Every so often, on time scales measured in days or weeks, a solar flare appears in the sun's outermost layers. Many flares produce coronal mass ejections (CMEs is the term of art). Although the sun continuously pushes charged particles outward to form the solar wind," a CME outburst produces many times the normal particle flux, and contains particles moving at higher speeds than the ordinary solar-wind particles possess. Consider the greatest CME recorded on Earth: the "Carrington event" that ended August 1859. Intense streams of charged particles from the sun ... shook the Earth's magnetic field, inducing electrical currents that put telegraph systems on the fritz around the world. Today, of course, we no longer rely on the telegraph. Instead, we routinely depend on transformers ... to regulate the electrical currents that power pretty much everything. A sufficiently powerful CME could burn out these transformers and deaden the electrical grid. It might also induce short circuits in anything that uses electrical circuits. The National Academy of Sciences estimates that the economic damage [from such an event] could reach $2 trillion. When can we expect the big one to arrive? Earlier this year, the physicist Peter Riley, ... published his estimate. Analyzing the records of CMEs for the past half-century, Riley estimates that the chance of a CME packing the punch of a Carrington-like event during the next decade at 12 percent, which he called "a sobering figure."