Zipping through space at close to the speed of light, Solar Energetic Particles, or SEPs, are one of the main challenges for the future of human spaceflight. Clouds of these tiny solar projectiles can make it to Earth—a 93 million mile journey—in under an hour. They can fry sensitive spacecraft electronics and pose serious risks to human astronauts. But their onset is extraordinarily hard to predict, in part because we still don't know exactly where on the Sun they come from.