The space rock, Asteroid 2019 OK, has never been tracked before and even top astronomers have said it was “uncomfortably close”.
According to NASA, the 130m space rock hurtled past at around 45,000 mph on July 25.
It was within 50,000 miles of Earth, just a fifth of the distance to the moon.
At this size and speed a direct hit would obliterate a city with the power of a thermonuclear weapon.
“If it had hit Columbus, Ohio, then there would be Columbus, Ohio, no more”
Prof Kris Stanek
“If it’s 100 metres in diameter, it would leave a crater about one kilometre in diameter, and the explosion energy would be equivalent to five megatons of TNT,” Prof Kris Stanek told El Reg.
“If it had hit Columbus, Ohio, then there would be Columbus, Ohio, no more.
“But if it hits somewhere where there is nothing, then not much would happen. If it hits the ocean, it may incite a tsunami, but not one that’s life threatening.”
Astronomers said it was amazing that nobody had seen the large asteroid, with worrying implications.
Alan Duffy, lead scientist at the Royal Institution of Australia, said he was baffled when his phone was flooded with calls.
“I thought everyone was getting worried about something we knew was coming,” he told The Washington Post.
When he realised the “uncomfortably close” asteroid was an unknown rogue, he could scarcely believe it.
“I was stunned,” he said. “This was a true shock.”
“It snuck up on us pretty quickly,” Michael Brown, an associate professor in Australia with Monash University’s School of Physics and Astronomy, added.
He told the title: “People are only sort of realising what happened pretty much after it’s already flung past us.”