One of the biggest disasters we face would begin about 18 hours after the sun spit out a 10-billion-ton ball of plasma--something it has done before and is sure to do again. When the ball, a charged cloud of particles called a coronal mass ejection (CME), struck the Earth, electrical currents would spike through the power grid. Transformers would be destroyed. Lights would go out. Food would spoil and--since the entire transportation system would also be shut down--go unrestocked.

Within weeks, backup generators at nuclear power plants would have run down, and the electric pumps that supply water to cooling ponds, where radioactive spent fuel rods are stored, would shut off. Multiple meltdowns would ensue. “Imagine 30 Chernobyls across the U.S.,” says electrical engineer John Kappenman, an expert on the grid’s vulnerability to space weather. A CME big enough to take out a chunk of the grid is what scientists and insurers call a high-consequence, low-frequency event. Many space-weather scientists say the Earth is due for one soon. Although CMEs can strike anytime, they are closely correlated to highs in the 11-year sunspot cycle. The current cycle will peak in July 2013.