Risk, she explains, is what you get when you multiply probability and consequence.

The probability of an EMP attack gets larger every day, she says, because "a whole lot of people are developing weapons, and some of these people are not very responsible." The consequences are increasing all the time, too, she says, as we become more and more dependent on solid-state electronics.

Even a country with one low-yield bomb, deployed from a missile shot from a freighter in U.S. coastal waters to an altitude of 200 miles, could turn the United States into a Third World country within moments, she says. The damage is so swift that most lightning-protective devices are useless.

An EMP isn't dangerous to our health. (When a nuclear bomb explodes in the atmosphere rather than on the ground, there is no fallout, blast or thermal effect.) But the consequences of the EMP will destroy us just the same, Packer says. We won't be able to start our cars, use our phones, fly planes or use computers. We won't be able to pump gas, harvest or deliver food, collect garbage, purify water or flush most of our toilets. Our backup fuel to run our backup generators will last only a short while, she says. We will quickly run out of water, food and other necessities of life, and the entire infrastructure of the nation will fail.

To survive, each of us will need our own supply of water or a way to purify the water we'll be forced to retrieve from streams. We'll need a supply of food that won't spoil and a way to cook it. We'll need a way to stay warm. We'll need a stockpile of medications and a high-frequency amateur radio that has been carefully protected in a metal Faraday cage. And we'll need all these things until the power system has been restored, which will most likely take more than a year, she says. One example of how hard it will be to patch the infrastructure: The United States does not produce or stockpile any large power transformers; instead it imports them from overseas, and we'd need thousands of them to restore our power. And even if we did import them, we'd have no way to truck them to remote locations.

Personal preparation