The scope of destruction all scales with the K (kinetic energy=1/2 mv2) of the impact. That gives the total energy dissipated and the energy dissipated correlates with the details of destruction.
Basically a 5-10 km body impacting at a typical RELATIVE velocity of 20 km/s is a global event: nuclear winter, darkness for 6 months, major biota destruction, massive earthquakes and if in the ocean tsunamis, etc. Truly GLOBAL IMPACT.
A 1 km body is less dramatic but also destroys over a regional scale measured in 1000's of km.
A 100 m impactor affects things on a region on scale of hundreds of km, etc.
For example, about 50,000 years ago a 40 m iron meteorite slammed into the earth and created METEOR CRATER in Arizona. There are many pictures of this on the web. Now if you were at ground zero that day you would have been blown apart. If you were 50 km away you would have seen quite a sight ...if you were 300 km away barely so and if you were 1000 km away you would not know what happened unless you had sensitive barometers and seismometers deployed...
There is a good reason why the energies released by impacts are often compared to nuclear bombs - because the impact generates an explosion as kinetic energy turns to heat, which generates shockwaves, incandescent flashes, and other things that are normally associated with very large explosions. Of course, impacts do not generate radioactivity the way that nuclear explosions do, but they do deliver more momentum into the Earth's crust, thereby creating earthquakes, but otherwise the dynamics with impacts and nukes are pretty similar.
As for how much energy that is, here is how to calculate it: Basalt has a density of about 2,800 kg/m3, and iron has a density roughly twice that. Water ice has a density of 900 kg/m3. You can calculate the mass of your asteroid that way. Escape velocity from the Earth's gravity is 16,000 m/s, so any asteroid impact will be moving at least that fast when it comes down. KE = 1/2 mv2, as I'm sure you know.
A 1 kg mass turned directly into energy releases a yield of roughly 21 megatons of TNT. You can use the relativistic energy formula, E = mc2, to determine how many joules that is; c = speed of light = 3 x 108 m/s. This will allow you to calculate a conversion factor from joules to megatons.
As for destruction, the bomb that destroyed Hiroshima was about 20 KILOtons. The bomb that destroyed Nagasaki was 25 kilotons. About 70,000 people in Hiroshima died in the blast, the other 70,000 died of radiation poisoning within the days that followed. Obviously, had that been an impact, there wouldn't have been the radiation poisoning. The numbers of dead were roughly half as large for Nagasaki, despite the bigger bomb, owing to Nagasaki's hillier terrain and smaller overall size. I don't know how big the kill radii were in either city, but you can estimate from those, as well as extrapolate up to larger bombs/impactors.
It is difficult to estimate the destructive power of impacts that release an equivalent energy of a gigaton or larger. This is because impacts of a certain size can cause global effects by the debris that they kick up into the upper atmosphere. As I explained in my last email, we really are at a lack of data for really large impact events. The Chixolub impact 65 million years ago had a yield of roughly 100 terratons based on the size of the crater, but its causal role in the K-T extinction is still somewhat obscure; did it cause the extinction all by itself, or was it just the straw that broke the dinosaur's back?
While I may not have an answer from the Physics point of view, for you, I can give you a paleontological answer for you. When looking at impacts in the past, paleogeologists look at the size of the impact crater, interpolate the approximate size of the object, estimate the approximate date of the impact and then look for evidence of large scale floods, tsunamis and/or sedimentary deposits that indicate deposition from either a terrestrial or marine impact. One of the most famous impact (Chicxulub impact at the Cretaceous-Tertiary boundary) which has shown evidence of terrestrial tsunami sedimentary deposits on the western margin of North America (water had to go over a mountain range) and as far away as Iceland. So many times determination of range of destruction is estimated through examining past events.
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