It all began on 1 July, when an asteroid-hunting telescope in Chile spotted a tiny white dot hurtling against the background stars. The online chatter began almost immediately as astronomers tracked the object in earlier images and amateur observations. Soon it was clear the dot was moving through the Solar System so fast—209,000 kilometers per hour—that it could not be orbiting the Sun. That night, says Michigan State University (MSU) astronomer Darryl Seligman, “I woke up at 1 a.m. [and] saw m...