The Earth had a surprise visitor Thursday, and not a welcome one. An asteroid big enough to earn the nickname "city-killer" from astronomers whizzed by at a close distance, and it wasn't detected soon enough to take any defensive action had that been necessary, reports the Washington Post. Another outlet, CNET, likens the pass-by of Asteroid 2019 OK to having a "baseball [whistle] by your ear" as you walk down the street. The asteroid is about 330 feet wide and came within about 45,000 miles of Earth, one fifth the distance to the moon. At its closest point, it would have been visible with binoculars, writes astronomer Michael Brown in the Conversation. “It snuck up on us pretty quickly,” Brown tells the Post. "People are only sort of realizing what happened pretty much after it’s already flung past us."
Had the asteroid struck, it would have landed with the force of a large nuclear weapon, or about 10 megatons of TNT. Two astronomy teams picked up on its presence on Wednesday; Brown says the details about how big it was and where it was traveling were disseminated hours before it cruised past us. So how did asteroid-trackers nearly miss it? Its size, for one thing. While big enough to cause lots of damage, 2019 OK is not that big in the grand scheme—nothing on the scale of the famous dinosaur-killer. So yes, it could be spotted with binoculars at its closest point, but "three days before that it was 1,000 times fainter, and thus harder to spot," writes Brown. It also has an "eccentric orbit," he says, adding that it's unlikely to come this close to Earth in the future. (Japan's space agency just pulled off a remarkable feat with a different asteroid.)