How Big Does an Asteroid Need to Be to End Humanity?

While small space rocks burn up in our atmosphere every day, scientists are tracking the larger ones that pose a genuine threat. An impact from a “medium” asteroid like Bennu could trigger a global winter, but what would it take to cause total extinction? From mile-wide asteroids creating mega-tsunamis to planet-sterilizing comets that could boil the oceans, this is the science behind an apocalyptic impact.