A giant 'city-killer' asteroid that just whizzed past earth seemingly appeared "out of nowhere" has stunned astronomers after only being discovered last week, days before it flew within around 45,000 miles from earth - or less than 20% of the distance to the moon, according to the Washington Post.

"I was stunned," said Alan Duffy - lead scientist at the Royal Institution of Australia. "This was a true shock."

This asteroid wasn’t one that scientists had been tracking, and it had seemingly appeared from “out of nowhere,” Michael Brown, a Melbourne-based observational astronomer, told The Washington Post. According to data from NASA, the craggy rock was large, an estimated 57 to 130 meters wide (187 to 427 feet), and moving fast along a path that brought it within about 73,000 kilometers (45,000 miles) of Earth. That’s less than one-fifth of the distance to the moon and what Duffy considers “uncomfortably close.” -Washington Post

"It snuck up on us pretty quickly," said Michael Brown, an associate professor at Australia's Monash University School of Physics and Astronomy, adding later "People are only sort of realizing what happened pretty much after it’s already flung past us."

This is the video of the close encounter of Asteroid 2019 OK we have been Twitting all day with the Earth: https://t.co/bjT7uhQJuO pic.twitter.com/3e4UyPcdPl — ASAS-SN (@SuperASASSN) July 25, 2019

The asteroid was discovered by separate astronomy teams in the United States and Brazil - while information on the 'city-killer' was announced only hours before it shot past Earth.

"It shook me out my morning complacency," said Brown. "It’s probably the largest asteroid to pass this close to Earth in quite a number of years."

How did we not see this coming?

For starters, while Asteroid 2019 OK (as it's been named) is large enough to destroy a city, it's nowhere near the half-mile-wide or larger asteroids which NASA and its international partners have scientists think they've identified 90% of.

"Nothing this size is easy to detect," said Duffy. "You’re really relying on reflected sunlight, and even at closest approach it was barely visible with a pair of binoculars."

Brown said the asteroid’s “eccentric orbit” and speed were also likely factors in what made spotting it ahead of time challenging. Its “very elliptical orbit” takes it “from beyond Mars to within the orbit of Venus,” which means the amount of time it spends near Earth where it is detectable isn’t long, he said. As it approached Earth, the asteroid was traveling at about 24 kilometers per second, he said, or nearly 54,000 mph. By contrast, other recent asteroids that flew by Earth clocked in between 4 and 19 kilometers per second (8,900 to 42,500 mph). “It’s faint for a long time,” Brown said of Asteroid 2019 OK. “With a week or two to go, it’s getting bright enough to detect, but someone needs to look in the right spot. Once it’s finally recognized, then things happen quickly, but this thing’s approaching quickly so we only sort of knew about it very soon before the flyby.” -Washington Post

"It should worry us all, quite frankly," Duffy added. "It’s not a Hollywood movie. It is a clear and present danger."

The reason Asteroid 2019 OK is referred to as a 'city-killer' is because it's large enough that if it struck earth, most of it would likely have reached the ground, resulting in catastrophic damage.

"It would have gone off like a very large nuclear weapon," with enough energy to level a city," said Duffy. "Many megatons, perhaps in the ballpark of 10 megatons of TNT, so something not to be messed with."

In 2013, a much smaller meteor (around 65 feet across) broke up over the Russian city of Chelyabinsk - the shockwave from which shattered windows, collapsed roofs, caused car accidents, and provided some amazing footage to boot. Around 1200 people were injured.

According to the report, "The last space rock to strike Earth similar in size to Asteroid 2019 OK was more than a century ago, Brown said. That asteroid, known as the Tunguska event, caused an explosion that leveled 2,000 square kilometers (770 square miles) of forest land in Siberia."

What to do?

Turning his attention to the topic of planetary defense, Duffy warns against trying to "blast it with a nuke" to avert disaster.

"It makes for a great Hollywood film," he said. "The challenge with a nuke is that it may or may not work, but it would definitely make the asteroid radioactive."

Instead, he recommends a 'gravity tractor' which would use the gravity of a spacecraft - something Duffy calls an "elegant solution."

In light of Asteroid 2019 OK, Duffy stressed the importance of investing in a “global dedicated approach” to detecting asteroids because “sooner or later there will be one with our name on it. It’s just a matter of when, not if.” “We don’t have to go the way of the dinosaurs,” he said. “We actually have the technology to find and deflect certainly these smaller asteroids if we commit to it now.” Emily Lakdawalla, senior editor of the Planetary Society, which promotes space exploration, said the recent near miss is a reminder that “it’s an important activity to be watching the skies.” The more that can be learned about an asteroid, the better prepared people can be to prevent potential disasters, she told The Post. -Washington Post

"It’s the kind of thing where you learn about something that you didn’t know about, like things flying close by us, and your inclination is to be scared," said Emily Lakdawalla. "But just like sharks in the ocean, they’re really not going to hurt you and they’re really fascinating to look at."

Sure, until one lands on your house.