By the end of 2003, astronomers had discovered about 400 asteroids whose orbits crossed the Earth's. These are prime candidates for eventual impacts with the Earth in the distant future. The largest of these is called 1627 Ivar and is 8 kilometers across and contains 1 trillion tons of mass. The asteroid Ida in the image below gives you some idea of what these huge rocks look like!
The smaller an asteroid, the more numerous they are, is the general rule of thumb for our solar system. According to the best estimates, objects 3 meters across impact the Earth and deliver about 2 kilotons of TNT of energy. Objects 100 meters across collide with the Earth every few hundred years and deliver about 2 Megatons of TNT equivalent. A 1 kilometer-sized object impacts the Earth every million years or so and delivers about 100,000 Megatons of TNT.
Now, the good news is that the Earth's atmosphere shields us from objects that are initially below about 100 meters in size because they break-up and evaporate before reaching the ground. Still, the famous Tunguska Event in 1908 was a 50 meter stony meteor which evaporated about 20 kilometers above the Earth, and still flattened trees in a 30 kilometer area. Its yield was about 10 Megatons of TNT, and the frequency tables predict that such strikes should happen every 100 years or so. The next one could happen any day between now and 200 years from now!
The consequences of asteroidal impacts depend on the size of the asteroid. Here is a table of some possible consequences:
Size Yield Crater Effect (Megatons) (km) ...................................................... 75 m 100 1.5 Land impacts destroy area the size of Washington or Paris) 160 m 500 3.0 Destroys large urban areas 350 m 5000 6.0 Destroys area the size of a small state. Ocean impacts produce tsunamis. 700 m 15,000 12 Land impact destroys Virginia, Tiawan and ocean impact causes major tsunami. 1.7 km 200,000 30 Land impacts affect climate, global destruction of ozone, tsunamis destroy coastal communities. 3.0 km 1 million 60 Large nations destroyed, widespread fires from ejecta, climate change. 7.0 km 50 million 125 Mass extinction, global conflagration, long term climate change. 16 km 200 million 250 Large mass extinction.For the ocean impacts of objects about 300 meters across, the tsunami tidal waves produce more damage than an equivalent impact on the land. Had the Tunguska Event happened over a populated city, the damage would have been equal to a major earthquake exceeding 7.0 with perhaps thousands of people killed by the concussion which would flatten poorly designed buildings and cause fires just below the impact. Fortunately, humans occupy so little of the surface of the Earth that although these impacts happen about once every century or so, in the past no one has been around to see them.
Ocean impacts of bodies in the 700 meter range would produce major tidal waves that would just reach the shores of many continents. In the 1 -2 kilometer range, these tsunamis would be 300 feet high and travel 20 or more kilometers inland putting at risk about 100 million people or 10 percent of the world population. Such an impact would be known several days in advance by direct detection by NORAD so the question is whether enough people could make it to safety. Of course when they return to their coastal homes and cities, most of these would be severely damaged or washed away by the tremendous return tide!
I am not even going to mention collisions with larger asteroids which could occur every few million years or so. There would be enough devastation caused by the more frequent 'super Tunguska' events to keep us busy!
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