Two historic incidents often figure in the discussion:
• On July 9, 1962, the Atomic Energy Commission and the Defense Atomic Support Agency detonated the Starfish Prime, a 1.4-megaton H-bomb test at an altitude of 250 miles, some 900 miles southwest of Hawaii over the Pacific Ocean. The pulse shorted out streetlights in Oahu.
• On March 9, 1989, the sun spat a million-mile-wide blast of high-temperature charged solar gas straight at the Earth. The "coronal mass ejection" struck the planet three days later, triggering a geomagnetic storm that made the northern lights visible in Texas. The storm also induced currents in Quebec's power grid that knocked out power for 6 million people in Canada and the USA for at least nine hours.
"A lot of the questions are what steps does it make sense to take," Legge says. "We could effectively gold-plate every component in the system, but the cost would mean that people can't afford the rates that would result to pay for it."
"The high-altitude nuclear-weapon-generated electromagnetic pulse is one of a small number of threats that has the potential to hold our society seriously at risk," concluded a 2008 EMP Commission report headed by William Graham, a former science adviser to President Reagan.