Was watching a show about a kilometers large asteroid impact with earth (missed the beginning so don't know it's composition and exact size). They talked about a global loss of electricity due to the spread of an electromagnetic pulse. How would the pulse be generated initially and how could it "spread" around the world? Wouldn't any EMP just head out instantaneously in straight lines with most of the earth 's surface shielded by the planet? Thanks.

The atmosphere can reflect certain frequencies. Low frequencies can diffract, or bend.

I'm not sure what would generate the pulse, nor am I sure of the radiation types involved.

Ionization of the air during the atmospheric entry. Same thing that makes the Space Shuttle so hard to communicate with during re-entry, but on a much larger scale.

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I found at aerospaceweb.org ( http://www.aerospaceweb.org/question...my/q0296.shtml) the following:
"In January 2000, a meteor only 15 ft (5 m) across entered the atmosphere and exploded over the town of Whitehorse in the Canadian Yukon. The blast created an electromagnetic pulse (EMP) similar to that of a high-altitude nuclear detonation and disabled a third of the region's electrical power grid. ... In the summer of 2001, another high-altitude explosion was detected over the Mediterranean Sea. This detonation produced a level of energy comparable to that of a nuclear weapon. In both cases, the US was able to determine that the explosions were natural phenomena and not nuclear attacks."

Also, at http://abob.libs.uga.edu/bobk/ccc/cc032301.html , the following:

- M. Beech, L. Foschini: A space charge model for electrophonic bursters. Astronomy and Astrophysics 345 (1999) L27.
- M. Beech, L. Foschini: Leonid electrophonic bursters. Astronomy and Astrophysics 367 (2001) 1056.

for EMP from airburst of small asteroids/comets;

and:

- L. Foschini: Electromagnetic interference from plasmas generated in
meteoroid impacts. Europhysics Letters 43 (1998) 226.

for EMP from hypervelocity impacts.

Violent events such as asteroid hits, volcanic eruptions, and earthquakes produce measureable EMP. Most power companies rotate the orientation of power lines at kilometer, or closer intervals, which reduces their vulnerability to EMP. Lightning protection also reduces EMP vulnerability. The EMP pulse would likely circle the Earth in less then a second, and be destructive only near the center of the destruction. The 5 megaton H bomb test allegedly tripped some circuit breakers 800 miles away. Neil

Asteroidal EMP's would be minimal, at best, unless the asteroid approached the size of our Moon.

If that were the case, we'd have a bit more to worry about than EMPs...