One mystery in planetary science is a satisfying origin story for Mars’s moons, Phobos and Deimos. Were they chunks of Mars blasted into space by a meteor impact? Were they captured asteroids from the belt? A new supercomputer simulation found that a reasonable explanation could come from a massive asteroid passing just close enough to Mars that it was torn into pieces. Over time, chunks and debris would have settled into a disk around Mars and clumped into moons.