In 2016, NASA took on a new responsibility: defending our planet from devastating impacts by asteroids and comets that approach the Earth, or near-Earth objects. That event, which followed the prominent Chelyabinsk meteor explosion in 2013, reflected a growing interest in, and concern about, the threat of celestial impacts. In ancient times, the solar system's small bodies-asteroids and comets-were sometimes seen as ill omens and warnings from...