Large sun-grazing comets could bring on the sort of global electronics meltdown usually associated with electromagnetic pulse weapons or a full-scale nuclear exchange.

FORBES: Sun-Grazing Comets As Triggers For Electromagnetic Armageddon

Military planners understand the vulnerability of the grid to upset, but have been more concerned with man-made threats such as the electromagnetic pulse that would be generated by a high-altitude nuclear detonation.

FORBES: Intelligence Community Warns Solar Storms Could Collapse Electric Grid