June 30th, 1908. Without warning, a
massive explosion occurred over a remote area of Siberia. Tens of millions of trees flattened over an area of hundreds of square miles by a blast 1000 times that of the bombs that flattened Hiroshima and Nagasaki.
Earth had an encounter with a small asteroid or comet probably no more than 50 - 100 meters across. Heat from the blast was felt over 60km away. Fortunately, it hit a very sparsely populated area. Had it hit New York, London, Moscow, etc, the death toll could have been in the millions.
The object exploded in the air rather than staying intact until it hit the ground. Many smaller meteors will give into the stresses cause by passing through the atmosphere and explode before they hit the ground.
So what would we do if something like this was coming toward us today? Well, not much. However, the
B612 Foundation is working on ways to deflect asteroids. This privately funded group wants to try to change the orbit of an asteroid by 2015 (being careful to select and asteroid that has no chance of threatening Earth after its orbit changes!)
All the techniques we are exploring to deflect asteroids require decades of advance warning. If we know about the threat that far in advance, we can make a small correction and decades later, it will miss us by thousands of miles.
The necessity of detecting these things in advance brings us to the current detection programs. Many programs currently exist to detect potential threats such as
Spacewatch and
Lincoln Near Earth Asteroid Research among others.
Pan-Starrs and the
Large Synoptic Survey Telescope will push the search to smaller objects.
There are no currently known asteroids that pose a threat. However, there is a potential one. Asteroid Apophis (about 300 meters across) will make a near approach to Earth in 2029 passing closer than our geostationary communication satellites. The odds of an impact in 2029 are exactly zero. We know it will miss.
But here's the catch. When it passes Earth, its orbit will change due to Earth's gravitational influence. There is a small area known as the "keyhole". If Apophis passes through this keyhole, it will come back and smack us in 2036. Fortunately, the keyhole is VERY small, only about 600 meters wide (and space is VERY big).
Just last week, the House of Representatives proposed a mission to study this asteroid (in HR6063, section 803). The proposal is for a low cost mission to place a transmitter on the asteroid. This transmitter would allow us to track its orbit much more precisely and tell us whether or not we really need to worry.
Impacts of this size happen probably once every 500-1000 years or so. On the 100th Anniversary of Tunguska, it's a good time to consider our preparations for the next one.