Hundreds of people can walk by your front door every day.

The only time you may notice someone, however, is when they knock. Last week, the universe knocked on Earth’s door.

Thousands of asteroids and meteoroids streak pass planet Earth every year. Some of them enter Earth’s atmosphere where most of them simply burn up—those are called meteors. Any that land on Earth are called meteorites.

When a meteorite landed in the city of Chelyabinsk, Russia last Friday, the world took notice. That’s because it was a particularly large meteorite; one chunk was about the size of a van.