Asteroids are small planets, found mostly between the planets Mars and Jupiter. They are the oldest elements in the solar system. A very long time ago (4.5 billion years), when the Sun was formed, there was only dust and gas around it.
The dust specks stuck together and made big objects, which might be as big as a truck, your school or even your town. These are the asteroids. Most of them were then able to stick together and make planets. But, as we said, there are still a few, especially between Mars and Jupiter.
Asteroids contain rock but also metal, often in small grains. There are some where the metal forms a big core in the center, and even some that are all metal (probably because they are pieces of broken big asteroid cores).
We know this thanks to space probes which have brought back samples. By the way, there is one currently, sent by Japan, called Hayabusa 2, which explores an asteroid called “Ryugu”, and gives us a lot of exciting images. Another probe, American, “Osiris-rex” has also just approached the asteroid Bennu, which looks a lot like it.
We also know this by looking at meteorites, which are pieces of asteroids.
