The Chelyabinsk meteor was a 20-metre rock that unexpectedly hurtled into the Earth’s atmosphere on 15 February 2013. It exploded with the force of about 600 kilotonnes of TNT, creating a shock wave that shattered windows and knocked people off their feet. The event sparked increased global interest in meteor warning systems.
Millions of smallish rocks are thought to swing near Earth as they orbit the sun. They are tough to find and study because they reflect very little sunlight and can only be spotted when they are close. Even then, there might not be a telescope trained on that part of the sky at the right time.