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.