"This asteroid wasn’t one that scientists had been tracking and it had seemingly appeared from 'out of nowhere'..."

"According to data from NASA, the craggy rock was large, roughly 100 meters wide, and moving quickly along a path that brought it within about 73,000 kilometers of Earth. That’s one-fifth of the distance to the moon and what [one scientist said was] 'uncomfortably close.' 'It snuck up on us pretty quickly,' said [another scientist] 'People are only sort of realizing what happened pretty much after it’s already flung past us.'... Information about its size and path was announced just hours before it rocketed past Earth.... Asteroid 2019 OK is a sizable chunk of rock, but it’s nowhere near as big as the ones capable of causing an event like the dinosaurs’ extinction. More than 90 percent of those asteroids, which are 1 kilometer or larger, have already been identified by NASA and its partners.  'Nothing this size is easy to detect'... 'It would have gone off like a very large nuclear weapon' with enough force to destroy a city, he said. 'Many megatons, perhaps in the ballpark of 10 megatons of TNT, so something not to be messed with.'... The last space rock to strike Earth similar in size to Asteroid 2019 OK was more than a century ago.... That asteroid, known as the Tunguska event, caused an explosion that leveled 2,000 square kilometers of forest land in Siberia."

WaPo reports.