More than 1,000 people were injured last February in Chelyabinsk, Russia, when a meteor exploded over the city. The collision shattered windows and pelted startled residents with shards of glass and debris. In the aftermath, the world was transfixed by extraordinary videos of the huge fireball as it streaked across the sky. Many wondered, why on earth did no one see it coming?

“The odds of asteroid impacts are much higher than people realize,” said Ed Lu, a former astronaut and chief executive officer of the B612 Foundation, which searches for asteroids that could potentially hit the Earth and cause human devastation. He said that there is a 30 percent chance of a city-destroying asteroid hitting the Earth in the next 100 years.

The primary source of meteors like the one that exploded over Russia is the asteroid belt between the orbits of Jupiter and Mars. The gravitational pull of giant Jupiter causes space debris to collide repeatedly, breaking into smaller and smaller fragments that became asteroids.

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