The small pieces of rock that enter Earth's atmosphere at speeds of 15 to 70 kilometers per second and burn up are called meteors.

Here's why:

* Meteoroid: This is the term for the small piece of rock *before* it enters Earth's atmosphere.

* Meteor: This is the term for the bright streak of light we see when the meteoroid burns up in the atmosphere.

* Meteorite: If a meteoroid survives its fiery journey through the atmosphere and actually hits the Earth's surface, it is then called a meteorite.

So, the word you're looking for is meteor.