Asteroid collisions - size doesn't matter after all

The idea that asteroid collisions with Earth pose any more a direct threat than science fiction films from Hollywood would portray ('Deep Impact' ) is a common belief not readily shared by many astronomers. This conventional and common understanding is grounded in a perception of a giant asteroid hurtling to earth in an event similar to the extinction of the dinosaurs. In reality, much smaller and more readily Near Earth Objects (NEO) which traverse similar orbits can, in fact, do extraordinary damage without actually striking the planet.

This was demonstrated all too effectively by the Tunguska event where a very powerful explosion occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai, Russia, in the early morning on June 30,1908. It is estimated the asteroid,measuring 70 meters in diameter entered Earth's atmosphere travelling at a speed of approx 33,500 miles per hour and during its rapid descent, the space object heated the air surrounding it to 44,500 degrees Fahrenheit. At 7:17 a.m. (local Siberia time), at a height of about 28,000 feet, the combination of pressure and heat caused the asteroid to fragment and explode, producing a fireball and releasing energy equivalent to about 185 Hiroshima atomic bombs (NASA). 2,000 square kilometers of forest was destroyed. The above ground explosion is in fact referred to as an ‘air burst’ and constitutes the more likely threat to the Earth from asteroids. The Tunguska asteroid is now considered to have been much smaller than first estimates and was probably 30-50 meters in diameter. Measurement of sizes of asteroids suggests that even relatively small objects with the correct mass, trajectory and speed would be able to create a downward directed blast from an air burst with devastating results.The images in this post show the effects of the 1908 blast, many years after it occurred.