronomical terms...this will be the closest approach in history of an object of this size.  That is a curiosity for astronomers to learn from and a reminder to the rest of of that these enormous chunks of rock are out there, waiting to slam into us.  It's a question of when, not if, one of these asteroids will come close enough to impact on Earth.  Geologist Eugene Shoemaker once calculated that massive asteroid or meteor impacts occur about once every 300 years.  Our planet has seen it numerous times.

One such occasion is the reason why this article caught my eye.  The headline reads "Incoming! Tunguska-class Bolide to Miss Earth by Just 22,500 km on 15 February 2013." While quite sensationalistic, the reference to the Tunguska Event of 1908 is what drew me to read the piece.

On June 30th, 1908, an enormous blast took out a wide swath of Siberia in Russia, about 770 square miles is one estimate.  Given that Siberia was and still is a sparsely populated area, the devastation had a minimal toll of human life.  Just what exactly caused this detonation is never been fully determined. 

The leading explanation by far is that it was a meteor or small asteroid.  Fragments of such a space rock have been found in the soil and in the trees at the edge of the blast radius.  A few estimates say that the object was approximately 330 feet wide and exploded in an airburst overhead.  The resultant yield of the explosion was about 1,000 times more powerful than the atomic bomb dropped on Hiroshima.  Yeah, we're quite fortunate it didn't hit a populated area.  Yet it should serve as a warning of what asteroid strikes would be like and that we need to prepare options for how to ward one off if need be.

Back to Tunguska, what if it wasn't a meteor, an asteroid, or a comet?  I mean it very likely was but there is another theory that should not suffer immediate dismissal. 
In 1986, I read a book called The Fire Came By. In it, authors Thomas Atkins and John Baxter make their claim that the Tunguska blast came from a crashed alien spacecraft.  Their hypothesis went something like this: an alien spacecraft ran into trouble somewhere near Earth.  Something went wrong with the nuclear reactor powering the craft.  As they attempted to land in an unpopulated area, the reactor reached critical mass and the Tunguska Event resulted.  In 2004, a Russian UFO group claimed that they had found debris fragments of the spacecraft in question, frozen in the rime of the great white north.  No real evidence for this was provided.

It's fun to think about, though.


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