This week, Earth had a close call with what some scientists call a "city-killer" asteroid, which, if it had made impact, would have hit the planet with 30 times the power of the atomic bomb that destroyed Hiroshima.

Asteroid 2019 OK sped by Earth on Wednesday, flying some 45,000 miles away, inside the orbit of the moon.

Scientists were shocked to discover the asteroid only within days of its passing, and only announced its presence hours before it became visible. According to The Washington Post, Asteroid 2019 OK was discovered by two astronomy teams in Brazil and the United States.

Michael Brown, a Melbourne, Australia-based observational astronomer, told The Washington Post that scientists had not been tracking the asteroid, and described it as appearing from "out of nowhere."

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Experts are still unsure of all of the details about the newly uncovered asteroid, with few specifics on its exact orbit known. The asteroid is estimated to measure between 187 and 426 feet, or 57 to 130 meters, across.

“It would have gone off like a very large nuclear weapon” with enough force to destroy a city, Alan Duffy, a lead scientist for the Royal Institution of Australia, told the Post. “Many megatons, perhaps in the ballpark of 10 megatons of TNT, so something not to be messed with.”

While the impact would have been large, it would not have been a global event. The asteroid theorized to have killed the dinosaurs more than 60 million years ago was, in comparison, 10 miles across or possibly larger.

NASA regularly monitors asteroids and other so-called near-Earth objects, but because this asteroid approached from the direction of the sun it was hard to spot. NASA is working on new technologies to possibly divert asteroids from colliding with Earth in the future.