Catastrophe Postponed?

“An EMP attack … only requires but one nuclear weapon, detonated 300 miles above the middle of the United States. One bomb.”

The Silver Bear Cafe has an article by William R. Forstchen, PhD on electromagnetic pulse (EMP) threat to the USA. If you don’t know what EMP is then watch Goldeneye, but basically it is a sharp pulse of electromagnetic energy that damages electronics. A widespread EMP can be caused by detonating a nuclear weapon in the lower reaches of space, and the article suggests that is a real threat, and that a single bomb exploded a three hundred miles above the USA would effectively wipe out the country.

That seemed a little pessimistic to me, so I did a quick Ask Jeeves calculation. I looked up the energy in TNT, a little over 4kJ per gramme, multiplied to a million tonnes to give the blast energy of a one megaton bomb (i.e. a big one) being about 4 million gigajoules. However that is the thermodynamic blast energy, and it is radiation we are interested in. According to Forstchen it is the gamma rays that cause EMP, and from our friend Jeeves we can find that 85% of the energy from a nuclear bomb is blast or thermal, and 5% gamma, so the gamma energy is around 250,000 gigajoules. Assume all the energy is converted to EMP, as I don’t know the rate but suspect it would be high.

I divided the energy by the the rough area of the contiguous USA, estimating 8 million square kilometres that is around 30 megajoules per square kilometre, or an average of 30 joules per square metre. But of course more than half the energy goes up into space, so that landing averages less than 15 joules per square metre. Of course it would be far more intense just below, but most of the US population lives on the seaboards where intensity would be far lower if the bomb was over central USA.

Sorry, Dr Forstchen, I am not buying it. The article suggests that a car hit by this energy will no longer work due to fried electrics, but a car hit by lightening still works, and far more EM energy is produced. The car is a Faraday Cage, and although not a perfect one for the cabin the engine electronics are well protected. Of course a fibreglass sports car might have some troubles, but TVR drivers probably know how to cope without the car.

Yes EMP is a hazard and a potential threat from a rogue state, but far more localised than Dr Forstchen suggests. Ironically Goldeneye was far more realistic in that sense. This does mean more should be done to plan for it, as a plan could be effective, which it could not in the case of national destruction.

Can anyone see a flaw in my reasoning? It is only a rough calculation, but the order of magnitude should be right as far as I can see.

Hat tip to Instapundit for the link.

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Edit: just to add a link to a follow-on post.