In February of 2013, skywatchers around the world turned their attention toward asteroid 2012 DA14, a cosmic rock about 150 feet (50 meters) in diameter that was going to fly closer to Earth than the spacecraft that bring us satellite TV.

Little did they realize as they prepared for the once-in-several-decades event that another bit of celestial debris was hurtling toward Earth, with a more direct heading. On February 15, 2013, the Chelyabinsk meteor, a roughly 62-foot (19 meter)-diameter asteroid exploded over the city of Chelyabinsk, Russia, as it entered Earth’s atmosphere at a shallow angle. The blast shattered windows and damaged buildings, and nearly two thousand people were hurt, though thankfully no one died.

“There is a large asteroid or comet lurking in our solar system with ‘Earth’ written on it. We just don’t know where it is or when it will hit.”

“It turned out that two completely independent asteroids were coming by that day,” says Philip Lubin, professor of physics at the University of California, Santa Barbara, and one of the many scientists anticipating 2012 DA14’s near-Earth rendezvous. “One of them we knew was going to miss the Earth. The other one, we didn’t even know it was coming.”

For Lubin and scientists like him, incidents like these underline the importance of robust planetary defense—the detection, tracking, characterization, and ultimately defense against potentially dangerous asteroids and comets. City-threatening events like Chelyabinsk are rare, happening about once every 50 to 100 years, but they are potentially devastating.

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