On 15 April 2020, a wave of gamma rays, nature’s most powerful kind of light, washed across the Solar System like a storm front. First contact came above Mars, where photons at energies comparable to the radiation from a nuclear bomb peppered a Russian particle detector on NASA’s Mars Odyssey probe. Six minutes later, the burst of light lit up a solar wind probe between the Sun and Earth. Five seconds after that, the signal splashed into specialized detectors on Earth’s surface.