On July 25, 2015, the near-Earth asteroid 1999 JD6 made its closest pass of Earth in at least over a century, coming within 4.5 million miles of our planet (7.2 million kilometers, or about 19 times the distance between Earth and the moon) and traveling at a relative velocity of 45,410 mph (20.3 km/s). As the 1.2-mile (2 km) asteroid zipped by, NASA aimed two of its largest radio telescopes at it, bouncing radar waves off its surface to measure its size, shape, and rotation. ...1999 JD6 is what’s known as a “contact binary,” a peanut-shaped world that’s likely the...