Our planet Earth is like a target in a shooting gallery. It could be hit by a big space rock at any time. In fact, we were hit by one five years ago today, when a small asteroid exploded over Russia. A shockwave injured more than 1500 and damaged more than 7,000 buildings.

The Chelyabinsk asteroid was about 60 feet wide and weighed more than 10,000 tons when it hit Earth’s atmosphere. Its high-speed plunge heated the air around it, forming a fireball that was visible across thousands of square miles.

As the asteroid plowed deeper, it ran into thicker layers of air. At such high speed, that was almost like running into a concrete wall — the pressure caused the space rock to explode. Fragments fell across a wide area. The largest one plummeted into an ice-covered lake. It weighed about 1400 pounds.

Asteroids pass close to Earth all the time. Every month, in fact, several pass within a few million miles of us. And occasionally, one even passes within the Moon’s orbit around Earth. Many of those asteroids are much bigger that the one that hit Chelyabinsk, so their potential destructive power is much greater.

So far, astronomers have discovered and plotted the orbits of thousands of asteroids whose orbits bring them close to Earth’s orbit. None of those asteroids is on a collision course. But as Chelyabinsk demonstrates, smaller asteroids can slip through without detection — taking dead aim at planet Earth.

Script by Damond Benningfield