A sizable asteroid zipped near our planet this month without anyone noticing because it traveled through an astronomical blind spot, scientists said.
The space boulder passed Earth within 288,000 miles (461,000 kilometers) -- or 1.2 times the distance to the moon -- on March 8, but since it came from the direction of the sun, scientists did not observe it until four days later.
The object, slightly larger than one that flattened a vast expanse of Siberia in 1908, was one of the 10 closest known asteroids to approach Earth, astronomers said.
"Asteroid 2002 EM7 took us by surprise. It is yet another reminder of the general impact hazard we face," said Benny Peiser, a European scientist who monitors the threat of Earth-asteroid collisions.
If it pierced the atmosphere, the approximately 70-meter-long rock could have disintegrated and unleashed the energy equivalent of a 4-megaton nuclear bomb, researchers said.
"If it were over a populated area, like Atlanta, it would have basically flattened it," said Gareth Williams, associate director of the International Astronomical Union Minor Planet Center in Boston, Massachusetts.
The rock is considerably smaller than dozens of potential planet killers 1-kilometer in size or larger that lurk in the inner solar system.
Like its larger siblings, asteroid 2002 EM7 follows an elliptical orbit with an extremely low risk of Earth collision in the coming decades or centuries.
Nonetheless, astronomers maintain that constant surveillance is necessary to identify more killer rocks in our neighborhood and ensure that none take our planet by surprise, in particular those traveling near the blinding light of the sun.
"If one comes from the direction of the sun, we're not going to see it," Williams said.
"Often these objects are outside of the Earth's orbit for a significant amount of time. The key is to detect them when they are outside the Earth's orbit and predict whether they might hit us in the future from the sun side."
Even lesser rocks such as 2002 EM7 could do serious damage by plunging into the ocean and unleashing monster tsunamis on coastal cities, he said.
According to NASA's Jet Propulsion Laboratory, 2002 EM7 could smack into Earth in 2093.
But don't tell the grandchildren to head to the hills just yet. The odds of a collision are currently 1 in 10 million and could become even more remote with more refined calculations.