It came from beyond our solar system. But the sun wasn’t content to let it leave in peace, or in one piece. Comet 2I/Borisov, an Eiffel Tower-sized clod of dust and ice, plunged into our solar system last fall, exhaling vapor as it buzzed nearest to our sun around Christmas. This alien visitor must have formed around a distant and unknown star. It slumbered as it crossed the frozen gulf of interstellar space. But now, suddenly, the sleeper is awake and kicking. To the simultaneous delight and frustration of the world’s astronomers, Borisov has sloughed off at least one fragment...