A gripping account of the "city-killer" asteroids that could threaten Earth and the race to build a planetary defense system. There are approximately 25,000 "city killer" asteroids in near-Earth orbit--and most are yet to be found. Small enough to evade detection, they are capable of large-scale destruction, and represent our greatest cosmic threat. But in September 2022, against all odds, NASA's Double Asteroid Redirection Test (DART) mission deliberately crashed a spacecraft into a carefully selected city killer, altering the asteroid's orbit and proving that we stand a chance against them. In How to Kill an Asteroid, award-winning science journalist Robin George Andrews--who was at DART mission control when it happened--reveals the development of the technology that made it possible, from spotting elusive asteroids and comets to figuring out their geologic defenses and orchestrating a deflection campaign. In a propulsive narrative that plays like a sci-fi thriller, Andrews tells the story of the planetary defense movement, and introduces the international team of scientists and engineers now working to protect Earth.