There is a scene in Dances with Wolves after Costner's character has arrived at the deserted Dakota base in which he discovers the company's garbage pile. He gives it a disappointing, scrutinizing look as he recognizes it's another harbinger of what is to come for the plains and its people. Fast forward 150 years to a different kind of frontier: space, in near-Earth orbit. There, we find a similar garbage pile, only this one is traveling at 30,000 miles per hour and threatens all the satellites and telescopes and space stations floating about.

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.