An asteroid big enough to earn the moniker "city killer" zipped by the Earth Wednesday night -- closer than the Earth is to the moon. Nobody saw it coming until just a few days ago.

It serves as a reminder that, for as much as astronomers keep staring at the sky for potential threats, there's a lot of sky to look at.

"Asteroid 2019 OK" passed by Earth at 9:22 p.m. EDT Wednesday. It came within 45,000 miles of Earth, or about one-fifth of the distance between Earth and the moon.

According to the International Astronomical Union's Minor Planet Center, the asteroid was first observed on June 28 by the Pan-STARRS observatory in Hawaii.  There were six observations on that day, but not another until July 7. Then there was another two-week gap before the next observations on July 21 by ATLAS (Asteroid Terrestrial-impact Last Alert System) also in Hawaii. 

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That was the last time anyone saw it until the SONEAR Observatory in Brazil picked it up just before it passed by Wednesday night. That's when astronomers learned of its size and what path it was taking.

Michael Brown, an Australian astronomer, told The Washington Post the rock was an estimated 187 to 427 feet wide -- not big enough to be an extinction-level event, but big enough to do some major damage if it struck Earth, even after partially burning up in the atmosphere.

“It would have gone off like a very large nuclear weapon” with enough force to destroy a city, Brown said, adding this is the largest asteroid to pass this close to Earth the past few years.

So how was it nobody saw this coming? Australian astronomer Alan Duffy lists a few reasons, according to the Post.

  • Size: While Asteroid 2019 OK is big, it's not as big as those which could cause a global catastrophe. NASA says more than 90 percent of those have been identified.
  • Orbit: Its elliptical orbit took it past Mars, but within the orbit of Venus, so it does not spend much time near Earth.
  • Speed: Most recent asteroids have passed Earth at anywhere from 8,900 to 42,500 mph. This was traveling 54,000 mph.

The fact that this asteroid got this close with almost no warning is a wake-up call.

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

Scientists are trying to develop ways to deflect or redirect asteroids that are a threat to Earth, but the technology is not there, yet. Duffy suggests using a nuclear weapon is not the way to go, however, because there is no guarantee it would work and it would make the asteroid radioactive.