GREENBELT, Md. (SCIENCE@NASA) When the sun rose over Russias Ural Mountains on Friday, Feb. 15th, many residents of nearby Chelyabinsk already knew that a space rock was coming. Later that day, an asteroid named 2012 DA14 would pass by Earth only 17,200 miles above Indonesia. There was no danger of a collision, NASA assured the public.

Maybe thats why, when the morning sky lit up with a second sun and a shock wave shattered windows in hundreds of buildings around Chelyabinsk, only a few people picking themselves off the ground figured it out right away. This was not a crashing plane or a rocket attack.

It was a meteor strikethe most powerful since the Tunguska event of 1908, says Bill Cooke of NASAs Meteoroid Environment Office.

In a coincidence that still has NASA experts shaking their heads, a small asteroid completely unrelated to 2012 DA14 struck Earth only hours before the publicized event. The impactor flew out of the blue, literally from the direction of the sun where no telescope could see it, and took everyone by surprise.

These are rare events and it is incredible to see them happening on the same day, says Paul Chodas of NASAs near-Earth Object Program at JPL.

Researchers have since pieced together what happened. The most telling information came from a network of infrasound sensors operated by the Comprehensive Test Ban Treaty Organization (CTBTO). Their purpose is to monitor nuclear explosions.

Infrasound is a type of very low-frequency sound wave that only elephants and a few other animals can hear. It turns out that meteors entering Earths atmosphere cause ripples of infrasound to spread through the air of our planet. By analyzing infrasound records, it is possible to learn how long a meteor was in the air, which direction it traveled, and how much energy it unleashed.

The Russian meteors infrasound signal was was the strongest ever detected by the CTBTO network. The furthest station to record the sub-audible sound was 15,000km away in Antarctica.

Western Ontario Professor of Physics Peter Brown analyzed the data: The asteroid was about 17 meters in diameter and weighed approximately 10,000 metric tons, he reports. It struck Earths atmosphere at 40,000 mph and broke apart about 12 to 15 miles above Earths surface. The energy of the resulting explosion exceeded 470 kilotons of TNT. For comparison, the first atomic bombs produced only 15 to 20 kilotons.

Based on the trajectory of the fireball, analysts have also plotted its orbit. It came from the asteroid belt, about 2.5 times farther from the sun than Earth, says Cooke.

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NASA: What exploded over Russia?