For millions of years, a frozen wanderer drifted between the stars before slipping into our solar system as 3I/ATLAS—only the third known interstellar comet ever spotted. When scientists turned NASA’s Swift Observatory toward it, they caught the first-ever hint of water from such an object, detected through a faint ultraviolet glow of hydroxyl gas. Even more surprising, the comet was blasting out water at a rate of about 40 kilograms per second while still far from the Sun—much farther than where most comets “switch on.”