Asteroid Lesson for Kids: Definition & Facts

Instructor: Josh Corbat

Josh has taught Earth Science and Physical Science at the High School level and holds a Master of Education degree from UNC-Chapel Hill.

Space is not just empty. It is full of rocks, dust, and even ice. In this lesson, we will talk about one type of these objects in space: Asteroids. We will talk about the size of asteroids and what they are made of.

What is an Asteroid?

In space (and in our own Solar System) there are many different kinds of objects. You probably already know about stars and planets (our Sun is a star, and Earth is a planet), but many people have never heard about some of the smaller objects out there.

A small object made of ice that revolves around a star (like the Sun) is called a comet. It is unique from most of the other objects out there because it is made of ice. Everything else is made mostly of rock and metal. The smallest of these objects are called meteoroids. You probably wouldn't be able to tell the difference between a meteoroid and a rock you see on the ground here on Earth! They look really similar (because they are!).

Any rock in space that is bigger than about 10 meters is called an asteroid. Asteroids can be found anywhere in space, but usually are not found near large planets. This is because the planet's gravity will attract most asteroids (and meteoroids) into it, burning it up in its atmosphere. That is, unless it's really big! Then the planet would have a problem. Just ask the dinosaurs about that! (Which you obviously can't do, because an asteroid wiped them all out 65 million years ago!).

This is an actual picture of an asteroid. We have sent many spacecraft out into space to take pictures of asteroids so we can learn more about them.

How do Asteroids Form?

Most asteroids form over many years as small amount of dust gather together and stick to each other. This happens faster as the asteroid gets larger because it will have more gravity, allowing it to attract more dust. Over time, the object becomes large enough (greater than 10 meters) to be called an asteroid.

Sometimes, objects in space collide. If two really large objects run into each other (like two large asteroids, or even two planets), they will most likely break apart. What's left over from these collisions are often many millions of asteroids. This is a leading theory behind how the Asteroid Belt formed in our Solar System between the orbits of Mars and Jupiter.

This image shows two large asteroids compared our own Moon. As you can see, some asteroids can be really big!
Asteroid Moon comparison

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