Letter Re: Potential Range of Nuclear Weapon EMP?

Mr. Rawles:

I’m confused. Some things that I’ve read say that the maximum range of [nuclear weapon electromagnetic pulse] EMP is about 60 miles, but others say 200 or 250 miles. Which of them is right? Wouldn’t a terrorist bomb at ground level have shorter range EMP than a nuke touched off at high altitude or low orbit? (With a wider horizon.) Thanks, – Lance in Nebraska

JWR Replies: You aren’t the first SurvivalBlog reader to ask about the greatest potential effective range of an EMP-optimized nuclear detonation. I first discussed this in SurvivalBlog back in October of 2005. The answer is both easy and impossible to determine. Let me explain. First, the easy part. The basic line of sight (LOS) footprint range calculation is really simple. It is essentially the same as the calculation that is used to determine the maximum effective range for a VHF or UHF radio onboard an aircraft. Referring back to one of my unclassified notebooks from my Electronic Warfare (5M) course at Fort Huachuca, I find: Assuming level terrain, the maximum potential radius of LOS in nautical miles (nmi) = square root of the emitter’s altitude (in feet) x 1.056. Hence, that would be 149.3 nmi at 20,000 feet above sea level (ASL), or 191.8 nmi at 33,000 feet ASL. (A typical jet or C-130’s service ceiling.) SurvivalBlog reader “Flighter” mentioned: “…some of the larger business jets such as the Airbus ACJ, Gulfstream, Challenger, and Citation are certificated to fly at or above 41,000 feet. The Sino Swearingen SJ30, is perhaps the highest flyer with a certificated ceiling of 49,000 feet. Hypothetically, a dangerous parabolic flight profile could with supplemental oxygen for the flight crew and perhaps even supplemental JATO rockets might push apogee to 75,000 feet in a few aircraft models. (Hey, it would be a suicidal flight anyway.) That is probably the highest altitude that could be expected for a terrorist to touch off a nuke–at least in the near future. That would equate to a footprint with a 280 mile radius. Oh, yes, they might also get really creative and use an unmanned balloon. (The word’s record for those was 51.82 km (170,000 feet / 32.2 miles) But that is highly unlikely. What is likely? A ground level detonation. The EMP footprint of fission bomb detonated near ground level on dead level ground (plains country) might be no more than a 45 mile radius.

Now on to the part that is impossible to predict: long range linear coupling.  Because telephone lines, power lines, and railroad tracks will act as giant antennas for EMP, the EMP waveforms will be coupled through those structures for many, many miles beyond line of sight (BLOS). Just how many miles BLOS is not yet known. I believe that if it were not for the advent of the Partial Test Ban Treaty in 1963 (which banned atmospheric and space nuclear weapons tests), the DOD and AEC would have had the opportunity to conduct far more extensive tests to further characterize the panoply of potential EMP effects. But those test bans have kept us in the dark. In the absence of practical data, there is a lot guesswork, even among “applied physics” expert nuclear weapons physicists. We may not know the full extent of the EMP risk until after we see that bright flash on the horizon.

For planning purposes, you can probably safely assume that if you are living more than 280 miles from a major city, then your vehicle electronics will be safe from a terrorist  nuke’s EMP. (Since you will be BLOS to the EMP footprint of a nuke that is set off below 75,000 feet ASL.) Your home electronics, however, anywhere in CONUS might be at risk due to long range linear coupling–that is if your house is on grid power. This, BTW, is one more good reason for you to set up your own off-grid self sufficient photovoltaic (PV) power system. The folks at Ready Made Resources. offer free consulting on PV system sizing, site selection, and design.