The discovery of asteroid 2002 NT7, the space rock that astronomers believe could collide with Earth on February 1, 2019, wasn't the worst cosmic news of the summer. More unsettling was the announcement on June 17 that a football-field-size asteroid had whizzed by Earth three days before. It missed us by 750,000 miles—less than a third of the distance to the moon—and had the destructive pop of a nuclear weapon.