A powerful solar flare erupts in this image from a Sun-watching spacecraft. The flare is an explosion on the Sun's surface, and it produces a torrent of radiation and charged particles. The most powerful flares can knock out satellites, disrupt some radio transmissions, and trigger blackouts. A recent study says that, more than a millennium ago, the Sun produced two outbursts that were more powerful than anything seen in the modern era, while another study says the Sun could produce flares that are even more powerful, threatening extensive damage to our technology. [NASA/GSFC]